InnoDB Plugin 1.0 for MySQL 5.1 User`s Guide

InnoDB Plugin 1.0 for MySQL 5.1 User`s Guide
InnoDB Plugin 1.0 for MySQL 5.1 User's Guide
InnoDB Plugin 1.0 for MySQL 5.1 User's Guide
Abstract
This is the User's Guide for InnoDB Plugin 1.0.8 for MySQL 5.1.
Starting with version 5.1, MySQL AB has promoted the idea of a “pluggable” storage engine architecture , which
permits multiple storage engines to be added to MySQL. Beginning with MySQL version 5.1, it is possible for
users to swap out one version of InnoDB and use another. The pluggable storage engine architecture also permits
Innobase Oy to release new versions of InnoDB containing bug fixes and new features independently of the
release cycle for MySQL.
This User's Guide documents the installation and removal procedures and the additional features of the InnoDB
Plugin 1.0.8 for MySQL 5.1.
WARNING: Because the InnoDB Plugin introduces a new file format, restrictions apply to the use of
a database created with the InnoDB Plugin with earlier versions of InnoDB, when using mysqldump
or MySQL replication and if you use the InnoDB Hot Backup utility. See Section 1.5, “Operational
Restrictions”.
For legal information, see the Legal Notices.
Document generated on: 2014-01-30 (revision: 37573)
Table of Contents
Preface and Legal Notices ........................................................................................................... vii
1 Introduction to the InnoDB Plugin ............................................................................................... 1
1.1 Overview ........................................................................................................................ 1
1.2 Features of the InnoDB Plugin ......................................................................................... 1
1.3 Obtaining and Installing the InnoDB Plugin ....................................................................... 2
1.4 Viewing the InnoDB Plugin Version Number ..................................................................... 3
1.5 Operational Restrictions .................................................................................................. 3
2 Fast Index Creation in the InnoDB Storage Engine ...................................................................... 5
2.1 Overview of Fast Index Creation ...................................................................................... 5
2.2 Examples ........................................................................................................................ 5
2.3 Implementation ............................................................................................................... 6
2.4 Concurrency Considerations ............................................................................................ 7
2.5 Crash Recovery .............................................................................................................. 7
2.6 Limitations ...................................................................................................................... 8
3 InnoDB Data Compression ......................................................................................................... 9
3.1 Overview of Table Compression ...................................................................................... 9
3.2 Enabling Compression for a Table ................................................................................... 9
3.2.1 Configuration Parameters for Compression .......................................................... 10
3.2.2 SQL Compression Syntax Warnings and Errors ................................................... 11
3.3 Tuning InnoDB Compression ......................................................................................... 12
3.3.1 When to Use Compression ................................................................................. 12
3.3.2 Monitoring Compression at Runtime .................................................................... 15
3.4 How Compression Works in InnoDB .............................................................................. 15
3.4.1 Compression Algorithms ..................................................................................... 15
3.4.2 InnoDB Data Storage and Compression .............................................................. 16
3.4.3 Compression and the InnoDB Buffer Pool ............................................................ 17
3.4.4 Compression and the InnoDB Log Files ............................................................... 18
4 InnoDB File-Format Management ............................................................................................. 19
4.1 Overview of InnoDB File Formats .................................................................................. 19
4.2 Named File Formats ...................................................................................................... 19
4.3 Enabling File Formats ................................................................................................... 20
4.4 File Format Compatibility ............................................................................................... 20
4.4.1 Startup File Format Compatibility Checking .......................................................... 21
4.4.2 Table-Access File Format Compatibility Checking ................................................ 22
4.5 Identifying the File Format in Use .................................................................................. 23
4.6 Downgrading the File Format ......................................................................................... 23
4.7 Future InnoDB File Formats .......................................................................................... 23
5 InnoDB Row Storage and Row Formats ................................................................................... 25
5.1 Storage of Variable-Length Columns .............................................................................. 25
5.2 COMPACT and REDUNDANT Row Formats ....................................................................... 25
5.3 DYNAMIC Row Format ................................................................................................... 25
5.4 Specifying a Table's Row Format ................................................................................... 26
6 InnoDB INFORMATION_SCHEMA Tables .................................................................................... 27
6.1 Overview of InnoDB Support in INFORMATION_SCHEMA ................................................. 27
6.2 Information Schema Tables about Compression ............................................................. 27
6.2.1 INNODB_CMP and INNODB_CMP_RESET .............................................................. 27
6.2.2 INNODB_CMPMEM and INNODB_CMPMEM_RESET .................................................. 28
6.2.3 Using the Compression Information Schema Tables ............................................. 28
6.3 Information Schema Tables about Transactions .............................................................. 29
6.3.1 INNODB_TRX ...................................................................................................... 29
6.3.2 INNODB_LOCKS .................................................................................................. 30
6.3.3 INNODB_LOCK_WAITS ........................................................................................ 31
6.3.4 Using the Transaction Information Schema Tables ............................................... 31
6.4 Notes on Locking in InnoDB .......................................................................................... 35
6.4.1 Understanding InnoDB Locking ........................................................................... 35
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InnoDB Plugin 1.0 for MySQL 5.1 User's Guide
6.4.2 Rapidly Changing Internal Data ........................................................................... 35
6.4.3 Possible Inconsistency with PROCESSLIST .......................................................... 36
7 Performance and Scalability Enhancements .............................................................................. 37
7.1 Overview ....................................................................................................................... 37
7.2 Faster Locking for Improved Scalability .......................................................................... 38
7.3 Using Operating System Memory Allocators ................................................................... 38
7.4 Controlling InnoDB Insert Buffering ................................................................................ 39
7.5 Controlling Adaptive Hash Indexing ................................................................................ 39
7.6 Changes Regarding Thread Concurrency ....................................................................... 40
7.7 Changes in the Read Ahead Algorithm .......................................................................... 41
7.8 Multiple Background I/O Threads ................................................................................... 42
7.9 Group Commit .............................................................................................................. 42
7.10 Controlling the Master Thread I/O Rate ........................................................................ 43
7.11 Controlling the Flushing Rate of Dirty Pages ................................................................. 43
7.12 Using the PAUSE instruction in InnoDB spin loops ........................................................ 44
7.13 Control of Spin Lock Polling ......................................................................................... 44
7.14 Making Buffer Cache Scan Resistant ........................................................................... 44
7.14.1 Guidelines for innodb_old_blocks_pct and innodb_old_blocks_time ..... 45
7.15 Improvements to Crash Recovery Performance ............................................................ 46
8 Changes for Flexibility, Ease of Use and Reliability ................................................................... 47
8.1 Overview ....................................................................................................................... 47
8.2 Enabling New File Formats ............................................................................................ 47
8.3 Dynamic Control of System Configuration Parameters .................................................... 48
8.3.1 Dynamically Changing innodb_file_per_table ............................................. 48
8.3.2 Dynamically Changing innodb_stats_on_metadata ........................................ 48
8.3.3 Dynamically Changing innodb_lock_wait_timeout ........................................ 49
8.3.4 Dynamically Changing innodb_adaptive_hash_index .................................... 49
8.4 TRUNCATE TABLE Reclaims Space .............................................................................. 49
8.5 InnoDB Strict Mode ....................................................................................................... 49
8.6 Controlling Optimizer Statistics Estimation ...................................................................... 50
8.7 Better Error Handling when Dropping Indexes ................................................................ 51
8.8 More Compact Output of SHOW ENGINE INNODB MUTEX .............................................. 52
8.9 More Read Ahead Statistics .......................................................................................... 52
9 Installing the InnoDB Plugin ..................................................................................................... 53
9.1 Overview of Installing the InnoDB Plugin ........................................................................ 53
9.2 Checking for Compatible Version Levels ........................................................................ 54
9.3 Installing the Precompiled InnoDB Plugin as a Shared Library ......................................... 54
9.3.1 Installing the InnoDB Plugin as a Shared Library on Unix or Linux ......................... 55
9.3.2 Installing the Binary InnoDB Plugin as a Shared Library on Microsoft Windows ....... 58
9.3.3 Errors When Installing the InnoDB Plugin on Microsoft Windows ........................... 60
9.4 Building the InnoDB Plugin from Source Code ................................................................ 61
9.4.1 Building the InnoDB Plugin on Linux or Unix ........................................................ 61
9.4.2 Building the InnoDB Plugin on Microsoft Windows ................................................ 62
9.5 Configuring the InnoDB Plugin ....................................................................................... 64
9.6 Frequently Asked Questions about Plugin Installation ...................................................... 64
9.6.1 Should I use the InnoDB-supplied plugin or the one that is included with MySQL
5.1.38 or higher? ........................................................................................................ 64
9.6.2 Why doesn't the MySQL service on Windows start after the replacement? .............. 64
9.6.3 The Plugin is installed... now what? ..................................................................... 65
9.6.4 Once the Plugin is installed, is it permanent? ....................................................... 65
10 Upgrading the InnoDB Plugin ................................................................................................. 67
10.1 Upgrading the Dynamic InnoDB Plugin ......................................................................... 67
10.2 Upgrading a Statically Built InnoDB Plugin .................................................................... 67
10.3 Converting Compressed Tables Created Before Version 1.0.2 ....................................... 68
11 Downgrading from the InnoDB Plugin ..................................................................................... 69
11.1 Overview ..................................................................................................................... 69
11.2 The Built-in InnoDB, the Plugin and File Formats .......................................................... 69
11.3 How to Downgrade ...................................................................................................... 70
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InnoDB Plugin 1.0 for MySQL 5.1 User's Guide
11.3.1 Converting Tables ............................................................................................ 70
11.3.2 Adjusting the Configuration ............................................................................... 70
11.3.3 Uninstalling a Dynamic Library .......................................................................... 70
11.3.4 Uninstalling a Statically Built InnoDB Plugin ....................................................... 71
11.4 Possible Problems ....................................................................................................... 71
11.4.1 Accessing COMPRESSED or DYNAMIC Tables ..................................................... 71
11.4.2 Issues with UNDO and REDO ........................................................................... 71
11.4.3 Issues with the Doublewrite Buffer ..................................................................... 72
11.4.4 Issues with the Insert Buffer .............................................................................. 73
12 InnoDB Plugin Change History ............................................................................................... 75
12.1 Changes in InnoDB Plugin 1.0.9 and Higher ................................................................. 75
12.2 Changes in InnoDB Plugin 1.0.8 (May, 2010) ............................................................... 75
12.3 Changes in InnoDB Plugin 1.0.7 (April, 2010) ............................................................... 75
12.4 Changes in InnoDB Plugin 1.0.6 (November 27, 2009) .................................................. 76
12.5 Changes in InnoDB Plugin 1.0.5 (November 18, 2009) .................................................. 76
12.6 Changes in InnoDB Plugin 1.0.4 (August 11, 2009) ....................................................... 77
12.7 Changes in InnoDB Plugin 1.0.3 (March 11, 2009) ........................................................ 78
12.8 Changes in InnoDB Plugin 1.0.2 (December 1, 2008) .................................................... 78
12.9 Changes in InnoDB Plugin 1.0.1 (May 8, 2008) ............................................................ 79
12.10 Changes in InnoDB Plugin 1.0.0 (April 15, 2008) ........................................................ 79
A Third-Party Software ................................................................................................................ 81
A.1 Performance Patches from Google ................................................................................ 81
A.2 Multiple Background I/O Threads Patch from Percona .................................................... 82
A.3 Performance Patches from Sun Microsystems ................................................................ 82
B Using the InnoDB Plugin with MySQL 5.1.30 or Earlier .............................................................. 85
C List of Parameters Changed in the InnoDB Plugin 1.0 ............................................................... 87
C.1 New Parameters ........................................................................................................... 87
C.2 Deprecated Parameters ................................................................................................ 89
C.3 Parameters with New Defaults ...................................................................................... 89
InnoDB Glossary ......................................................................................................................... 91
Index ........................................................................................................................................ 145
v
vi
Preface and Legal Notices
This is the User's Guide for the InnoDB Plugin 1.0.8 for MySQL 5.1.
Legal Notices
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vii
Legal Notices
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viii
Chapter 1 Introduction to the InnoDB Plugin
Table of Contents
1.1
1.2
1.3
1.4
1.5
Overview ................................................................................................................................
Features of the InnoDB Plugin ................................................................................................
Obtaining and Installing the InnoDB Plugin ...............................................................................
Viewing the InnoDB Plugin Version Number .............................................................................
Operational Restrictions ..........................................................................................................
1
1
2
3
3
1.1 Overview
The unique architecture of MySQL permits multiple storage engines with different capabilities to
be accessed through the same SQL language and APIs. Starting with version 5.1, MySQL AB has
promoted the idea of a “pluggable” storage engine architecture, which permits multiple storage
engines to be added to MySQL. Currently, however, most users have accessed only those storage
engines that are distributed by MySQL AB, and are linked into the binary (executable) releases.
Since 2001, MySQL AB has distributed the InnoDB transactional storage engine with its releases (both
source and binary). Beginning with MySQL version 5.1, it is possible for users to swap out one version
of InnoDB and use another. The pluggable storage engine architecture also permits Innobase Oy to
release new versions of InnoDB containing bug fixes and new features independently of the release
cycle for MySQL. Users can thus take advantage of these new versions of InnoDB in the context of
their deployed MySQL installations.
1.2 Features of the InnoDB Plugin
The InnoDB Plugin for MySQL contains several important new features:
• Viewing the InnoDB Plugin version number
• Fast index creation: add or drop indexes without copying the data
• Data compression: shrink tables, to significantly reduce storage and I/O
• New row format: fully off-page storage of long BLOB, TEXT, and VARCHAR columns
• File format management: protects upward and downward compatibility
• INFORMATION_SCHEMA tables: information about compression and locking
• Performance and scalability enhancements:
• Section 7.2, “Faster Locking for Improved Scalability”
• Section 7.3, “Using Operating System Memory Allocators”
• Section 7.4, “Controlling InnoDB Insert Buffering”
• Section 7.5, “Controlling Adaptive Hash Indexing”
• Section 7.6, “Changes Regarding Thread Concurrency”
• Section 7.7, “Changes in the Read Ahead Algorithm”
• Section 7.8, “Multiple Background I/O Threads”
• Section 7.9, “Group Commit”
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Obtaining and Installing the InnoDB Plugin
• Section 7.10, “Controlling the Master Thread I/O Rate”
• Section 7.11, “Controlling the Flushing Rate of Dirty Pages”
• Section 7.12, “Using the PAUSE instruction in InnoDB spin loops”
• Section 7.13, “Control of Spin Lock Polling”
• Section C.3, “Parameters with New Defaults”
• Section 7.14, “Making Buffer Cache Scan Resistant”
• Section 7.15, “Improvements to Crash Recovery Performance”
• Other changes for flexibility, ease of use and reliability:
• Section 8.2, “Enabling New File Formats”
• Section 8.3, “Dynamic Control of System Configuration Parameters”
• Section 8.4, “TRUNCATE TABLE Reclaims Space”
• Section 8.5, “InnoDB Strict Mode”
• Section 8.6, “Controlling Optimizer Statistics Estimation”
• Section 8.7, “Better Error Handling when Dropping Indexes”
• Section 8.8, “More Compact Output of SHOW ENGINE INNODB MUTEX”
• Section 8.9, “More Read Ahead Statistics”
Note that the ability to use data compression and the new row format require the use of a new InnoDB
file format called “Barracuda”. The previous file format, used by the built-in InnoDB in MySQL versions
5.0 and 5.1 is now called “Antelope” and does not support these features, but does support the other
features introduced with the InnoDB Plugin.
The InnoDB Plugin is upward compatible from standard InnoDB as built in to, and distributed
with, MySQL. Existing databases can be used with the InnoDB Plugin for MySQL. As described in
Section 9.5, “Configuring the InnoDB Plugin”, the new parameter innodb_file_format can help
protect upward and downward compatibility between InnoDB versions and database files, allowing
users to enable or disable use of new features that can only be used with certain versions of InnoDB.
The built-in InnoDB in MySQL since version 5.0.21 has a safety feature that prevents it from opening
tables that are in an unknown format. However, as noted in Section 11.2, “The Built-in InnoDB, the
Plugin and File Formats”, the system tablespace may contain references to new-format tables that will
confuse the built-in InnoDB in MySQL. These references will be cleared in a “slow” shutdown of the
InnoDB Plugin.
With previous versions of InnoDB, no error would be returned until you try to access a table that is in
a format “too new” for the software. Beginning with version 1.0.1 of the InnoDB Plugin, however, to
provide early feedback, InnoDB will check the system tablespace to ensure that the file format used in
the database is enabled for use before it will start. See Section 4.4.1, “Startup File Format Compatibility
Checking” for the details.
1.3 Obtaining and Installing the InnoDB Plugin
From the MySQL download site, you can download the MySQL Server 5.1.38 and up, containing an
additional, upgraded version of InnoDB with additional features beyond the built-in InnoDB provided by
MySQL. You can configure your server to use the InnnoDB Plugin rather than the built-in InnoDB, to
2
Viewing the InnoDB Plugin Version Number
take advantage of the extra features and performance enhancements in the Plugin. The InnoDB Plugin
for MySQL is licensed under the same license that MySQL uses (GPLv2). It is available at no charge
for use and can be freely redistributed, subject to the same requirements of the GPL as is MySQL
itself (see the GNU General Public License, version 2). Support is available for the InnoDB Plugin for
MySQL through the normal MySQL support channel, the MySQL bug database.
In many environments, the InnoDB plugin can dynamically be added to a MySQL instance without
relinking the MySQL server. The plugin version of InnoDB then “takes over” from the statically linked
InnoDB that is part of the mysqld binary. In other environments, it is currently necessary to build the
entire MySQL server, including the InnoDB plugin, from source code.
On Linux, Unix and Windows, it is a simple matter of installing the InnoDB Plugin for MySQL using
the INSTALL PLUGIN statement. When the InnoDB Plugin for MySQL is installed, it replaces the
statically-linked version of InnoDB that is incorporated in the MySQL binary as distributed by MySQL
AB.
On platforms where the dynamic plugin is not available, users must download the source code for
the InnoDB Plugin for MySQL and build MySQL from source. Building from source also facilitates
the distribution of a MySQL server where the InnoDB Plugin is already “installed”, so all users of an
organization can use the new capabilities without the INSTALL step. The procedure for building from
source code is documented in Section 9.4, “Building the InnoDB Plugin from Source Code”.
Full instructions are provided in Chapter 9, Installing the InnoDB Plugin.
1.4 Viewing the InnoDB Plugin Version Number
InnoDB Plugin releases are numbered with version numbers independent of MySQL release numbers.
The initial release of the InnoDB Plugin is version 1.0, and it is designed to work with MySQL 5.1.
• The first component of the InnoDB Plugin version number designates a major release level.
• The second component corresponds to the MySQL release. The digit 0 corresponds to MySQL 5.1.
• The third component indicates the specific release of the InnoDB Plugin (at a given major release
level and for a specific MySQL release); only bug fixes and minor functional changes are introduced
at this level.
Once you have installed the InnoDB Plugin, you can check its version number in three ways:
• In the error log, it is printed during startup.
• SELECT * FROM information_schema.plugins;
• SELECT @@innodb_version;
The InnoDB Plugin writes its version number to the error log, which can be helpful in diagnosis of
errors:
091105 12:28:06 InnoDB Plugin 1.0.5 started; log sequence number 46509
Note that the PLUGIN_VERSION column in the table INFORMATION_SCHEMA.PLUGINS does not
display the third component of the version number, only the first and second components, as in 1.0.
1.5 Operational Restrictions
Because the InnoDB Plugin introduces a new file format, with new on-disk data structures
within both the database and log files, there are important restrictions on the use of the plugin
in typical user environments. Specifically, you should pay special attention to the information
presented here about file format compatibility with respect to the following scenarios:
3
Operational Restrictions
• Downgrading from the InnoDB Plugin to the built-in InnoDB, or otherwise using different versions of
InnoDB with database files created by the InnoDB Plugin
• Using mysqldump.
• Using MySQL replication.
• Using InnoDB Hot Backup.
WARNING: Once you use the InnoDB Plugin on a set of database files, take care to avoid crashes
and corruptions when using those files with an earlier version of InnoDB, as might happen by opening
the database with MySQL when the plugin is not installed. It is strongly recommended that you use
a “slow shutdown” (SET GLOBAL innodb_fast_shutdown=0) when stopping the MySQL server
when the InnoDB Plugin is enabled. This will ensure log files and other system information written by
the plugin will not cause problems when using a prior version of InnoDB. See Section 11.3, “How to
Downgrade”.
Because of these considerations, and although it may be useful in certain circumstances to use the
plugin in a temporary way as just described, many users will find it preferable to test their application
with the plugin and use it on an on-going basis, without reverting back to the standard, built-in InnoDB.
WARNING: If you dump a database containing compressed tables with mysqldump, the dump
file may contain CREATE TABLE commands that attempt to create compressed tables, or those
using ROW_FORMAT=DYNAMIC in the new database. Therefore, you should be sure the new
database is running the InnoDB Plugin, with the proper settings for innodb_file_format and
innodb_file_per_table, if you want to have the tables re-created as they exist in the original
database. Typically, however, when the mysqldump file is loaded, MySQL and InnoDB will ignore
CREATE TABLE options they do not recognize, and the table(s) will be created in a format used by the
running server.
WARNING: If you use MySQL replication, you should be careful to ensure all slaves are
configured with the InnoDB Plugin, with the same settings for innodb_file_format and
innodb_file_per_table. If you do not do so, and you create tables that require the new
“Barracuda” file format, replication errors may occur. If a slave MySQL server is running the builtin InnoDB, it will ignore the CREATE TABLE options to create a compressed table or one with
ROW_FORMAT=DYNAMIC, and create the table uncompressed, with ROW_FORMAT=COMPACT.
WARNING: The current version of InnoDB Hot Backup does not support the new “Barracuda”
file format. Using InnoDB Hot Backup Version 3 to backup databases in this format will cause
unpredictable behavior. A future version of InnoDB Hot Backup will support databases used with the
InnoDB Plugin. As an alternative, you may back up such databases with mysqldump.
4
Chapter 2 Fast Index Creation in the InnoDB Storage Engine
Table of Contents
2.1
2.2
2.3
2.4
2.5
2.6
Overview of Fast Index Creation ..............................................................................................
Examples ................................................................................................................................
Implementation .......................................................................................................................
Concurrency Considerations ....................................................................................................
Crash Recovery ......................................................................................................................
Limitations ..............................................................................................................................
2.1 Overview of Fast Index Creation
In MySQL versions up to 5.0, adding or dropping an index on a table with existing data can be very
slow if the table has many rows. The CREATE INDEX and DROP INDEX commands work by creating
a new, empty table defined with the requested set of indexes. It then copies the existing rows to
the new table one-by-one, updating the indexes as it goes. Inserting entries into the indexes in this
fashion, where the key values are not sorted, requires random access to the index nodes, and is far
from optimal. After all rows from the original table are copied, the old table is dropped and the copy is
renamed with the name of the original table.
Beginning with version 5.1, MySQL allows a storage engine to create or drop indexes without copying
the contents of the entire table. The standard built-in InnoDB in MySQL version 5.1, however, does not
take advantage of this capability. With the InnoDB Plugin, however, users can in most cases add and
drop indexes much more efficiently than with prior releases.
In InnoDB, the rows of a table are stored in a clustered (or primary key) index, forming what some
database systems call an “index-organized table”. Changing the clustered index requires copying the
data, even with the InnoDB Plugin. However, adding or dropping a secondary index with the InnoDB
Plugin is much faster, since it does not involve copying the data.
This new mechanism also means that you can generally speed the overall process of creating and
loading an indexed table by creating the table with only the clustered index, and adding the secondary
indexes after the data is loaded.
Although no syntax changes are required in the CREATE INDEX or DROP INDEX commands, some
factors affect the performance, space usage, and semantics of this operation (see Section 2.6,
“Limitations”).
Because the ability to create and drop indexes does not require use of a new on-disk file format,
it is possible to temporarily use the InnoDB Plugin to create or drop an index, and then fall back to
using the standard built-in InnoDB in MySQL for normal operations if you wish. See Chapter 11,
Downgrading from the InnoDB Plugin for more information.
2.2 Examples
It is possible to create multiple indexes on a table with one ALTER TABLE command. This is relatively
efficient, because the clustered index of the table needs to be scanned only once (although the data is
sorted separately for each new index). For example:
CREATE TABLE T1(A INT PRIMARY KEY,
B INT, C CHAR(1)) ENGINE=InnoDB;
INSERT INTO T1 VALUES
(1,2,'a'), (2,3,'b'), (3,2,'c'), (4,3,'d'), (5,2,'e');
COMMIT;
ALTER TABLE T1 ADD INDEX (B), ADD UNIQUE INDEX (C);
5
5
5
6
7
7
8
Implementation
The above commands will create table T1 with the clustered index (primary key) on column A, insert
several rows, and then build two new indexes on columns B and C. If there were many rows inserted
into T1 before the ALTER TABLE command, this approach would be much more efficient than creating
the table with all its indexes before loading the data.
You may also create the indexes one at a time, but then the clustered index of the table is scanned (as
well as sorted) once for each CREATE INDEX command. Thus, the following commands are not as
efficient as the ALTER TABLE command above, even though neither requires recreating the clustered
index for table T1.
CREATE INDEX B ON T1 (B);
CREATE UNIQUE INDEX C ON T1 (C);
Dropping indexes in the InnoDB Plugin does not require any copying of table data. Thus, you can
equally quickly drop multiple indexes with a single ALTER TABLE command or multiple DROP INDEX
commands:
ALTER TABLE T1 DROP INDEX B, DROP INDEX C;
or
DROP INDEX B ON T1;
DROP INDEX C ON T1;
Restructuring the clustered index in InnoDB always requires copying the data in the table. For example,
if you create a table without a primary key, InnoDB chooses one for you, which may be the first
UNIQUE key defined on NOT NULL columns, or a system-generated key. Defining a PRIMARY KEY
later causes the data to be copied, as in the following example:
CREATE TABLE T2 (A INT, B INT) ENGINE=InnoDB;
INSERT INTO T2 VALUES (NULL, 1);
ALTER TABLE T2 ADD PRIMARY KEY (B);
Note that when you create a UNIQUE or PRIMARY KEY index, InnoDB must do some extra work.
For UNIQUE indexes, InnoDB checks that the table contains no duplicate values for the key. For a
PRIMARY KEY index, InnoDB also checks that none of the PRIMARY KEY columns contains a NULL. It
is best to define the primary key when you create a table, so you need not rebuild the table later.
2.3 Implementation
InnoDB has two types of indexes: the clustered index and secondary indexes. Since the clustered
index contains the data values in its B-tree nodes, adding or dropping a clustered index does involve
copying the data, and creating a new copy of the table. A secondary index, however, contains only
the index key and the value of the primary key. This type of index may be created or dropped without
copying the data in the clustered index. Furthermore, because the secondary index contains the values
of the primary key (used to access the clustered index when needed), when you change the definition
of the primary key, thus recreating the clustered index, all secondary indexes are recreated as well.
Dropping a secondary index is simple. Only the internal InnoDB system tables and the MySQL data
dictionary tables need to be updated to reflect the fact that the index no longer exists. InnoDB returns
the storage used for the index to the tablespace that contained it, so that new indexes or additional
table rows may use the space.
To add a secondary index to an existing table, InnoDB scans the table, and sorts the rows using
memory buffers and temporary files in order by the value(s) of the secondary index key column(s).
The B-tree is then built in key-value order, which is more efficient than inserting rows into an index in
random order with respect to the key values. Because the B-tree nodes are split when they fill, building
the index in this way results in a higher fill-factor for the index, making it more efficient for subsequent
access.
6
Concurrency Considerations
2.4 Concurrency Considerations
While a secondary index is being created or dropped, the table is locked in shared mode. That is, any
writes to the table are blocked, but the data in the table may be read. When you alter the clustered
index of a table, however, the table is locked in exclusive mode, because the data must be copied.
Thus, during the creation of a new clustered index, all operations on the table are blocked.
Before it can start executing, a CREATE INDEX or ALTER TABLE command must always wait for
currently executing transactions that are accessing the table to commit or rollback before it can
proceed. In addition, ALTER TABLE commands that create a new clustered index must wait for all
SELECT statements that access the table to complete (or their containing transactions to commit). Even
though the original index exists throughout the creation of the new clustered index, no transactions
whose execution spans the creation of the index can be accessing the table, because the original table
must be dropped when clustered index is restructured.
Once a CREATE INDEX or ALTER TABLE command that creates a secondary index begins executing,
queries may access the table for read access, but may not update the table. If an ALTER TABLE
command is changing the clustered index, all queries must wait until the operation completes.
A newly-created secondary index contains only data that is current in the table as of the time the
CREATE INDEX or ALTER TABLE command begins to execute. Specifically, a newly-created index
contains only the versions of data as of the most-recently committed transactions prior to the creation
of the index. The index thus does not contain any rows that were deleted (and therefore marked for
deletion) by transactions that completed before the CREATE INDEX or ALTER TABLE began. Similarly,
the index contains only current versions of every row, and none of the old versions of rows that were
updated by transactions that ran before the index was created.
Because a newly-created index contains only information about data current at the time the index
was created, queries that need to see data that was deleted or changed before the index was created
cannot use the index. The only queries that could be affected by this limitation are those executing in
transactions that began before the creation of the index was begun. For such queries, unpredictable
results could occur. Newer queries can use the index.
2.5 Crash Recovery
No data is lost if the server crashes while an ALTER TABLE command is executing. Recovery,
however, is different for clustered indexes and secondary indexes.
If the server crashes while creating a secondary index, upon recovery, InnoDB drops any partially
created indexes. All you need to do to create the index is to re-run the ALTER TABLE or CREATE
INDEX command.
However, when a crash occurs during the creation of a clustered index, recovery is somewhat
more complicated, because the data in the table must be copied to an entirely new clustered index.
Remember that all InnoDB tables are stored as clustered indexes. In the following discussion, we use
the word table and clustered index interchangeably.
The InnoDB Plugin creates the new clustered index by copying the existing data from the original table
to a temporary table that has the desired index structure. Once the data is completely copied to this
temporary table, the original table is renamed with a different temporary table name. The temporary
table comprising the new clustered index is then renamed with the name of the original table, and the
original table is then dropped from the database.
If a system crash occurs while creating a new clustered index, no data is lost, but users must complete
the recovery process using the temporary tables that exist during the process.
Users rarely re-create a clustered index or re-define primary keys on large tables. Because system
crashes are uncommon and the situation described here is rare, this manual does not provide
information on recovering from this scenario. Instead, contact MySQL support.
7
Limitations
2.6 Limitations
Take the following considerations into account when creating or dropping indexes using the InnoDB
Plugin:
• During index creation, files are written to the temporary directory ($TMPDIR on Unix, %TEMP% on
Windows, or the value of --tmpdir configuration variable). Each temporary file is large enough to
hold one column that makes up the new index, and each one is removed as soon as it is merged into
the final index.
• Due to a limitation of MySQL, the table is copied, rather than using “Fast Index Creation” when you
create an index on a TEMPORARY TABLE. This has been reported as MySQL Bug #39833.
• To avoid consistency issues between the InnoDB data dictionary and the MySQL data dictionary,
the table is copied, rather than using Fast Index Creation when you use the ALTER TABLE ...
RENAME COLUMN syntax.
• The command ALTER IGNORE TABLE t ADD UNIQUE INDEX does not delete duplicate rows.
This has been reported as MySQL Bug #40344. The IGNORE keyword is ignored. If any duplicate
rows exist, the operation fails with the following error message:
ERROR 23000: Duplicate entry '347' for key 'pl'
• As noted above, a newly-created index contains only information about data current at the time the
index was created. Therefore, you should not run queries in a transaction that might use a secondary
index that did not exist at the beginning of the transaction. There is no way for InnoDB to access
“old” data that is consistent with the rest of the data read by the transaction. See the discussion of
locking in Section 2.4, “Concurrency Considerations”.
Prior to InnoDB Plugin 1.0.4, unexpected results could occur if a query attempts to use an index
created after the start of the transaction containing the query. If an old transaction attempts to access
a “too new” index, InnoDB Plugin 1.0.4 and later reports an error:
ERROR HY000: Table definition has changed, please retry transaction
As the error message suggests, committing (or rolling back) the transaction, and restarting it, cures
the problem.
• InnoDB Plugin 1.0.2 introduces some improvements in error handling when users attempt to drop
indexes. See section Section 8.7, “Better Error Handling when Dropping Indexes” for details.
• MySQL 5.1 does not support efficient creation or dropping of FOREIGN KEY constraints. Therefore,
if you use ALTER TABLE to add or remove a REFERENCES constraint, the child table will be copied,
rather than using “Fast Index Creation”.
8
Chapter 3 InnoDB Data Compression
Table of Contents
3.1 Overview of Table Compression .............................................................................................. 9
3.2 Enabling Compression for a Table ........................................................................................... 9
3.2.1 Configuration Parameters for Compression .................................................................. 10
3.2.2 SQL Compression Syntax Warnings and Errors ........................................................... 11
3.3 Tuning InnoDB Compression ................................................................................................. 12
3.3.1 When to Use Compression ......................................................................................... 12
3.3.2 Monitoring Compression at Runtime ............................................................................ 15
3.4 How Compression Works in InnoDB ...................................................................................... 15
3.4.1 Compression Algorithms ............................................................................................. 15
3.4.2 InnoDB Data Storage and Compression ...................................................................... 16
3.4.3 Compression and the InnoDB Buffer Pool .................................................................... 17
3.4.4 Compression and the InnoDB Log Files ...................................................................... 18
3.1 Overview of Table Compression
Over the years, processors and cache memories have become much faster, but mass storage based
on rotating magnetic disks has not kept pace. While the storage capacity of disks has grown by about a
factor of 1,000 in the past decade, random seek times and data transfer rates are still severely limited
by mechanical constraints. Therefore, many workloads are I/O-bound. The idea of data compression is
to pay a small cost in increased CPU utilization for the benefit of smaller databases and reduced I/O to
improve throughput, potentially significantly.
The ability to compress user data is an important new capability of the InnoDB Plugin. Compressed
tables reduce the size of the database on disk, resulting in fewer reads and writes needed to access
the user data. For many InnoDB workloads and many typical user tables (especially with readintensive applications where sufficient memory is available to keep frequently-used data in memory),
compression not only significantly reduces the storage required for the database, but also improves
throughput by reducing the I/O workload, at a modest cost in processing overhead. The storage cost
savings can be important, but the reduction in I/O costs can be even more valuable. Compression can
be especially important for SSD storage devices, because they tend to have lower capacity than HDD
devices.
3.2 Enabling Compression for a Table
The usual (uncompressed) size of InnoDB data pages is 16KB. Beginning with the InnoDB Plugin, you
can use the attributes ROW_FORMAT=COMPRESSED, KEY_BLOCK_SIZE, or both in the CREATE TABLE
and ALTER TABLE statements to enable table compression. Depending on the combination of option
values, InnoDB attempts to compress each page to 1KB, 2KB, 4KB, 8KB, or 16KB.
Note
The term KEY_BLOCK_SIZE does not refer to a “key”, but simply specifies the
size of compressed pages to use for the table. Likewise, in the InnoDB Plugin,
compression is applicable to tables, not to individual rows, despite the option
name ROW_FORMAT. Because the InnoDB storage engine cannot add syntax to
SQL statements, the InnoDB Plugin re-uses the clauses originally defined for
MyISAM.
To create a compressed table, you might use a statement like this:
CREATE TABLE name
(column1 INT PRIMARY KEY)
9
Configuration Parameters for Compression
ENGINE=InnoDB
ROW_FORMAT=COMPRESSED
KEY_BLOCK_SIZE=4;
If you specify ROW_FORMAT=COMPRESSED but not KEY_BLOCK_SIZE, the default compressed
page size of 8KB is used. If KEY_BLOCK_SIZE is specified, you can omit the attribute
ROW_FORMAT=COMPRESSED.
Setting KEY_BLOCK_SIZE=16 most often does not result in much compression, since the normal
InnoDB page size is 16KB. However, this setting may be useful for tables with many long BLOB,
VARCHAR or TEXT columns, because such values often do compress well, and might therefore require
fewer “overflow” pages as described later in this section.
Note that compression is specified on a table-by-table basis. All indexes of a table (including the
clustered index) are compressed using the same page size, as specified on the CREATE TABLE or
ALTER TABLE statement. Table attributes such as ROW_FORMAT and KEY_BLOCK_SIZE are not part
of the CREATE INDEX syntax, and are ignored if they are specified (although you see them in the
output of the SHOW CREATE TABLE statement).
3.2.1 Configuration Parameters for Compression
Compressed tables are stored in a format that previous versions of InnoDB cannot process. To
preserve downward compatibility of database files, compression can be specified only when the
“Barracuda” data file format is enabled using the configuration parameter innodb_file_format.
Table compression is also not available for the InnoDB system tablespace. The system tablespace
(space 0, the ibdata* files) may contain user data, but it also contains internal InnoDB system
information, and therefore is never compressed. Thus, compression applies only to tables (and
indexes) stored in their own tablespaces.
To use compression, enable the “file per table” mode using the configuration parameter
innodb_file_per_table and enable the “Barracuda” disk file format using the parameter
innodb_file_format. You can set these parameters in the MySQL option file my.cnf or my.ini,
but both are dynamic parameters that you can change with the SET statement without shutting down
the MySQL server, as noted in Section 9.5, “Configuring the InnoDB Plugin”.
Specifying ROW_FORMAT=COMPRESSED or a KEY_BLOCK_SIZE in the CREATE TABLE or ALTER
TABLE statements if the “Barracuda” file format has not been enabled produces these warnings that
you can view with the SHOW WARNINGS statement:
Level
Code Message
Warning
1478 InnoDB: KEY_BLOCK_SIZE requires innodb_file_per_table.
Warning
1478 InnoDB: KEY_BLOCK_SIZE requires innodb_file_format=1.
Warning
1478 InnoDB: ignoring KEY_BLOCK_SIZE=4.
Warning
1478 InnoDB: ROW_FORMAT=COMPRESSED requires innodb_file_per_table.
Warning
1478 InnoDB: assuming ROW_FORMAT=COMPACT.
Note
These messages are only warnings, not errors, and the table is created as if
the options were not specified. Enabling InnoDB “strict mode” (see Section 8.5,
“InnoDB Strict Mode”) causes InnoDB to generate an error, not a warning, for
these cases. In strict mode, the table is not created if the current configuration
does not permit using compressed tables.
The “non-strict” behavior is intended to permit you to import a mysqldump file into a database that
does not support compressed tables, even if the source database contained compressed tables. In
that case, the InnoDB Plugin creates the table in ROW_FORMAT=COMPACT instead of preventing the
operation.
10
SQL Compression Syntax Warnings and Errors
When you import the dump file into a new database, if you want to have the tables re-created as they
exist in the original database, ensure the server is running the InnoDB Plugin with the proper settings
for the configuration parameters innodb_file_format and innodb_file_per_table,
3.2.2 SQL Compression Syntax Warnings and Errors
The attribute KEY_BLOCK_SIZE is permitted only when ROW_FORMAT is specified as COMPRESSED
or is omitted. Specifying a KEY_BLOCK_SIZE with any other ROW_FORMAT generates a warning
that you can view with SHOW WARNINGS. However, the table is non-compressed; the specified
KEY_BLOCK_SIZE is ignored).
Level
Code Message
Warning
1478 InnoDB: ignoring KEY_BLOCK_SIZE=n unless
ROW_FORMAT=COMPRESSED.
If you are running in InnoDB strict mode, the combination of a KEY_BLOCK_SIZE with any
ROW_FORMAT other than COMPRESSED generates an error, not a warning, and the table is not created.
Table 3.1, “Meaning of CREATE TABLE and ALTER TABLE Options” summarizes how the various
options on CREATE TABLE and ALTER TABLE are handled.
Table 3.1 Meaning of CREATE TABLE and ALTER TABLE Options
Option
Usage
Description
ROW_FORMAT=Storage format used prior to
REDUNDANT MySQL 5.0.3
Less efficient than ROW_FORMAT=COMPACT; for
backward compatibility
ROW_FORMAT=Default storage format since
COMPACT
MySQL 5.0.3
Stores a prefix of 768 bytes of long column values in
the clustered index page, with the remaining bytes
stored in an overflow page
ROW_FORMAT=Available only with
DYNAMIC
innodb_file
_format=Barracuda
Store values within the clustered index page if they
fit; if not, stores only a 20-byte pointer to an overflow
page (no prefix)
ROW_FORMAT=Available only with
COMPRESSED innodb_file
_format=Barracuda
Compresses the table and indexes using zlib to
default compressed page size of 8K bytes; implies
ROW_FORMAT=DYNAMIC
KEY_BLOCK_ Available only with
SIZE=n
innodb_file
_format=Barracuda
Specifies compressed page size of 1, 2, 4, 8 or
16K bytes; implies ROW_FORMAT=DYNAMIC and
ROW_FORMAT=COMPRESSED
Table 3.2, “CREATE/ALTER TABLE Warnings and Errors when InnoDB Strict Mode is OFF”
summarizes error conditions that occur with certain combinations of configuration parameters and
options on the CREATE TABLE or ALTER TABLE statements, and how the options appear in the output
of SHOW TABLE STATUS.
When InnoDB strict mode is OFF, InnoDB creates or alters the table, but may ignore certain settings, as
shown below. You can see the warning messages in the MySQL error log. When InnoDB strict mode
is ON, these specified combinations of options generate errors, and the table is not created or altered.
You can see the full description of the error condition with SHOW ERRORS. For example:
mysql> CREATE TABLE x (id INT PRIMARY KEY, c INT)
-> ENGINE=INNODB KEY_BLOCK_SIZE=33333;
ERROR 1005 (HY000): Can't create table 'test.x' (errno: 1478)
mysql> SHOW ERRORS;
+-------+------+-------------------------------------------+
| Level | Code | Message
|
11
Tuning InnoDB Compression
+-------+------+-------------------------------------------+
| Error | 1478 | InnoDB: invalid KEY_BLOCK_SIZE=33333.
|
| Error | 1005 | Can't create table 'test.x' (errno: 1478) |
+-------+------+-------------------------------------------+
2 rows in set (0.00 sec)
Table 3.2 CREATE/ALTER TABLE Warnings and Errors when InnoDB Strict Mode is OFF
Syntax
Warning or Error Condition
ROW_FORMAT=REDUNDANT None
ROW_FORMAT=COMPACT
Resulting ROW_FORMAT,
as shown in SHOW TABLE
STATUS
REDUNDANT
None
COMPACT
ROW_FORMAT=COMPRESSEDIgnored unless you override the default
or
settings for innodb_file_format and
ROW_FORMAT=DYNAMIC
innodb_file_per_table
or KEY_BLOCK_SIZE is
specified
COMPACT
Invalid KEY_BLOCK_SIZE KEY_BLOCK_SIZE is ignored
is specified (not 1, 2, 4, 8
or 16)
the requested one, or
COMPACT by default
ROW_FORMAT=COMPRESSEDNone; KEY_BLOCK_SIZE specified is used, COMPRESSED
and valid
not the 8K default
KEY_BLOCK_SIZE are
specified
KEY_BLOCK_SIZE
is specified with
REDUNDANT, COMPACT or
DYNAMIC row format
KEY_BLOCK_SIZE is ignored
ROW_FORMAT is not one
Ignored if recognized by the MySQL
of REDUNDANT, COMPACT, parser. Otherwise, an error is issued.
DYNAMIC or COMPRESSED
REDUNDANT, COMPACT or
DYNAMIC
COMPACT or N/A
When InnoDB strict mode is ON (innodb_strict_mode=1), the InnoDB Plugin rejects invalid
ROW_FORMAT or KEY_BLOCK_SIZE parameters. For compatibility with the built-in InnoDB in MySQL,
InnoDB strict mode is not enabled by default, and in this default non-strict mode, the InnoDB Plugin
issues warnings (not errors) for ignored invalid parameters.
Note that it is not possible to see the chosen KEY_BLOCK_SIZE using SHOW TABLE STATUS. The
statement SHOW CREATE TABLE displays the KEY_BLOCK_SIZE (even if it was ignored by InnoDB).
The real compressed page size inside InnoDB cannot be displayed by MySQL.
3.3 Tuning InnoDB Compression
Most often, the internal optimizations in InnoDB described in a later portion of this section, ensure that
the system runs well with compressed data. However, because the efficiency of compression depends
on the nature of your data, there are some factors you should consider to get best performance. You
need to choose which tables to compress, and what compressed page size to use. You may also want
to adjust the size of the buffer pool based on run-time performance characteristics such as the amount
of time the system spends compressing and uncompressing data.
3.3.1 When to Use Compression
In general, compression works best on tables that include a reasonable number of character string
columns and where the data is read far more often than it is written. Because there are no guaranteed
ways to predict whether or not compression benefits a particular situation, always test with a specific
12
When to Use Compression
workload and data set running on a representative configuration. Consider the following factors when
deciding which tables to compress.
3.3.1.1 Data Characteristics and Compression
A key determinant of the efficiency of compression in reducing the size of data files is the nature of
the data itself. Recall that compression works by identifying repeated strings of bytes in a block of
data. Completely randomized data is the worst case. Typical data often has repeated values, and so
compresses effectively. Character strings often compress well, whether defined in CHAR, VARCHAR,
TEXT or BLOB columns. On the other hand, tables containing mostly binary data (integers or floating
point numbers) or data that is previously compressed (for example JPEG or PNG images) may not
generally compress well, significantly or at all.
Compression is chosen on a table by table basis with the InnoDB Plugin, and a table and all of its
indexes use the same (compressed) page size. It might be that the primary key (clustered) index,
which contains the data for all columns of a table, compresses more effectively than the secondary
indexes. For those cases where there are long rows, the use of compression may result in long column
values being stored “off-page”, as discussed in Section 5.3, “DYNAMIC Row Format”. Those overflow
pages may compress well. Given these considerations, for many applications, some tables compress
more effectively than others, and you may find that your workload performs best only with a subset of
tables compressed.
Experimenting is the only way to determine whether or not to compress a particular table. InnoDB
compresses data in 16K chunks corresponding to the uncompressed page size, and in addition to
user data, the page format includes some internal system data that is not compressed. Compression
utilities compress an entire stream of data, and so may find more repeated strings across the entire
input stream than InnoDB would find in a table compressed in 16K chunks. But you can get a sense
of how compression efficiency by using a utility that implements LZ77 compression (such as gzip or
WinZip) on your data file.
Another way to test compression on a specific table is to copy some data from your uncompressed
table to a similar, compressed table (having all the same indexes) and look at the size of the resulting
file. When you do so (if nothing else using compression is running), you can examine the ratio of
successful compression operations to overall compression operations. (In the INNODB_CMP table,
compare COMPRESS_OPS to COMPRESS_OPS_OK. See INNODB_CMP for more information.) If a high
percentage of compression operations complete successfully, the table might be a good candidate for
compression.
3.3.1.2 Compression and Application and Schema Design
Decide whether to compress data in your application or in the InnoDB table. It is usually not sensible to
store data that is compressed by an application in an InnoDB compressed table. Further compression
is extremely unlikely, and the attempt to compress just wastes CPU cycles.
Compressing in the Database
The InnoDB table compression is automatic and applies to all columns and index values. The columns
can still be tested with operators such as LIKE, and sort operations can still use indexes even when
the index values are compressed. Because indexes are often a significant fraction of the total size
of a database, compression could result in significant savings in storage, I/O or processor time. The
compression and decompression operations happen on the database server, which likely is a powerful
system that is sized to handle the expected load.
Compressing in the Application
If you compress data such as text in your application, before it is inserted into the database, You might
save overhead for data that does not compress well by compressing some columns and not others.
This approach uses CPU cycles for compression and uncompression on the client machine rather
than the database server, which might be appropriate for a distributed application with many clients, or
where the client machine has spare CPU cycles.
13
When to Use Compression
Hybrid Approach
Of course, it is possible to combine these approaches. For some applications, it may be appropriate to
use some compressed tables and some uncompressed tables. It may be best to externally compress
some data (and store it in uncompressed InnoDB tables) and allow InnoDB to compress (some of) the
other tables in the application. As always, up-front design and real-life testing are valuable in reaching
the right decision.
3.3.1.3 Workload Characteristics and Compression
In addition to choosing which tables to compress (and the page size), the workload is another key
determinant of performance. If the application is dominated by reads, rather than updates, fewer
pages need to be reorganized and recompressed after the index page runs out of room for the perpage “modification log” that InnoDB maintains for compressed data. If the updates predominantly
change non-indexed columns or those containing BLOBs or large strings that happen to be stored “offpage”, the overhead of compression may be acceptable. If the only changes to a table are INSERTs
that use a monotonically increasing primary key, and there are few secondary indexes, there is little
need to reorganize and recompress index pages. Since InnoDB can “delete-mark” and delete rows
on compressed pages “in place” by modifying uncompressed data, DELETE operations on a table are
relatively efficient.
For some environments, the time it takes to load data can be as important as run-time retrieval.
Especially in data warehouse environments, many tables may be read-only or read-mostly. In those
cases, it might or might not be acceptable to pay the price of compression in terms of increased load
time, unless the resulting savings in fewer disk reads or in storage cost is significant.
Fundamentally, compression works best when the CPU time is available for compressing and
uncompressing data. Thus, if your workload is I/O bound, rather than CPU-bound, you may find that
compression can improve overall performance. Therefore when you test your application performance
with different compression configurations, it is important to test on a platform similar to the planned
configuration of the production system.
3.3.1.4 Configuration Characteristics and Compression
Reading and writing database pages from and to disk is the slowest aspect of system performance.
Therefore, compression attempts to reduce I/O by using CPU time to compress and uncompress data,
and thus is most effective when I/O is a relatively scarce resource compared to processor cycles.
This is often especially the case when running in a multi-user environment with fast, multi-core CPUs.
When a page of a compressed table is in memory, InnoDB often uses an additional 16K in the buffer
pool for an uncompressed copy of the page. The adaptive LRU algorithm in the InnoDB Plugin
attempts to balance the use of memory between compressed and uncompressed pages to take into
account whether the workload is running in an I/O-bound or CPU-bound manner. Nevertheless, a
configuration with more memory dedicated to the InnoDB buffer pool tends to run better when using
compressed tables than a configuration where memory is highly constrained.
3.3.1.5 Choosing the Compressed Page Size
The optimal setting of the compressed page size depends on the type and distribution of data that the
table and its indexes contain. The compressed page size should always be bigger than the maximum
record size, or operations may fail as noted in the discussion of compression internals later in this
section.
Setting the compressed page size too large wastes some space, but the pages do not have to be
compressed as often. If the compressed page size is set too small, inserts or updates may require
time-consuming recompression, and the B-tree nodes may have to be split more frequently, leading to
bigger data files and less efficient indexing.
Typically, one would set the compressed page size to 8K or 4K bytes. Given that the maximum InnoDB
record size is around 8K, KEY_BLOCK_SIZE=8 is usually a safe choice.
14
Monitoring Compression at Runtime
3.3.2 Monitoring Compression at Runtime
The current version of the InnoDB Plugin provides only a limited means to monitor the performance of
compression at runtime. Overall application performance, CPU and I/O utilization and the size of disk
files are the best indicators of how effective compression is for your application.
The InnoDB Plugin does include some Information Schema tables (see Example 6.1, “Using the
Compression Information Schema Tables”) that reflect the internal use of memory and the rates of
compression used overall. The INNODB_CMP tables report information about compression activity for
each compressed page size (KEY_BLOCK_SIZE) in use. The information in these tables is systemwide, and includes summary data across all compressed tables in your database. You can use this
data to help decide whether or not to compress a table by examining these tables when no other
compressed tables are being accessed.
The key statistics to consider are the number of, and amount of time spent performing, compression
and uncompression operations. Since InnoDB must split B-tree nodes when they are too full to contain
the compressed data following a modification, you should also compare the number of “successful”
compression operations with the number of such operations overall. Based on the information in the
INNODB_CMP tables and overall application performance and hardware resource utilization, you may
decide to make changes in your hardware configuration, adjust the size of the InnoDB buffer pool,
choose a different page size, or select a different set of tables to compress.
If the amount of CPU time required for compressing and uncompressing is high, changing to faster
CPUs, or those with more cores, can help improve performance with the same data, application
workload and set of compressed tables. You may also benefit by increasing the size of the InnoDB
buffer pool, so that more uncompressed pages can stay in memory, reducing the need to uncompress
pages which exist in memory only in compressed form.
A large number of compression operations overall (compared to the number of INSERT, UPDATE and
DELETE operations in your application and the size of the database) could indicate that some of your
compressed tables are being updated too heavily for effective compression. You may want to choose a
larger page size, or be more selective about which tables you compress.
If the number of “successful” compression operations (COMPRESS_OPS_OK) is a high percentage of
the total number of compression operations (COMPRESS_OPS), then the system is likely performing
well. However, if the ratio is low, then InnoDB is being caused to reorganize, recompress and split Btree nodes more often than is desirable. In this case, you may want to avoid compressing some tables
or choose a larger KEY_BLOCK_SIZE for some of the tables for which you are using compression.
You may not want to compress tables which cause the number of “compression failures” in your
application to be more than 1% or 2% of the total (although this may be acceptable during a data load,
for example, if your application does not encounter such a ratio during normal operations).
3.4 How Compression Works in InnoDB
This section describes some internal implementation details about compression in InnoDB. The
information presented here may be helpful in tuning for performance, but is not necessary to know for
basic use of compression.
3.4.1 Compression Algorithms
Some operating systems implement compression at the file system level. Files are typically divided into
fixed-size blocks that are compressed into variable-size blocks, which easily leads into fragmentation.
Every time something inside a block is modified, the whole block is recompressed before it is written
to disk. These properties make this compression technique unsuitable for use in an update-intensive
database system.
The InnoDB Plugin implements a novel type of compression with the help of the well-known zlib
library, which implements the LZ77 compression algorithm. This compression algorithm is mature,
robust, and efficient in both CPU utilization and in reduction of data size. The algorithm is “lossless”,
15
InnoDB Data Storage and Compression
so that the original uncompressed data can always be reconstructed from the compressed form. LZ77
compression works by finding sequences of data that are repeated within the data to be compressed.
The patterns of values in your data determine how well it compresses, but typical user data often
compresses by 50% or more.
Unlike compression performed by an application, or compression features of some other database
management systems, InnoDB compression applies both to user data and to indexes. In many cases,
indexes can constitute 40-50% or more of the total database size, so this difference is significant.
When compression is working well for a data set, the size of the InnoDB data files (the .ibd files) is
25% to 50% of the uncompressed size or possibly smaller. Depending on the workload, this smaller
database can in turn lead to a reduction in I/O, and an increase in throughput, at a modest cost in
terms of increased CPU utilization.
3.4.2 InnoDB Data Storage and Compression
All user data in InnoDB is stored in pages comprising a B-tree index (the so-called clustered index).
In some other database systems, this type of index is called an “index-organized table”. Each row in
the index node contains the values of the (user-specified or system-generated) primary key and all the
other columns of the table.
Secondary indexes in InnoDB are also B-trees, containing pairs of values: the index key and a pointer
to a row in the clustered index. The pointer is in fact the value of the primary key of the table, which is
used to access the clustered index if columns other than the index key and primary key are required.
Secondary index records must always fit on a single B-tree page.
The compression of B-tree nodes (of both clustered and secondary indexes) is handled differently from
compression of overflow pages used to store long VARCHAR, BLOB, or TEXT columns, as explained in
the following sections.
3.4.2.1 Compression of B-Tree Pages
Because they are frequently updated, B-tree pages require special treatment. It is important to
minimize the number of times B-tree nodes are split, as well as to minimize the need to uncompress
and recompress their content.
One technique InnoDB uses is to maintain some system information in the B-tree node in
uncompressed form, thus facilitating certain in-place updates. For example, this allows rows to be
delete-marked and deleted without any compression operation.
In addition, InnoDB attempts to avoid unnecessary uncompression and recompression of index pages
when they are changed. Within each B-tree page, the system keeps an uncompressed “modification
log” to record changes made to the page. Updates and inserts of small records may be written to this
modification log without requiring the entire page to be completely reconstructed.
When the space for the modification log runs out, InnoDB uncompresses the page, applies the
changes and recompresses the page. If recompression fails, the B-tree nodes are split and the process
is repeated until the update or insert succeeds.
Generally, InnoDB requires that each B-tree page can accommodate at least two records. For
compressed tables, this requirement has been relaxed. Leaf pages of B-tree nodes (whether of the
primary key or secondary indexes) only need to accommodate one record, but that record must fit in
uncompressed form, in the per-page modification log. Starting with InnoDB Plugin version 1.0.2, and
if InnoDB strict mode is ON, the InnoDB Plugin checks the maximum row size during CREATE TABLE
or CREATE INDEX. If the row does not fit, the following error message is issued: ERROR HY000: Too
big row.
If you create a table when InnoDB strict mode is OFF, and a subsequent INSERT or UPDATE statement
attempts to create an index entry that does not fit in the size of the compressed page, the operation
fails with ERROR 42000: Row size too large. (This error message does not name the index for
16
Compression and the InnoDB Buffer Pool
which the record is too large, or mention the length of the index record or the maximum record size
on that particular index page.) To solve this problem, rebuild the table with ALTER TABLE and select
a larger compressed page size (KEY_BLOCK_SIZE), shorten any column prefix indexes, or disable
compression entirely with ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPACT.
3.4.2.2 Compressing BLOB, VARCHAR and TEXT Columns
In a clustered index, BLOB, VARCHAR and TEXT columns that are not part of the primary key may be
stored on separately allocated (“overflow”) pages. We call these “off-page columns” whose values are
stored on singly-linked lists of overflow pages.
For tables created in ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED, the values of BLOB,
TEXT or VARCHAR columns may be stored fully off-page, depending on their length and the length of
the entire row. For columns that are stored off-page, the clustered index record only contains 20-byte
pointers to the overflow pages, one per column. Whether any columns are stored off-page depends
on the page size and the total size of the row. When the row is too long to fit entirely within the page
of the clustered index, InnoDB chooses the longest columns for off-page storage until the row fits on
the clustered index page. As noted above, if a row does not fit by itself on a compressed page, an error
occurs.
Tables created in previous versions of InnoDB use the “Antelope” file format, which supports only
ROW_FORMAT=REDUNDANT and ROW_FORMAT=COMPACT. In these formats, InnoDB stores the first 768
bytes of BLOB, VARCHAR and TEXT columns in the clustered index record along with the primary key.
The 768-byte prefix is followed by a 20-byte pointer to the overflow pages that contain the rest of the
column value.
When a table is in COMPRESSED format, all data written to overflow pages is compressed “as is”; that is,
InnoDB applies the zlib compression algorithm to the entire data item. Other than the data, compressed
overflow pages contain an uncompressed header and trailer comprising a page checksum and a link
to the next overflow page, among other things. Therefore, very significant storage savings can be
obtained for longer BLOB, TEXT or VARCHAR columns if the data is highly compressible, as is often the
case with text data (but not previously compressed images).
The overflow pages are of the same size as other pages. A row containing ten columns stored offpage occupies ten overflow pages, even if the total length of the columns is only 8K bytes. In an
uncompressed table, ten uncompressed overflow pages occupy 160K bytes. In a compressed table
with an 8K page size, they occupy only 80K bytes. Thus, it is often more efficient to use compressed
table format for tables with long column values.
Using a 16K compressed page size can reduce storage and I/O costs for BLOB, VARCHAR or TEXT
columns, because such data often compress well, and might therefore require fewer “overflow” pages,
even though the B-tree nodes themselves take as many pages as in the uncompressed form.
3.4.3 Compression and the InnoDB Buffer Pool
In a compressed InnoDB table, every compressed page (whether 1K, 2K, 4K or 8K) corresponds to an
uncompressed page of 16K bytes. To access the data in a page, InnoDB must read the compressed
page from disk (unless it is already in memory), and then uncompress the page to its original 16K
byte form. This section describes how InnoDB manages the buffer pool with respect to pages of
compressed tables.
To minimize I/O and to reduce the need to uncompress a page, at times the buffer pool contains
both the compressed and uncompressed form of a database page. However, to make room for
other required database pages, InnoDB may “evict” from the buffer pool an uncompressed page,
while leaving the compressed page in memory. Or, if a page has not been accessed in a while, the
compressed form of the page may be written to disk, to free space for other data. Thus, at any given
time, the buffer pool may (a) not contain any copy of a given database page, (b) contain only the
compressed form of the page, or (c) contain both the compressed and uncompressed forms of the
page.
17
Compression and the InnoDB Log Files
InnoDB keeps track of which pages to retain in memory and which to evict using a least-recentlyused (LRU) list, so that “hot” or frequently accessed data tends to stay in memory. When compressed
tables are accessed, InnoDB uses an adaptive LRU algorithm to achieve an appropriate balance of
compressed and uncompressed pages in memory. This adaptive algorithm is sensitive to whether the
system is running in an I/O-bound or CPU-bound manner.
The essential idea is to avoid spending too much processing time uncompressing pages when the
CPU is busy, and to avoid doing excess I/O when the CPU has spare cycles that can be used for
uncompressing compressed pages (that may already be in memory). When the system is I/O-bound,
the algorithm prefers to evict the uncompressed copy of a page rather than both copies, to make more
room for other disk pages to become memory resident. When the system is CPU-bound, InnoDB
prefers to evict both the compressed and uncompressed page, so that more memory can be used for
“hot” pages and reducing the need to uncompress data in memory only in compressed form.
3.4.4 Compression and the InnoDB Log Files
Before (but not necessarily at the same time as) a compressed page is written to a database file,
InnoDB writes a copy of the page to the redo log (if it has been recompressed since the last time it was
written to the database). This is done to ensure that redo logs will always be usable, even if a future
version of InnoDB uses a slightly different compression algorithm. Therefore, some increase in the
size of log files, or a need for more frequent checkpoints, can be expected when using compression.
The amount of increase in the log file size or checkpoint frequency depends on the number of times
compressed pages are modified in a way that requires reorganization and recompression.
Note that the redo log file format (and the database file format) are different from previous releases
when using compression. The current release of InnoDB Hot Backup (version 3) therefore does not
support databases that use compression. Only databases using the file format “Antelope” can be
backed up online by InnoDB Hot Backup.
18
Chapter 4 InnoDB File-Format Management
Table of Contents
4.1
4.2
4.3
4.4
Overview of InnoDB File Formats ..........................................................................................
Named File Formats ..............................................................................................................
Enabling File Formats ...........................................................................................................
File Format Compatibility .......................................................................................................
4.4.1 Startup File Format Compatibility Checking ..................................................................
4.4.2 Table-Access File Format Compatibility Checking ........................................................
4.5 Identifying the File Format in Use ..........................................................................................
4.6 Downgrading the File Format .................................................................................................
4.7 Future InnoDB File Formats ..................................................................................................
19
19
20
20
21
22
23
23
23
4.1 Overview of InnoDB File Formats
As InnoDB evolves, new on-disk data structures are sometimes required to support new features.
This release of InnoDB introduces two such new data structures: compressed tables (see Chapter 3,
InnoDB Data Compression), and long variable-length columns stored off-page (see Chapter 5, InnoDB
Row Storage and Row Formats). These features both require use of the new Barracuda file format.
Note
These new data structures are not compatible with prior versions of InnoDB.
The other new features of the InnoDB Plugin are compatible with the original
Antelope file format and do not require the Barracuda file format.
In general, a newer version of InnoDB may create a table or index that cannot safely be read or
written with a prior version of InnoDB without risk of crashes, hangs, wrong results or corruptions. The
InnoDB Plugin introduces a new mechanism to guard against these conditions, and to help preserve
compatibility among database files and versions of InnoDB. This mechanism lets you take advantage
of some new features of an InnoDB release (e.g., performance improvements and bug fixes), and still
preserve the option of using your database with a prior version of InnoDB, by precluding the use of new
features that create downward incompatible on-disk data structures.
4.2 Named File Formats
The InnoDB Plugin introduces the idea of a named file format and a configuration parameter to enable
the use of features that require use of that format. The new file format is the Barracuda format, and
the file format supported by prior releases of InnoDB is known as Antelope. Compressed tables and
the new row format that stores long columns “off-page” require the use of the Barracuda file format or
newer. Future versions of InnoDB may introduce a series of file formats, identified with the names of
animals, in ascending alphabetic order.
Beginning with this release, every InnoDB per-table tablespace file is labeled with a file format
identifier. This does not apply to the system tablespace (the ibdata files) but only the files of separate
tablespaces (the *.ibd files where tables and indexes are stored in their own tablespace). As noted
below, however, the system tablespace is tagged with the “highest” file format in use in a group of
InnoDB database files, and this tag is checked when the files are opened.
In this release, when you create a compressed table, or a table with ROW_FORMAT=DYNAMIC, the file
header for the corresponding .ibd file and the table type in the InnoDB data dictionary are updated
with the identifier for the “Barracuda” file format. From that point forward, the table cannot be used with
a version of InnoDB that does not support this new file format. To protect against anomalous behavior,
InnoDB version 5.0.21 and later performs a compatibility check when the table is opened, as described
19
Enabling File Formats
below. (Note that the ALTER TABLE command in many cases, causes a table to be recreated and
thereby change its properties. The special case of adding or dropping indexes without rebuilding the
table is described in Chapter 2, Fast Index Creation in the InnoDB Storage Engine.)
If a version of InnoDB supports a particular file format (whether or not it is enabled), you can access
and even update any table that requires that format or an earlier format. Only the creation of new tables
using new features is limited based on the particular file format enabled. Conversely, if a tablespace
contains a table or index that uses a file format that is not supported by the currently running software,
it cannot be accessed at all, even for read access.
The only way to “downgrade” an InnoDB tablespace to an earlier file format is to copy the data to a new
table, in a tablespace that uses the earlier format. This can be done with the ALTER TABLE command,
as described in Section 4.6, “Downgrading the File Format”.
The easiest way to determine the file format of an existing InnoDB tablespace is to examine the
properties of the table it contains, using the SHOW TABLE STATUS command or querying the table
INFORMATION_SCHEMA.TABLES. If the Row_format of the table is reported as 'Compressed' or
'Dynamic', the tablespace containing the table uses the “Barracuda” format. Otherwise, it uses the
prior InnoDB file format, “Antelope”.
4.3 Enabling File Formats
The new configuration parameter innodb_file_format controls whether such commands as
CREATE TABLE and ALTER TABLE can be used to create tables that depend on support for the
“Barracuda” file format.
The file format is a dynamic, global parameter that can be specified in the MySQL option file (my.cnf
or my.ini) or changed with the SET GLOBAL command, as described in Section 9.5, “Configuring the
InnoDB Plugin”.
4.4 File Format Compatibility
To avoid confusion, for the purposes of this discussion we define the term “ib-file set” to mean the set
of operating system files that InnoDB manages as a unit. The ib-file set includes the following files:
• The system tablespace (one or more ibdata files) that contain internal system information
(including internal catalogs and undo information) and may include user data and indexes.
• Zero or more single-table tablespaces (also called “file per table” files, named *.ibd files).
• (Usually two) InnoDB log files (ib_logfile0 and ib_logfile1), used for crash recovery and in
backups.
This collection of files is transactionally consistent, and recoverable as a unit. An “ib-file set” specifically
does not include the related MySQL .frm files that contain metadata about InnoDB tables. The .frm
files are created and managed exclusively by MySQL, and can sometimes get out of sync with the
internal metadata in InnoDB.
Instead of “ib-file set”, we might call such a collection a “database”. However, MySQL uses the word
“database” to mean a logical collection of tables, what other systems term a “schema” or “catalog”.
Given MySQL terminology, multiple tables (even from more than one database) can be stored in a
single “ib-file set”.
The InnoDB Plugin incorporates several checks to guard against the possible crashes and data
corruptions that might occur if you use an ib-file set in a file format that is not supported by the software
release in use. These checks take place when the server is started, and when you first access a table.
This section describes these checks, how you can control them, and error and warning conditions that
may arise.
20
Startup File Format Compatibility Checking
4.4.1 Startup File Format Compatibility Checking
To prevent possible crashes or data corruptions when InnoDB Plugin opens an ib-file set, it checks
that it can fully support the file formats in use within the ib-file set. If the system is restarted following
a crash, or a “fast shutdown” (i.e., innodb_fast_shutdown is greater than zero), there may be ondisk data structures (such as redo or undo entries, or doublewrite pages) that are in a “too-new” format
for the current software. During the recovery process, serious damage can be done to your data files
if these data structures are accessed. The startup check of the file format occurs before any recovery
process begins, thereby preventing the problems described in Section 11.4, “Possible Problems”.
Beginning with version 1.0.1 of the InnoDB Plugin, the system tablespace records an identifier
or tag for the “highest” file format used by any table in any of the tablespaces that is part of the
ib-file set. Checks against this file format tag are controlled by the new configuration parameter
innodb_file_format_check, which is ON by default.
If the file format tag in the system tablespace is newer or higher than the highest version supported by
the particular currently executing software and if innodb_file_format_check is ON, the following
error is issued when the server is started:
InnoDB: Error: the system tablespace is in a file format that this version doesn't support
You can also set innodb_file_format to a file format name. Doing so prevents the InnoDB Plugin
from starting if the current software does not support the file format specified. It also sets the “high
water mark” to the value you specify. The ability to set innodb_file_format_check will be useful
(with future releases of InnoDB) if you manually “downgrade” all of the tables in an ib-file set (as
described in Chapter 11, Downgrading from the InnoDB Plugin). You can then rely on the file format
check at startup if you subsequently use an older version of InnoDB to access the ib-file set.
In some limited circumstances, you might want to start the server and use an ib-file set that is in a
“too new” format (one that is not supported by the software you are using). If you set the configuration
parameter innodb_file_format_check to OFF, the InnoDB Plugin opens the database, but issues
this warning message in the error log:
InnoDB: Warning: the system tablespace is in a
file format that this version doesn't support
Note
This is a very dangerous setting, as it permits the recovery process to run,
possibly corrupting your database if the previous shutdown was a crash
or “fast shutdown”. You should only set innodb_file_format_check
to OFF if you are sure that the previous shutdown was done with
innodb_fast_shutdown=0, so that essentially no recovery process occurs.
In a future release, this parameter setting may be renamed from OFF to
UNSAFE. (However, until there are newer releases of InnoDB that support
additional file formats, even disabling the startup checking is in fact “safe”.)
Note that the parameter innodb_file_format_check affects only what happens when a database
is opened, not subsequently. Conversely, the parameter innodb_file_format (which enables a
specific format) only determines whether or not a new table can be created in the enabled format and
has no effect on whether or not a database can be opened.
The file format tag is a “high water mark”, and as such it is increased after the server is started, if a
table in a “higher” format is created or an existing table is accessed for read or write (assuming its
format is supported). If you access an existing table in a format higher than the format the running
software supports, the system tablespace tag is not updated, but table-level compatibility checking
applies (and an error is issued), as described in Section 4.4.2, “Table-Access File Format Compatibility
Checking”. Any time the high water mark is updated, the value of innodb_file_format_check is
updated as well, so the command SELECT @@innodb_file_format_check; displays the name of
21
Table-Access File Format Compatibility Checking
the newest file format known to be used by tables in the currently open ib-file set and supported by the
currently executing software.
To best illustrate this behavior, consider the scenario described in Table 4.1, “InnoDB Data File
Compatibility and Related InnoDB Parameters”. Imagine that some future version of InnoDB supports
the “Cheetah” format and that an ib-file set has been used with that version.
Table 4.1 InnoDB Data File Compatibility and Related InnoDB Parameters
innodb innodb file
file
format
format
check
Highest
file format
used in ibfile set
Highest
Result
file format
supported
by InnoDB
OFF
Antelope or Barracuda Barracuda Database can be opened; tables can be created
Barracuda
which require “Antelope” or “Barracuda” file format
OFF
Antelope or Cheetah
Barracuda
OFF
Cheetah
ON
Antelope or Barracuda Barracuda Database can be opened; tables can be created
Barracuda
which require “Antelope” or “Barracuda” file format
ON
Antelope or Cheetah
Barracuda
ON
Cheetah
Barracuda Database can be opened with a warning, since
the database contains files in a “too new” format;
tables can be created which require “Antelope” or
“Barracuda” file format; tables in “Cheetah” format
cannot be accessed
Barracuda Barracuda Database cannot be opened;
innodb_file_format cannot be set to “Cheetah”
Barracuda Database cannot be opened, since the database
contains files in a “too new” format (“Cheetah”)
Barracuda Barracuda Database cannot be opened;
innodb_file_format cannot be set to “Cheetah”
4.4.2 Table-Access File Format Compatibility Checking
When a table is first accessed, InnoDB (including some releases prior to InnoDB Plugin 1.0) check
that the file format of the tablespace in which the table is stored is fully supported. This check prevents
crashes or corruptions that would otherwise occur when tables using a “too new” data structure are
encountered.
Note that all tables using any file format supported by a release can be read or written
(assuming the user has sufficient privileges). The setting of the system configuration parameter
innodb_file_format can prevent creating a new table that uses specific file formats, even if they
are supported by a given release. Such a setting might be used to preserve backward compatibility, but
it does not prevent accessing any table that uses any supported format.
As noted in Section 4.2, “Named File Formats”, versions of InnoDB older than 5.0.21 cannot reliably
use database files created by newer versions if a new file format was used when a table was created.
To prevent various error conditions or corruptions, InnoDB checks file format compatibility when it
opens a file (e.g., upon first access to a table). If the currently running version of InnoDB does not
support the file format identified by the table type in the InnoDB data dictionary, MySQL reports the
following error:
ERROR 1146 (42S02): Table 'test.t1'doesn't exist
Furthermore, InnoDB writes a message to the error log:
InnoDB: table test/t1: unknown table type 33
The table type should be equal to the tablespace flags, which contains the file format version as
discussed in Section 4.5, “Identifying the File Format in Use”.
22
Identifying the File Format in Use
Versions of InnoDB prior to 4.1 did not include table format identifiers in the database files, and
versions prior to 5.0.21 did not include a table format compatibility check. Therefore, there is no way to
ensure proper operations if a table in a “too new” format is used with versions of InnoDB prior to 5.0.21.
The new file format management capability introduced with the InnoDB Plugin (comprising tablespace
tagging and run-time checks) allows InnoDB to verify as soon as possible that the running version of
software can properly process the tables existing in the database.
If you permit InnoDB to open a database containing files in a format it does not support (by setting the
parameter innodb_file_format_check to OFF), the table-level checking described in this section
still applies.
Users are strongly urged not to use database files that contain “Barracuda” file format tables with
releases of InnoDB older than the InnoDB Plugin. It is possible to “downgrade” such tables to the
“Antelope” format (that the built-in InnoDB in MySQL up to version 5.1 supports) with the procedure
described in Section 4.6, “Downgrading the File Format”.
4.5 Identifying the File Format in Use
Although you may have enabled a given innodb_file_format at a particular time, unless you
create a new table, the database file format is unchanged. If you do create a new table, the tablespace
containing the table is tagged with the “earliest” or “simplest” file format that is required for the table's
features. For example, if you enable file format “Barracuda”, and create a new table that is not
compressed and does not use ROW_FORMAT=DYNAMIC, the new tablespace that contains the table is
tagged as using file format “Antelope”.
It is easy to identify the file format used by a given tablespace or table. The table uses the “Barracuda”
format if the Row_format reported by SHOW CREATE TABLE or INFORMATION_SCHEMA.TABLES
is one of 'Compressed' or 'Dynamic'. (Please note that the Row_format is a separate column,
and ignore the contents of the Create_options column, which may contain the string ROW_FORMAT.)
If the table in a tablespace uses neither of those features, the file uses the format supported by prior
releases of InnoDB, now called file format “Antelope”. Then, the Row_format is one of 'Redundant'
or 'Compact'.
The file format identifier is written as part of the tablespace flags (a 32-bit number) in the *.ibd file in
the 4 bytes starting at position 54 of the file, most significant byte first. (The first byte of the file is byte
zero.) On some systems, you can display these bytes in hexadecimal with the command od -t x1
-j 54 -N 4 tablename.ibd. If all bytes are zero, the tablespace uses the “Antelope” file format
(which is the format used by the standard built-in InnoDB in MySQL up to version 5.1). Otherwise, the
least significant bit should be set in the tablespace flags, and the file format identifier is written in the
bits 5 through 11. (Divide the tablespace flags by 32 and take the remainder after dividing the integer
part of the result by 128.)
4.6 Downgrading the File Format
Each InnoDB tablespace file (with a name matching *.ibd) is tagged with the file format used to
create its table and indexes. The way to downgrade the tablespace is to re-create the table and its
indexes. The easiest way to recreate a table and its indexes is to use the command:
ALTER TABLE t ROW_FORMAT=COMPACT;
on each table that you want to downgrade. The COMPACT row format uses the file format “Antelope”. It
was introduced in MySQL 5.0.3.
4.7 Future InnoDB File Formats
The file format used by the standard built-in InnoDB in MySQL 5.1 is the “Antelope” format, and the
new file format introduced with the InnoDB Plugin 1.0 is the “Barracuda” format. No definitive plans
23
Future InnoDB File Formats
have been made to introduce new features that would require additional new file formats. However, the
file format mechanism introduced with the InnoDB Plugin allows for further enhancements.
For the sake of completeness, these are the file format names that might be used for future file formats:
Antelope, Barracuda, Cheetah, Dragon, Elk, Fox, Gazelle, Hornet, Impala, Jaguar, Kangaroo, Leopard,
Moose, Nautilus, Ocelot, Porpoise, Quail, Rabbit, Shark, Tiger, Urchin, Viper, Whale, Xenops, Yak and
Zebra. These file formats correspond to the internal identifiers 0..25.
24
Chapter 5 InnoDB Row Storage and Row Formats
Table of Contents
5.1
5.2
5.3
5.4
Storage of Variable-Length Columns ......................................................................................
COMPACT and REDUNDANT Row Formats ...............................................................................
DYNAMIC Row Format ...........................................................................................................
Specifying a Table's Row Format ...........................................................................................
25
25
25
26
5.1 Storage of Variable-Length Columns
All data in InnoDB is stored in database pages comprising a B-tree index (the so-called clustered index
or primary key index). The essential idea is that the nodes of the B-tree contain, for each primary key
value (whether user-specified or generated or chosen by the system), the values of the remaining
columns of the row as well as the key. In some other database systems, a clustered index is called
an “index-organized table”. Secondary indexes in InnoDB are also B-trees, containing pairs of values
of the index key and the value of the primary key, which acts as a pointer to the row in the clustered
index.
There is an exception to this rule. Variable-length columns (such as BLOB and VARCHAR) that are too
long to fit on a B-tree page are stored on separately allocated disk (“overflow”) pages. We call these
“off-page columns”. The values of such columns are stored on singly-linked lists of overflow pages, and
each such column has its own list of one or more overflow pages. In some cases, all or a prefix of the
long column values is stored in the B-tree, to avoid wasting storage and eliminating the need to read a
separate page.
The new “Barracuda” file format provides a new option (KEY_BLOCK_SIZE) to control how much
column data is stored in the clustered index, and how much is placed on overflow pages.
5.2 COMPACT and REDUNDANT Row Formats
Previous versions of InnoDB used an unnamed file format (now called “Antelope”) for database files.
With that format, tables were defined with ROW_FORMAT=COMPACT (or ROW_FORMAT=REDUNDANT) and
InnoDB stored up to the first 768 bytes of variable-length columns (such as BLOB and VARCHAR) in the
index record within the B-tree node, with the remainder stored on the overflow page(s).
To preserve compatibility with those prior versions, tables created with the InnoDB Plugin use the prefix
format, unless one of ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED is specified (or implied)
on the CREATE TABLE command.
With the “Antelope” file format, if the value of a column is not longer than 768 bytes, no overflow page
is needed, and some savings in I/O may result, since the value is in the B-tree node. This works well
for relatively short BLOBs, but may cause B-tree nodes to fill with data rather than key values, thereby
reducing their efficiency. Tables with many BLOB columns could cause B-tree nodes to become too full
of data, and contain too few rows, making the entire index less efficient than if the rows were shorter or
if the column values were stored off-page.
5.3 DYNAMIC Row Format
When innodb_file_format is set to “Barracuda” and a table is created with
ROW_FORMAT=DYNAMIC or ROW_FORMAT=COMPRESSED, long column values are stored fully off-page,
and the clustered index record contains only a 20-byte pointer to the overflow page.
Whether any columns are stored off-page depends on the page size and the total size of the row.
When the row is too long, InnoDB chooses the longest columns for off-page storage until the clustered
index record fits on the B-tree page.
25
Specifying a Table's Row Format
The DYNAMIC row format maintains the efficiency of storing the entire row in the index node if it fits
(as do the COMPACT and REDUNDANT formats), but this new format avoids the problem of filling Btree nodes with a large number of data bytes of long columns. The DYNAMIC format is predicated on
the idea that if a portion of a long data value is stored off-page, it is usually most efficient to store all
of the value off-page. With DYNAMIC format, shorter columns are likely to remain in the B-tree node,
minimizing the number of overflow pages needed for any given row.
5.4 Specifying a Table's Row Format
The row format used for a table is specified with the ROW_FORMAT clause of the CREATE TABLE and
ALTER TABLE commands. Note that COMPRESSED format implies DYNAMIC format. See Section 3.2,
“Enabling Compression for a Table” for more details on the relationship between this clause and other
clauses of these commands.
26
Chapter 6 InnoDB INFORMATION_SCHEMA Tables
Table of Contents
6.1 Overview of InnoDB Support in INFORMATION_SCHEMA .........................................................
6.2 Information Schema Tables about Compression .....................................................................
6.2.1 INNODB_CMP and INNODB_CMP_RESET ......................................................................
6.2.2 INNODB_CMPMEM and INNODB_CMPMEM_RESET ..........................................................
6.2.3 Using the Compression Information Schema Tables .....................................................
6.3 Information Schema Tables about Transactions ......................................................................
6.3.1 INNODB_TRX ..............................................................................................................
6.3.2 INNODB_LOCKS ..........................................................................................................
6.3.3 INNODB_LOCK_WAITS ................................................................................................
6.3.4 Using the Transaction Information Schema Tables .......................................................
6.4 Notes on Locking in InnoDB ..................................................................................................
6.4.1 Understanding InnoDB Locking ...................................................................................
6.4.2 Rapidly Changing Internal Data ...................................................................................
6.4.3 Possible Inconsistency with PROCESSLIST ..................................................................
27
27
27
28
28
29
29
30
31
31
35
35
35
36
6.1 Overview of InnoDB Support in INFORMATION_SCHEMA
The INFORMATION_SCHEMA tables INNODB_BUFFER_PAGE, INNODB_BUFFER_PAGE_LRU,
INNODB_BUFFER_POOL_STATS, INNODB_CMP, INNODB_CMP_RESET, INNODB_CMPMEM,
INNODB_CMPMEM_RESET, INNODB_TRX, INNODB_LOCKS and INNODB_LOCK_WAITS contain live
information about the InnoDB buffer pool, compressed InnoDB tables, the compressed InnoDB buffer
pool, all transactions currently executing inside InnoDB, the locks that transactions hold and those that
are blocking transactions waiting for access to a resource (a table or row).
Note that the Information Schema tables are themselves plugins to the MySQL server. As such they
need to be INSTALLed as described in Chapter 9, Installing the InnoDB Plugin. If they are installed, but
the InnoDB storage engine plugin is not installed, these tables appear to be empty.
Following is a description of the new Information Schema tables introduced in the InnoDB Plugin, and
some examples of their use.
6.2 Information Schema Tables about Compression
Two new pairs of Information Schema tables provided by the InnoDB Plugin can give you some insight
into how well compression is working overall. One pair of tables contains information about the number
of compression operations and the amount of time spent performing compression. Another pair of
tables contains information on the way memory is allocated for compression.
6.2.1 INNODB_CMP and INNODB_CMP_RESET
The tables INNODB_CMP and INNODB_CMP_RESET contain status information on the operations
related to compressed tables, which are covered in Chapter 3, InnoDB Data Compression. The
compressed page size is in the column PAGE_SIZE.
These two tables have identical contents, but reading from INNODB_CMP_RESET resets the
statistics on compression and uncompression operations. For example, if you archived the output
of INNODB_CMP_RESET every 60 minutes, it would show the hourly statistics. If you never read
INNODB_CMP_RESET and monitored the output of INNODB_CMP instead, it would show the cumulated
statistics since InnoDB was started.
Table 6.1 Columns of INNODB_CMP and INNODB_CMP_RESET
Column name
Description
PAGE_SIZE
Compressed page size in bytes.
27
INNODB_CMPMEM and INNODB_CMPMEM_RESET
Column name
Description
COMPRESS_OPS
Number of times a B-tree page of the size PAGE_SIZE has been
compressed. Pages are compressed whenever an empty page is created
or the space for the uncompressed modification log runs out.
COMPRESS_OPS_OK
Number of times a B-tree page of the size PAGE_SIZE has
been successfully compressed. This count should never exceed
COMPRESS_OPS.
COMPRESS_TIME
Total time in seconds spent in attempts to compress B-tree pages of the
size PAGE_SIZE.
UNCOMPRESS_OPS
Number of times a B-tree page of the size PAGE_SIZE has been
uncompressed. B-tree pages are uncompressed whenever compression
fails or at first access when the uncompressed page does not exist in the
buffer pool.
UNCOMPRESS_TIME
Total time in seconds spent in uncompressing B-tree pages of the size
PAGE_SIZE.
6.2.2 INNODB_CMPMEM and INNODB_CMPMEM_RESET
You may consider the tables INNODB_CMPMEM and INNODB_CMPMEM_RESET as the status information
on the compressed pages that reside in the buffer pool. Please consult Chapter 3, InnoDB Data
Compression for further information on compressed tables and the use of the buffer pool. The tables
INNODB_CMP and INNODB_CMP_RESET should provide more useful statistics on compression.
The InnoDB Plugin uses a so-called “buddy allocator” system to manage memory allocated to pages
of various sizes, from 1KB to 16KB. Each row of the two tables described here corresponds to a single
page size.
These two tables have identical contents, but reading from INNODB_CMPMEM_RESET resets
the statistics on relocation operations. For example, if every 60 minutes you archived the
output of INNODB_CMPMEM_RESET, it would show the hourly statistics. If you never read
INNODB_CMPMEM_RESET and monitored the output of INNODB_CMPMEM instead, it would show the
cumulated statistics since InnoDB was started.
Table 6.2 Columns of INNODB_CMPMEM and INNODB_CMPMEM_RESET
Column name
Description
PAGE_SIZE
Block size in bytes. Each record of this table describes blocks of this size.
PAGES_USED
Number of blocks of the size PAGE_SIZE that are currently in use.
PAGES_FREE
Number of blocks of the size PAGE_SIZE that are currently available for
allocation. This column shows the external fragmentation in the memory
pool. Ideally, these numbers should be at most 1.
RELOCATION_OPS
Number of times a block of the size PAGE_SIZE has been relocated.
The buddy system can relocate the allocated “buddy neighbor” of a freed
block when it tries to form a bigger freed block. Reading from the table
INNODB_CMPMEM_RESET resets this count.
RELOCATION_TIME
Total time in microseconds spent in relocating blocks of the size
PAGE_SIZE. Reading from the table INNODB_CMPMEM_RESET resets this
count.
6.2.3 Using the Compression Information Schema Tables
Example 6.1 Using the Compression Information Schema Tables
The following is sample output from a database that contains compressed tables (see Chapter 3,
InnoDB Data Compression, INNODB_CMP, and INNODB_CMPMEM).
28
Information Schema Tables about Transactions
The following table shows the contents of INFORMATION_SCHEMA.INNODB_CMP under a light
workload. The only compressed page size that the buffer pool contains is 8K. Compressing or
uncompressing pages has consumed less than a second since the time the statistics were reset,
because the columns COMPRESS_TIME and UNCOMPRESS_TIME are zero.
page size compress
ops
compress ops ok compress time uncompress
ops
uncompress time
1024
0
0
0
0
0
2048
0
0
0
0
0
4096
0
0
0
0
0
8192
1048
921
0
61
0
16384
0
0
0
0
0
According to INNODB_CMPMEM, there are 6169 compressed 8KB pages in the buffer pool.
The following table shows the contents of INFORMATION_SCHEMA.INNODB_CMPMEM under light
load. We can see that some memory is unusable due to fragmentation of the InnoDB memory
allocator for compressed pages: SUM(PAGE_SIZE*PAGES_FREE)=6784. This is because small
memory allocation requests are fulfilled by splitting bigger blocks, starting from the 16K blocks that
are allocated from the main buffer pool, using the buddy allocation system. The fragmentation is this
low because some allocated blocks have been relocated (copied) to form bigger adjacent free blocks.
This copying of SUM(PAGE_SIZE*RELOCATION_OPS) bytes has consumed less than a second
(SUM(RELOCATION_TIME)=0).
page size
pages used
pages free
relocation ops
relocation time
1024
0
0
0
0
2048
0
1
0
0
4096
0
1
0
0
8192
6169
0
5
0
16384
0
0
0
0
6.3 Information Schema Tables about Transactions
Three new Information Schema tables introduced in the InnoDB Plugin make it much easier to
monitor transactions and diagnose possible locking problems. The three tables are INNODB_TRX,
INNODB_LOCKS and INNODB_LOCK_WAITS.
6.3.1 INNODB_TRX
Contains information about every transaction currently executing inside InnoDB, including whether the
transaction is waiting for a lock, when the transaction started, and the particular SQL statement the
transaction is executing.
Table 6.3 INNODB_TRX Columns
Column name
Description
TRX_ID
Unique transaction ID number, internal to InnoDB.
TRX_WEIGHT
The weight of a transaction, reflecting (but not necessarily the
exact count of) the number of rows altered and the number of
rows locked by the transaction. To resolve a deadlock, InnoDB
selects the transaction with the smallest weight as the “victim” to
rollback. Transactions that have changed non-transactional tables
are considered heavier than others, regardless of the number of
altered and locked rows.
29
INNODB_LOCKS
Column name
Description
TRX_STATE
Transaction execution state. One of 'RUNNING', 'LOCK WAIT',
'ROLLING BACK' or 'COMMITTING'.
TRX_STARTED
Transaction start time; the transaction is created by executing a
transactional query.
TRX_REQUESTED_LOCK_ID
ID of the lock the transaction is currently waiting for (if TRX_STATE
is 'LOCK WAIT', otherwise NULL). Details about the lock can be
found by joining with INNODB_LOCKS on LOCK_ID.
TRX_WAIT_STARTED
Time when the transaction started waiting on the lock (if
TRX_STATE is 'LOCK WAIT', otherwise NULL).
TRX_MYSQL_THREAD_ID
MySQL thread ID. Can be used for joining with PROCESSLIST
on ID. See Section 6.4.3, “Possible Inconsistency with
PROCESSLIST”.
TRX_QUERY
The SQL query that is being executed by the transaction.
6.3.2 INNODB_LOCKS
Each transaction in InnoDB that is waiting for another transaction to release a lock
(INNODB_TRX.TRX_STATE='LOCK WAIT') is blocked by exactly one “blocking lock request”. That
blocking lock request is for a row or table lock held by another transaction in an incompatible mode.
The waiting or blocked transaction cannot proceed until the other transaction commits or rolls back,
thereby releasing the requested lock. For every blocked transaction, INNODB_LOCKS contains one row
that describes each lock the transaction has requested, and for which it is waiting. INNODB_LOCKS
also contains one row for each lock that is blocking another transaction, whatever the state of the
transaction that holds the lock ('RUNNING', 'LOCK WAIT', 'ROLLING BACK' or 'COMMITTING').
The lock that is blocking a transaction is always held in a mode (read vs. write, shared vs. exclusive)
incompatible with the mode of requested lock.
Table 6.4 INNODB_LOCKS Columns
Column name
Description
LOCK_ID
Unique lock ID number, internal to InnoDB. Should be treated as an opaque
string. Although LOCK_ID currently contains TRX_ID, the format of the data in
LOCK_ID is not guaranteed to remain the same in future releases. You should
not write programs that parse the LOCK_ID value.
LOCK_TRX_ID
ID of the transaction holding this lock. Details about the transaction can be found
by joining with INNODB_TRX on TRX_ID.
LOCK_MODE
Mode of the lock. One of 'S', 'X', 'IS', 'IX', 'S,GAP', 'X,GAP',
'IS,GAP', 'IX,GAP', or 'AUTO_INC' for shared, exclusive, intention
shared, intention exclusive row locks, shared and exclusive gap locks, intention
shared and intention exclusive gap locks, and auto-increment table level lock,
respectively. Refer to the sections InnoDB Lock Modes and The InnoDB
Transaction Model and Locking of the MySQL Manual for information on InnoDB
locking.
LOCK_TYPE
Type of the lock. One of 'RECORD' or 'TABLE' for record (row) level or table
level locks, respectively.
LOCK_TABLE
Name of the table that has been locked or contains locked records.
LOCK_INDEX
Name of the index if LOCK_TYPE='RECORD', otherwise NULL.
LOCK_SPACE
Tablespace ID of the locked record if LOCK_TYPE='RECORD', otherwise NULL.
LOCK_PAGE
Page number of the locked record if LOCK_TYPE='RECORD', otherwise NULL.
LOCK_REC
Heap number of the locked record within the page if LOCK_TYPE='RECORD',
otherwise NULL.
30
INNODB_LOCK_WAITS
Column name
Description
LOCK_DATA
Primary key of the locked record if LOCK_TYPE='RECORD', otherwise NULL.
This column contains the value(s) of the primary key column(s) in the locked row,
formatted as a valid SQL string (ready to be copied to SQL commands). If there is
no primary key then the InnoDB internal unique row ID number is used. When the
page containing the locked record is not in the buffer pool (in the case that it was
paged out to disk while the lock was held), InnoDB does not fetch the page from
disk, to avoid unnecessary disk operations. Instead, LOCK_DATA is set to NULL.
6.3.3 INNODB_LOCK_WAITS
Using this table, you can tell which transactions are waiting for a given lock, or for which lock a given
transaction is waiting. This table contains one or more rows for each blocked transaction, indicating the
lock it has requested and any locks that are blocking that request. The REQUESTED_LOCK_ID refers
to the lock that a transaction is requesting, and the BLOCKING_LOCK_ID refers to the lock (held by
another transaction) that is preventing the first transaction from proceeding. For any given blocked
transaction, all rows in INNODB_LOCK_WAITS have the same value for REQUESTED_LOCK_ID and
different values for BLOCKING_LOCK_ID.
Table 6.5 INNODB_LOCK_WAITS Columns
Column name
Description
REQUESTING_TRX_ID
ID of the requesting transaction.
REQUESTED_LOCK_ID
ID of the lock for which a transaction is waiting. Details about the lock can
be found by joining with INNODB_LOCKS on LOCK_ID.
BLOCKING_TRX_ID
ID of the blocking transaction.
BLOCKING_LOCK_ID
ID of a lock held by a transaction blocking another transaction from
proceeding. Details about the lock can be found by joining with
INNODB_LOCKS on LOCK_ID.
6.3.4 Using the Transaction Information Schema Tables
Example 6.2 Identifying Blocking Transactions
It is sometimes helpful to be able to identify which transaction is blocking another. You can use the
Information Schema tables to find out which transaction is waiting for another, and which resource is
being requested.
Suppose you have the following scenario, with three users running concurrently. Each user (or session)
corresponds to a MySQL thread, and executes one transaction after another. Consider the state of
the system when these users have issued the following commands, but none has yet committed its
transaction:
• User A:
BEGIN;
SELECT a FROM t FOR UPDATE;
SELECT SLEEP(100);
• User B:
SELECT b FROM t FOR UPDATE;
• User C:
SELECT c FROM t FOR UPDATE;
31
Using the Transaction Information Schema Tables
In this scenario, you can use this query to see who is waiting for whom:
SELECT r.trx_id waiting_trx_id,
r.trx_mysql_thread_id waiting_thread,
r.trx_query waiting_query,
b.trx_id blocking_trx_id,
b.trx_mysql_thread_id blocking_thread,
b.trx_query blocking_query
FROM
information_schema.innodb_lock_waits w
INNER JOIN information_schema.innodb_trx b ON
b.trx_id = w.blocking_trx_id
INNER JOIN information_schema.innodb_trx r ON
r.trx_id = w.requesting_trx_id;
waiting waiting waiting query
trx id thread
blockingblockingblocking query
trx id
thread
A4
6
SELECT b FROM t FOR
UPDATE
A3
5
SELECT SLEEP(100)
A5
7
SELECT c FROM t FOR
UPDATE
A3
5
SELECT SLEEP(100)
A5
7
SELECT c FROM t FOR
UPDATE
A4
6
SELECT b FROM t FOR
UPDATE
In the above result, you can identify users by the “waiting query” or “blocking query”. As you can see:
• User B (trx id 'A4', thread 6) and User C (trx id 'A5', thread 7) are both waiting for User A (trx id
'A3', thread 5).
• User C is waiting for User B as well as User A.
You can see the underlying data in the tables INNODB_TRX, INNODB_LOCKS, and
INNODB_LOCK_WAITS.
The following table shows some sample contents of INFORMATION_SCHEMA.INNODB_TRX.
trx trx
trx started
id state
trx
requested
lock id
trx wait started trx
trx mysql trx query
weight thread id
A3 RUN- 2008-01-15
NING 16:44:54
NULL
NULL
2
5
SELECT
SLEEP(100)
A4 LOCK 2008-01-15
WAIT 16:45:09
A4:1:3:2
2008-01-15
16:45:09
2
6
SELECT b FROM t
FOR UPDATE
A5 LOCK 2008-01-15
WAIT 16:45:14
A5:1:3:2
2008-01-15
16:45:14
2
7
SELECT c FROM t
FOR UPDATE
The following table shows some sample contents of INFORMATION_SCHEMA.INNODB_LOCKS.
lock id
lock lock lock type lock table
trx id mode
lock index
lock lock lock lock data
space page rec
A3:1:3:2
A3
X
RECORD
`test`.`t`
`PRIMARY`
1
3
2
0x0200
A4:1:3:2
A4
X
RECORD
`test`.`t`
`PRIMARY`
1
3
2
0x0200
A5:1:3:2
A5
X
RECORD
`test`.`t`
`PRIMARY`
1
3
2
0x0200
The following table shows some sample contents of INFORMATION_SCHEMA.INNODB_LOCK_WAITS.
requesting trx id
requested lock id
blocking trx id
blocking lock id
A4
A4:1:3:2
A3
A3:1:3:2
32
Using the Transaction Information Schema Tables
requesting trx id
requested lock id
blocking trx id
blocking lock id
A5
A5:1:3:2
A3
A3:1:3:2
A5
A5:1:3:2
A4
A4:1:3:2
Example 6.3 More Complex Example of Transaction Data in Information Schema Tables
Sometimes you would like to correlate the internal InnoDB locking information with session-level
information maintained by MySQL. For example, you might like to know, for a given InnoDB transaction
ID, the corresponding MySQL session ID and name of the user that may be holding a lock, and thus
blocking another transaction.
The following output from the INFORMATION_SCHEMA tables is taken from a somewhat loaded system.
As can be seen in the following tables, there are several transactions running.
The following INNODB_LOCKS and INNODB_LOCK_WAITS tables shows that:
• Transaction 77F (executing an INSERT) is waiting for transactions 77E, 77D and 77B to commit.
• Transaction 77E (executing an INSERT) is waiting for transactions 77D and 77B to commit.
• Transaction 77D (executing an INSERT) is waiting for transaction 77B to commit.
• Transaction 77B (executing an INSERT) is waiting for transaction 77A to commit.
• Transaction 77A is running, currently executing SELECT.
• Transaction E56 (executing an INSERT) is waiting for transaction E55 to commit.
• Transaction E55 (executing an INSERT) is waiting for transaction 19C to commit.
• Transaction 19C is running, currently executing an INSERT.
Note that there may be an inconsistency between queries shown in the two tables
INNODB_TRX.TRX_QUERY and PROCESSLIST.INFO. The current transaction ID for a thread, and the
query being executed in that transaction, may be different in these two tables for any given thread. See
Section 6.4.3, “Possible Inconsistency with PROCESSLIST” for an explanation.
The following table shows the contents of INFORMATION_SCHEMA.PROCESSLIST in a system running
a heavy workload.
ID
USER
HOST
DB
COMMAND
TIME
STATE
INFO
384
root
localhost
test
Query
10
update
insert into t2
values …
257
root
localhost
test
Query
3
update
insert into t2
values …
130
root
localhost
test
Query
0
update
insert into t2
values …
61
root
localhost
test
Query
1
update
insert into t2
values …
8
root
localhost
test
Query
1
update
insert into t2
values …
4
root
localhost
test
Query
0
preparing
SELECT * FROM
processlist
2
root
localhost
test
Sleep
566
NULL
The following table shows the contents of INFORMATION_SCHEMA.INNODB_TRX in a system running
a heavy workload.
33
Using the Transaction Information Schema Tables
trx
id
trx
state
trx started
trx
requested
lock id
trx wait
started
trx
trx
weight mysql
thread
id
trx query
77F
LOCK
WAIT
2008-01-15
13:10:16
77F:806
2008-01-15
13:10:16
1
876
insert into t09
(D, B, C) values
…
77E
LOCK
WAIT
2008-01-15
13:10:16
77E:806
2008-01-15
13:10:16
1
875
insert into t09
(D, B, C) values
…
77D
LOCK
WAIT
2008-01-15
13:10:16
77D:806
2008-01-15
13:10:16
1
874
insert into t09
(D, B, C) values
…
77B
LOCK
WAIT
2008-01-15
13:10:16
77B:733:12:1 2008-01-15
13:10:16
4
873
insert into t09
(D, B, C) values
…
77A
RUNNING
2008-01-15
13:10:16
NULL
NULL
4
872
select b, c from
t09 where …
E56
LOCK
WAIT
2008-01-15
13:10:06
E56:743:6:2
2008-01-15
13:10:06
5
384
insert into t2
values …
E55
LOCK
WAIT
2008-01-15
13:10:06
E55:743:38:2 2008-01-15
13:10:13
965
257
insert into t2
values …
19C
RUNNING
2008-01-15
13:09:10
NULL
NULL
2900
130
insert into t2
values …
E15
RUNNING
2008-01-15
13:08:59
NULL
NULL
5395
61
insert into t2
values …
51D
RUNNING
2008-01-15
13:08:47
NULL
NULL
9807
8
insert into t2
values …
The following table shows the contents of INFORMATION_SCHEMA.INNODB_LOCK_WAITS in a loaded
system
requesting trx id
requested lock id
blocking trx id
blocking lock id
77F
77F:806
77E
77E:806
77F
77F:806
77D
77D:806
77F
77F:806
77B
77B:806
77E
77E:806
77D
77D:806
77E
77E:806
77B
77B:806
77D
77D:806
77B
77B:806
77B
77B:733:12:1
77A
77A:733:12:1
E56
E56:743:6:2
E55
E55:743:6:2
E55
E55:743:38:2
19C
19C:743:38:2
The following table shows the contents of INFORMATION_SCHEMA.INNODB_LOCKS in a system
running a heavy workload.
lock id
lock lock
lock type lock table lock index
trx id mode
lock lock lock
space page rec
77F:806
77F
NULL NULL NULL NULL
AUTO
_INC
TABLE
`test`
.`t09`
34
NULL
lock data
Notes on Locking in InnoDB
lock id
lock lock
lock type lock table lock index
trx id mode
lock lock lock
space page rec
lock data
77E:806
77E
AUTO
_INC
TABLE
`test`
.`t09`
NULL
NULL NULL NULL NULL
77D:806
77D
AUTO
_INC
TABLE
`test`
.`t09`
NULL
NULL NULL NULL NULL
77B:806
77B
AUTO
_INC
TABLE
`test`
.`t09`
NULL
NULL NULL NULL NULL
77B:733
:12:1
77B
X
RECORD
`test`
.`t09`
`PRIMARY`
733
12
1
supremum
pseudorecord
77A:733
:12:1
77A
X
RECORD
`test`
.`t09`
`PRIMARY`
733
12
1
supremum
pseudorecord
E56:743
:6:2
E56
S
RECORD
`test`
.`t2`
`PRIMARY`
743
6
2
0, 0
E55:743
:6:2
E55
X
RECORD
`test`
.`t2`
`PRIMARY`
743
6
2
0, 0
E55:743
:38:2
E55
S
RECORD
`test`
.`t2`
`PRIMARY`
743
38
2
1922,
1922
19C:743
:38:2
19C
X
RECORD
`test`
.`t2`
`PRIMARY`
743
38
2
1922,
1922
6.4 Notes on Locking in InnoDB
6.4.1 Understanding InnoDB Locking
When a transaction updates a row in a table, or locks it with SELECT FOR UPDATE, InnoDB
establishes a list or queue of locks on that row. Similarly, InnoDB maintains a list of locks on a table
for table-level locks transactions hold. If a second transaction wants to update a row or lock a table
already locked by a prior transaction in an incompatible mode, InnoDB adds a lock request for the row
to the corresponding queue. For a lock to be acquired by a transaction, all incompatible lock requests
previously entered into the lock queue for that row or table must be removed (the transactions holding
or requesting those locks either commit or rollback).
A transaction may have any number of lock requests for different rows or tables. At any given time, a
transaction may be requesting a lock that is held by another transaction, in which case it is blocked by
that other transaction. The requesting transaction must wait for the transaction that holds the blocking
lock to commit or rollback. If a transaction is not waiting for a a lock, it is in the 'RUNNING' state. If a
transaction is waiting for a lock, it is in the 'LOCK WAIT' state.
The table INNODB_LOCKS holds one or more row for each 'LOCK WAIT' transaction, indicating any
lock requests that are preventing its progress. This table also contains one row describing each lock in
a queue of locks pending for a given row or table. The table INNODB_LOCK_WAITS shows which locks
already held by a transaction are blocking locks requested by other transactions.
6.4.2 Rapidly Changing Internal Data
The data exposed by the transaction and locking tables represent a glimpse into fast-changing data.
This is not like other (user) tables, where the data only changes when application-initiated updates
occur. The underlying data is internal system-managed data, and can change very quickly.
For performance reasons, and to minimize the chance of misleading JOINs between the
INFORMATION_SCHEMA tables, InnoDB collects the required transaction and locking information into
35
Possible Inconsistency with PROCESSLIST
an intermediate buffer whenever a SELECT on any of the tables is issued. This buffer is refreshed only
if more than 0.1 seconds has elapsed since the last time the buffer was read. The data needed to fill
the three tables is fetched atomically and consistently and is saved in this global internal buffer, forming
a point-in-time “snapshot”. If multiple table accesses occur within 0.1 seconds (as they almost certainly
do when MySQL processes a join among these tables), then the same snapshot is used to satisfy the
query.
A correct result is returned when you JOIN any of these tables together in a single query, because
the data for the three tables comes from the same snapshot. Because the buffer is not refreshed with
every query of any of these tables, if you issue separate queries against these tables within a tenth of
a second, the results are the same from query to query. On the other hand, two separate queries of the
same or different tables issued more than a tenth of a second apart may see different results, since the
data come from different snapshots.
Because InnoDB must temporarily stall while the transaction and locking data is collected, too frequent
queries of these tables can negatively impact performance as seen by other users.
As these tables contain sensitive information (at least INNODB_LOCKS.LOCK_DATA and
INNODB_TRX.TRX_QUERY), for security reasons, only the users with the PROCESS privilege are
allowed to SELECT from them.
6.4.3 Possible Inconsistency with PROCESSLIST
As just described, while the transaction and locking data is correct and consistent when these
INFORMATION_SCHEMA tables are populated, the underlying data changes so fast that similar
glimpses at other, similarly fast-changing data, may not be in sync. Thus, you should be
careful in comparing the data in the InnoDB transaction and locking tables with that in the The
INFORMATION_SCHEMA PROCESSLIST Table. The data from the PROCESSLIST table does not come
from the same snapshot as the data about locking and transactions. Even if you issue a single SELECT
(JOINing INNODB_TRX and PROCESSLIST, for example), the content of those tables is generally not
consistent. INNODB_TRX may reference rows that are not present in PROCESSLIST or the currently
executing SQL query of a transaction, shown in INNODB_TRX.TRX_QUERY may be different from
the one in PROCESSLIST.INFO. The query in INNODB_TRX is always consistent with the rest of
INNODB_TRX, INNODB_LOCKS and INNODB_LOCK_WAITS when the data comes from the same
snapshot.
36
Chapter 7 Performance and Scalability Enhancements
Table of Contents
7.1 Overview ............................................................................................................................... 37
7.2 Faster Locking for Improved Scalability .................................................................................. 38
7.3 Using Operating System Memory Allocators ........................................................................... 38
7.4 Controlling InnoDB Insert Buffering ........................................................................................ 39
7.5 Controlling Adaptive Hash Indexing ........................................................................................ 39
7.6 Changes Regarding Thread Concurrency ............................................................................... 40
7.7 Changes in the Read Ahead Algorithm .................................................................................. 41
7.8 Multiple Background I/O Threads ........................................................................................... 42
7.9 Group Commit ...................................................................................................................... 42
7.10 Controlling the Master Thread I/O Rate ................................................................................ 43
7.11 Controlling the Flushing Rate of Dirty Pages ......................................................................... 43
7.12 Using the PAUSE instruction in InnoDB spin loops ............................................................... 44
7.13 Control of Spin Lock Polling ................................................................................................ 44
7.14 Making Buffer Cache Scan Resistant ................................................................................... 44
7.14.1 Guidelines for innodb_old_blocks_pct and innodb_old_blocks_time ............. 45
7.15 Improvements to Crash Recovery Performance .................................................................... 46
7.1 Overview
InnoDB has always been highly efficient, and includes several unique architectural elements to
assure high performance and scalability. InnoDB Plugin 1.0.8 includes several new features that take
better advantage of recent advances in operating systems and hardware platforms, such as multicore processors and improved memory allocation systems. In addition, this release permits you to
better control the use of some InnoDB internal subsystems to achieve the best performance with your
workload.
InnoDB Plugin 1.0.8 includes new capabilities in these areas:
• Section 7.2, “Faster Locking for Improved Scalability”
• Section 7.3, “Using Operating System Memory Allocators”
• Section 7.4, “Controlling InnoDB Insert Buffering”
• Section 7.5, “Controlling Adaptive Hash Indexing”
• Section 7.6, “Changes Regarding Thread Concurrency”
• Section 7.7, “Changes in the Read Ahead Algorithm”
• Section 7.8, “Multiple Background I/O Threads”
• Section 7.9, “Group Commit”
• Section 7.10, “Controlling the Master Thread I/O Rate”
• Section 7.11, “Controlling the Flushing Rate of Dirty Pages”
• Section 7.12, “Using the PAUSE instruction in InnoDB spin loops”
• Section 7.13, “Control of Spin Lock Polling”
• Section 7.14, “Making Buffer Cache Scan Resistant”
• Section 7.15, “Improvements to Crash Recovery Performance”
37
Faster Locking for Improved Scalability
7.2 Faster Locking for Improved Scalability
In MySQL and InnoDB, multiple threads of execution access shared data structures. InnoDB
synchronizes these accesses with its own implementation of mutexes and read/write locks. InnoDB
has historically protected the internal state of a read/write lock with an InnoDB mutex. On Unix and
Linux platforms, the internal state of an InnoDB mutex is protected by a Pthreads mutex, as in IEEE
Std 1003.1c (POSIX.1c).
On many platforms, there is a more efficient way to implement mutexes and read/write locks. Atomic
operations can often be used synchronize the actions of multiple threads more efficiently than
Pthreads. Each operation to acquire or release a lock can be done in fewer CPU instructions, and thus
result in less wasted time when threads are contending for access to shared data structures. This in
turn means greater scalability on multi-core platforms.
Beginning with InnoDB Plugin 1.0.3, InnoDB implements mutexes and read/write locks with the built-in
functions provided by the GNU Compiler Collection (GCC) for atomic memory access instead of using
the Pthreads approach previously used. More specifically, an InnoDB Plugin that is compiled with GCC
version 4.1.2 or later will use the atomic builtins instead of a pthread_mutex_t to implement InnoDB
mutexes and read/write locks.
On 32-bit Microsoft Windows, InnoDB has implemented mutexes (but not read/write locks) with handwritten assembler instructions. Beginning with Microsoft Windows 2000, it is possible to use functions
for Interlocked Variable Access that are similar to the built-in functions provided by GCC. Beginning
with InnoDB Plugin 1.0.4, InnoDB makes use of the Interlocked functions on Windows. Unlike the old
hand-written assembler code, the new implementation supports read/write locks and 64-bit platforms.
Solaris 10 introduced library functions for atomic operations. Beginning with InnoDB Plugin 1.0.4, when
InnoDB is compiled on Solaris 10 with a compiler that does not support the built-in functions provided
by the GNU Compiler Collection (GCC) for atomic memory access, the library functions will be used.
This change improves the scalability of InnoDB on multi-core systems. Note that the user does not
have to set any particular parameter or option to take advantage of this new feature. This feature is
enabled out-of-the-box on the platforms where it is supported. On platforms where the GCC, Windows,
or Solaris functions for atomic memory access are not available, InnoDB will use the traditional
Pthreads method of implementing mutexes and read/write locks.
When MySQL starts, InnoDB will write a message to the log file indicating whether atomic memory
access will be used for mutexes, for mutexes and read/write locks, or neither. If suitable tools are used
to build the InnoDB Plugin and the target CPU supports the atomic operations required, InnoDB will
use the built-in functions for mutexing. If, in addition, the compare-and-swap operation can be used on
thread identifiers (pthread_t), then InnoDB will use the instructions for read-write locks as well.
Note: If you are building from source, see Section 9.4.1, “Building the InnoDB Plugin on Linux or
Unix” [62] to ensure that your build process properly takes advantage of your platform capabilities.
7.3 Using Operating System Memory Allocators
When InnoDB was developed, the memory allocators supplied with operating systems and run-time
libraries were often lacking in performance and scalability. At that time, there were no memory allocator
libraries tuned for multi-core CPUs. Therefore, InnoDB implemented its own memory allocator in the
mem subsystem. This allocator is guarded by a single mutex, which may become a bottleneck. InnoDB
also implements a wrapper interface around the system allocator (malloc and free) that is likewise
guarded by a single mutex.
Today, as multi-core systems have become more widely available, and as operating systems have
matured, significant improvements have been made in the memory allocators provided with operating
systems. New memory allocators perform better and are more scalable than they were in the past.
The leading high-performance memory allocators include Hoard, libumem, mtmalloc, ptmalloc,
38
Controlling InnoDB Insert Buffering
tbbmalloc, and TCMalloc. Most workloads, especially those where memory is frequently allocated
and released (such as multi-table joins) will benefit from using a more highly tuned memory allocator as
opposed to the internal, InnoDB-specific memory allocator.
Beginning with InnoDB Plugin 1.0.3, you control whether InnoDB uses its own memory allocator or
an allocator of the operating system, by setting the value of the new system configuration parameter
innodb_use_sys_malloc in the MySQL option file (my.cnf or my.ini). If set to ON or 1 (the
default), InnoDB will use the malloc and free functions of the underlying system rather than
manage memory pools itself. This parameter is not dynamic, and takes effect only when the system
is started. To continue to use the InnoDB memory allocator in InnoDB Plugin, you will have to set
innodb_use_sys_malloc to 0.
Note that when the InnoDB memory allocator is disabled, InnoDB will ignore the value of the parameter
innodb_additional_mem_pool_size. The InnoDB memory allocator uses an additional memory
pool for satisfying allocation requests without having to fall back to the system memory allocator. When
the InnoDB memory allocator is disabled, all such allocation requests will be fulfilled by the system
memory allocator.
Furthermore, since InnoDB cannot track all memory use when the system memory allocator is used
(innodb_use_sys_malloc is ON), the section “BUFFER POOL AND MEMORY” in the output of the
SHOW ENGINE INNODB STATUS command will only include the buffer pool statistics in the “Total
memory allocated”. Any memory allocated using the mem subsystem or using ut_malloc will be
excluded.
On Unix-like systems that use dynamic linking, replacing the memory allocator may be as easy as
making the environment variable LD_PRELOAD or LD_LIBRARY_PATH point to the dynamic library
that implements the allocator. On other systems, some relinking may be necessary. Please refer to the
documentation of the memory allocator library of your choice.
7.4 Controlling InnoDB Insert Buffering
When INSERTs are done to a table, often the values of indexed columns (particularly the values of
secondary keys) are not in sorted order. This means that the inserts of such values into secondary Btree indexes is “random”, and this can cause excessive i/o if the entire index does not fit in memory.
InnoDB has an insert buffer that caches changes to secondary index entries when the relevant page
is not in the buffer pool, thus avoiding I/O operations by not reading in the page from the disk. The
buffered changes are written into a special insert buffer tree and are subsequently merged when the
page is loaded to the buffer pool. The InnoDB main thread merges buffered changes when the server
is nearly idle.
Usually, this process will result in fewer disk reads and writes, especially during bulk inserts. However,
the insert buffer tree will occupy a part of the buffer pool. If the working set almost fits in the buffer
pool, it may be useful to disable insert buffering. If the working set entirely fits in the buffer pool, insert
buffering will not be used anyway, because the index would exist in memory.
Beginning with InnoDB Plugin 1.0.3, you can control whether InnoDB performs insert buffering
with the system configuration parameter innodb_change_buffering. The allowed values of
innodb_change_buffering are none (do not buffer any operations) and inserts (buffer insert
operations, the default). You can set the value of this parameter in the MySQL option file (my.cnf
or my.ini) or change it dynamically with the SET GLOBAL command, which requires the SUPER
privilege. Changing the setting affects the buffering of new operations; the merging of already buffered
entries is not affected.
7.5 Controlling Adaptive Hash Indexing
If a table fits almost entirely in main memory, the fastest way to perform queries on it is to use hash
indexes rather than B-tree lookups. InnoDB monitors searches on each index defined for a table. If it
notices that certain index values are being accessed frequently, it automatically builds an in-memory
39
Changes Regarding Thread Concurrency
hash table for that index. Based on the pattern of searches that InnoDB observes, it will build a hash
index using a prefix of the index key. The prefix of the key can be any length, and it may be that only
a subset of the values in the B-tree will appear in the hash index. InnoDB builds hash indexes on
demand for those pages of the index that are often accessed.
The adaptive hash index mechanism allows InnoDB to take advantage of large amounts of memory,
something typically done only by database systems specifically designed for databases that reside
entirely in memory. Normally, the automatic building and use of adaptive hash indexes will improve
performance. However, sometimes, the read/write lock that guards access to the adaptive hash index
may become a source of contention under heavy workloads, such as multiple concurrent joins.
You can monitor the use of the adaptive hash index and the contention for its use in the
“SEMAPHORES” section of the output of the SHOW ENGINE INNODB STATUS command. If you
see many threads waiting on an RW-latch created in btr0sea.c, then it might be useful to disable
adaptive hash indexing.
The configuration parameter innodb_adaptive_hash_index can be set to disable or enable the
adaptive hash index. See Section 8.3.4, “Dynamically Changing innodb_adaptive_hash_index”
for details.
7.6 Changes Regarding Thread Concurrency
InnoDB uses operating system threads to process requests from user transactions. (Transactions may
issue many requests to InnoDB before they commit or roll back.) On today's modern operating systems
and servers with multi-core processors, where context switching is efficient, most workloads will run
well without any limit on the number of concurrent threads. Thanks to several scalability improvements
in InnoDB Plugin 1.0.3, and further changes in release 1.0.4, there should be less need to artificially
limit the number of concurrently executing threads inside InnoDB.
However, for some situations, it may be helpful to minimize context switching between threads. InnoDB
can use a number of techniques to limit the number of concurrently executing operating system threads
(and thus the number of requests that are processed at any one time). When InnoDB receives a new
request from a user session, if the number of threads concurrently executing is at a pre-defined limit,
the new request will sleep for a short time before it tries again. A request that cannot be rescheduled
after the sleep is put in a first-in/first-out queue and eventually will be processed. Threads waiting for
locks are not counted in the number of concurrently executing threads.
The limit on the number of concurrent threads is given by the settable global variable
innodb_thread_concurrency. Once the number of executing threads reaches this limit,
additional threads will sleep for a number of microseconds, set by the system configuration parameter
innodb_thread_sleep_delay, before being placed into the queue.
The default value for innodb_thread_concurrency and the implied default limit on the number of
concurrent threads has been changed in various releases of MySQL and the InnoDB Plugin. Starting
with InnoDB Plugin 1.0.3, the default value of innodb_thread_concurrency is 0, so that by default
there is no limit on the number of concurrently executing threads, as shown in Table 7.1, “Changes to
innodb_thread_concurrency”.
Table 7.1 Changes to innodb_thread_concurrency
InnoDB Version
MySQL Version
Default Default limit
value
of concurrent
threads
Value to allow
unlimited
threads
Built-in
Earlier than 5.1.11
20
No limit
20 or higher
Built-in
5.1.11 and newer
8
8
0
InnoDB Plugin before 1.0.3
(corresponding to
Plugin)
8
8
0
40
Changes in the Read Ahead Algorithm
InnoDB Version
MySQL Version
Default Default limit
value
of concurrent
threads
Value to allow
unlimited
threads
InnoDB Plugin 1.0.3 and
newer
(corresponding to
Plugin)
0
0
No limit
Note that InnoDB will cause threads to sleep only when the number of concurrent threads is limited.
When there is no limit on the number of threads, all will contend equally to be scheduled. That is, if
innodb_thread_concurrency is 0, the value of innodb_thread_sleep_delay is ignored.
When there is a limit on the number of threads, InnoDB reduces context switching overhead by
permitting multiple requests made during the execution of a single SQL statement to enter InnoDB
without observing the limit set by innodb_thread_concurrency. Since an SQL statement (such
as a join) may comprise multiple row operations within InnoDB, InnoDB assigns “tickets” that allow a
thread to be scheduled repeatedly with minimal overhead.
When starting to execute a new SQL statement, a thread will have no tickets, and it must observe
innodb_thread_concurrency. Once the thread is entitled to enter InnoDB, it will be assigned
a number of tickets that it can use for subsequently entering InnoDB. If the tickets run out,
innodb_thread_concurrency will be observed again and further tickets will be assigned. The
number of tickets to assign is specified by the global option innodb_concurrency_tickets, which
is 500 by default. A thread that is waiting for a lock will be given one ticket once the lock becomes
available.
The correct values of these variables are dependent on your environment and workload. You will
need to try a range of different values to determine what value works for your applications. Before
limiting the number of concurrently executing threads, you should review configuration options that
may improve the performance of InnoDB on multi-core and multi-processor computers, such as
innodb_use_sys_malloc and innodb_adaptive_hash_index.
7.7 Changes in the Read Ahead Algorithm
A read ahead request is an I/O request to prefetch multiple pages in the buffer cache asynchronously
in anticipation that these pages will be needed in the near future. InnoDB has historically used two read
ahead algorithms to improve I/O performance.
Random read-ahead is a technique that predicts when pages might be needed soon based on pages
already in the buffer pool, regardless of the order in which those pages were read. If 13 consecutive
pages from the same extent are found in the buffer pool, InnoDB asynchronously issues a request to
prefetch the remaining pages of the extent. This feature was initially removed from InnoDB starting with
InnoDB Plugin 1.0.4 and turned off with MySQL 5.5. It is available once again starting in MySQL 5.1.59
and 5.5.16 and higher, turned off by default. To enable this feature, set the configuration variable
innodb_random_read_ahead.
Linear read ahead is based on the access pattern of the pages in the buffer cache, not just
their number. In releases before 1.0.4, if most pages belonging to some extent are accessed
sequentially, InnoDB will issue an asynchronous prefetch request for the entire next extent
when it reads in the last page of the current extent. Beginning with InnoDB Plugin 1.0.4, users
can control when InnoDB performs a read ahead, by adjusting the number of sequential page
accesses required to trigger an asynchronous read request using the new configuration parameter
innodb_read_ahead_threshold.
If the number of pages read from an extent of 64 pages is greater or equal to
innodb_read_ahead_threshold, InnoDB will initiate an asynchronous read ahead of the entire
following extent. Thus, this parameter controls how sensitive InnoDB is to the pattern of page accesses
within an extent in deciding whether to read the following extent asynchronously. The higher the value,
the more strict will be the access pattern check. For example, if you set the value to 48, InnoDB will
trigger a linear read ahead request only when 48 pages in the current extent have been accessed
41
Multiple Background I/O Threads
sequentially. If the value is 8, InnoDB would trigger an asynchronous read ahead even if as few as 8
pages in the extent were accessed sequentially.
The new configuration parameter innodb_read_ahead_threshold may be set to any value from
0-64. The default value is 56, meaning that an asynchronous read ahead is performed only when 56
of the 64 pages in the extent are accessed sequentially. You can set the value of this parameter in the
MySQL option file (my.cnf or my.ini), or change it dynamically with the SET GLOBAL command, which
requires the SUPER privilege.
Starting with InnoDB Plugin 1.0.5 more statistics are provided through SHOW ENGINE INNODB
STATUS command to measure the effectiveness of the read ahead algorithm. See Section 8.9, “More
Read Ahead Statistics” for more information.
7.8 Multiple Background I/O Threads
InnoDB uses background threads to service various types of I/O requests. Starting from InnoDB
Plugin 1.0.4, the number of background threads tasked with servicing read and write I/O on data
pages is configurable. In previous versions of InnoDB, there was only one thread each for read and
write on non-Windows platforms. On Windows, the number of background threads was controlled by
innodb_file_io_threads. The configuration parameter innodb_file_io_threads has been
removed in InnoDB Plugin 1.0.4. If you try to set a value for this parameter, a warning will be written to
the log file and the value will be ignored.
In place of innodb_file_io_threads, two new configuration parameters are introduced in
the InnoDB Plugin 1.0.4, which are effective on all supported platforms. The two parameters
innodb_read_io_threads and innodb_write_io_threads signify the number of background
threads used for read and write requests respectively. You can set the value of these parameters in
the MySQL option file (my.cnf or my.ini). These parameters cannot be changed dynamically. The
default value for these parameters is 4 and the permissible values range from 1-64.
The purpose of this change is to make InnoDB more scalable on high end systems. Each background
thread can handle up to 256 pending I/O requests. A major source of background I/O is the read
ahead requests. InnoDB tries to balance the load of incoming requests in such way that most of the
background threads share work equally. InnoDB also attempts to allocate read requests from the same
extent to the same thread to increase the chances of coalescing the requests together. If you have
a high end I/O subsystem and you see more than 64 times innodb_read_io_threads pending
read requests in SHOW ENGINE INNODB STATUS, then you may gain by increasing the value of
innodb_read_io_threads.
7.9 Group Commit
InnoDB, like any other ACID compliant database engine, is required to flush the redo log of a
transaction before it is committed. Historically InnoDB used group commit functionality to group multiple
such flush requests together to avoid one flush for each commit. With group commit, InnoDB can issue
a single write to the log file to effectuate the commit action for multiple user transactions that commit at
about the same time, significantly improving throughput.
Group commit in InnoDB worked until MySQL 4.x. With the introduction of support for the distributed
transactions and Two Phase Commit (2PC) in MySQL 5.0, group commit functionality inside InnoDB
was broken.
Beginning with InnoDB Plugin 1.0.4, the group commit functionality inside InnoDB works with the Two
Phase Commit protocol in MySQL. Re-enabling of the group commit functionality fully ensures that the
ordering of commit in the MySQL binlog and the InnoDB logfile is the same as it was before. It means it
is totally safe to use InnoDB Hot Backup with InnoDB Plugin 1.0.4.
Group commit is transparent to the user and nothing needs to be done by the user to take advantage of
this significant performance improvement.
42
Controlling the Master Thread I/O Rate
7.10 Controlling the Master Thread I/O Rate
The master thread in InnoDB performs various tasks in the background. Most of these tasks are I/
O related like flushing of the dirty pages from the buffer cache or writing the buffered inserts to the
appropriate secondary indexes. The master thread attempts to perform these tasks in a way that
does not adversely affect the normal working of the server. It tries to estimate the free I/O bandwidth
available and tune its activities to take advantage of this free capacity. Historically, InnoDB has used
a hard coded value of 100 IOPs (input/output operations per second) as the total I/O capacity of the
server.
Beginning with InnoDB Plugin 1.0.4, a new configuration parameter is introduced to indicate the
overall I/O capacity available to InnoDB. The new parameter innodb_io_capacity should be set to
approximately the number of I/O operations that the system can perform per second. The value will of
course depend on your system configuration. When innodb_io_capacity is set, the master threads
estimates the I/O bandwidth available for background tasks based on the set value. Setting the value to
100 reverts to the old behavior.
You can set the value of innodb_io_capacity to any number 100 or greater, and the default value
is 200. Typically, values around the previous default of 100 are appropriate for consumer-level storage
devices, such as hard drives up to 7200 RPMs. Faster hard drives, RAID configurations, and SSDs
benefit from higher values. You can set the value of this parameter in the MySQL option file (my.cnf
or my.ini) or change it dynamically with the SET GLOBAL command, which requires the SUPER
privilege.
7.11 Controlling the Flushing Rate of Dirty Pages
InnoDB performs certain tasks in the background, including flushing of dirty pages (those pages
that have been changed but are not yet written to the database files) from the buffer cache, a task
performed by the “master thread”. Currently, the master thread aggressively flushes buffer pool pages
if the percentage of dirty pages in the buffer pool exceeds innodb_max_dirty_pages_pct.
This behavior can cause temporary reductions in throughput when excessive buffer pool flushing takes
place, limiting the I/O capacity available for ordinary read and write activity. Beginning with release
1.0.4, InnoDB Plugin uses a new algorithm to estimate the required rate of flushing based on the speed
of redo log generation and the current rate of flushing. The intent of this change is to smooth overall
performance, eliminating steep dips in throughput, by ensuring that buffer flush activity keeps up with
the need to keep the buffer pool “clean”.
Remember that InnoDB uses its log files in a circular fashion. To make a log file (or a portion of it)
reusable, InnoDB must flush to disk all dirty buffer pool pages whose redo entries are contained in that
portion of the log file. When required, InnoDB performs a so-called “sharp checkpoint” by flushing the
appropriate dirty pages to make space available in the log file. If a workload is write intensive, it will
generate a lot of redo information (writes to the log file). In this case, it is possible that available space
in the log files will be used up, even though innodb_max_dirty_pages_pct is not reached. This will
cause a sharp checkpoint, causing a temporary reduction in throughput.
Beginning with release 1.0.4, InnoDB Plugin uses a new heuristic-based algorithm to avoid such a
scenario. The heuristic is a function of the number of dirty pages in the buffer cache and the rate at
which redo is being generated. Based on this heuristic, the master thread will decide how many dirty
pages to flush from the buffer cache each second. This self adapting heuristic is able to deal with
sudden changes in the workload.
The primary aim of this feature is to smooth out I/O activity, avoiding sudden dips in throughput when
flushing activity becomes high. Internal benchmarking has also shown that this algorithm not only
maintains throughput over time, but can also improve overall throughput significantly.
Because adaptive flushing is a new feature that can significantly affect the I/O pattern of a workload,
the InnoDB Plugin introduces a new configuration parameter that can be used to disable this feature.
The default value of the new boolean parameter innodb_adaptive_flushing is TRUE, enabling the
43
Using the PAUSE instruction in InnoDB spin loops
new algorithm. You can set the value of this parameter in the MySQL option file (my.cnf or my.ini)
or change it dynamically with the SET GLOBAL command, which requires the SUPER privilege.
7.12 Using the PAUSE instruction in InnoDB spin loops
Synchronization inside InnoDB frequently involves the use of spin loops (where, while waiting, InnoDB
executes a tight loop of instructions repeatedly to avoid having the InnoDB process and threads be
rescheduled by the operating system). If the spin loops are executed too quickly, system resources
are wasted, imposing a relatively severe penalty on transaction throughput. Most modern processors
implement the PAUSE instruction for use in spin loops, so the processor can be more efficient.
Beginning with 1.0.4, the InnoDB Plugin uses a PAUSE instruction in its spin loops on all platforms
where such an instruction is available. This technique increases overall performance with CPU-bound
workloads, and has the added benefit of minimizing power consumption during the execution of the
spin loops.
Using the PAUSE instruction in InnoDB spin loops is transparent to the user. User does not have to do
anything to take advantage of this performance improvement.
7.13 Control of Spin Lock Polling
Many InnoDB mutexes and rw-locks are reserved for a short amount of time. On a multi-core system,
it is often more efficient for a thread to actively poll a mutex or rw-lock for a while before sleeping. If the
mutex or rw-lock becomes available during this polling period, the thread may continue immediately,
in the same time slice. Alas, if a shared object is being polled too frequently by multiple threads, it may
result in “cache ping-pong”, the shipping of cache lines between processors. InnoDB tries to avoid this
by making threads busy, waiting a random time between subsequent polls. The delay is implemented
as a busy loop.
Starting with InnoDB Plugin 1.0.4, it is possible to control the maximum delay between sampling a
mutex or rw-lock using the new parameter innodb_spin_wait_delay. In the 100 MHz Pentium
era, the unit of delay used to be one microsecond. The duration of the delay loop depends on the
C compiler and the target processor. On a system where all processor cores share a fast cache
memory, it might be useful to reduce the maximum delay or disable the busy loop altogether by setting
innodb_spin_wait_delay=0. On a system that consists of multiple processor chips, the shipping of
cache lines can be slower and it may be useful to increase the maximum delay.
The default value of innodb_spin_wait_delay is 6. The spin wait delay is a dynamic, global
parameter that can be specified in the MySQL option file (my.cnf or my.ini) or changed at runtime
with the command SET GLOBAL innodb_spin_wait_delay=delay, where delay is the desired
maximum delay. Changing the setting requires the SUPER privilege.
7.14 Making Buffer Cache Scan Resistant
Historically, InnoDB has inserted newly read blocks into the middle of the list representing the buffer
cache, to avoid pollution of the cache due to excessive read-ahead. The idea is that the read-ahead
algorithm should not pollute the buffer cache by forcing the frequently accessed (“hot”) pages out of the
LRU list. To achieve this, InnoDB internally maintains a pointer at 3/8 from the tail of the LRU list, and
all newly read pages are inserted at this location in the LRU list. The pages are moved to the front of
the list (the most-recently used end) when they are accessed from the buffer cache for the first time.
Thus pages that are never accessed never make it to the front 5/8 of the LRU list.
The above arrangement logically divides the LRU list into two segments where the 3/8 pages
downstream of the insertion point are considered “old” and are desirable victims for LRU eviction.
Starting with InnoDB Plugin 1.0.5, this mechanism has been extended in two ways.
You can control the insertion point in the LRU list. A new configuration parameter
innodb_old_blocks_pct now controls the percentage of “old” blocks in the LRU list. The default
44
Guidelines for innodb_old_blocks_pct and innodb_old_blocks_time
value of innodb_old_blocks_pct is 37, corresponding to the original fixed ratio of 3/8. The
permissible value range is 5 to 95.
The optimization that keeps the buffer cache from being churned too much by read-ahead, is extended
to avoid similar problems resulting from table or index scans. During an index scan, a data page
is typically accessed a few times in quick succession and is then never touched again. InnoDB
Plugin 1.0.5 introduces a new configuration parameter innodb_old_blocks_time which specifies
the time window (in milliseconds) after the first access to a page during which it can be accessed
without being moved to the front (most-recently used end) of the LRU list. The default value of
innodb_old_blocks_time is 0, corresponding to the original behavior of moving a page to the MRU
end of the LRU list on first access in the buffer pool.
Both the new parameters innodb_old_blocks_pct and innodb_old_blocks_time are dynamic,
global and can be specified in the MySQL option file (my.cnf or my.ini) or changed at runtime with
the SET GLOBAL command. Changing the setting requires the SUPER privilege.
To help you gauge the effect of setting these parameters, some additional statistics are reported by
SHOW ENGINE INNODB STATUS command. The BUFFER POOL AND MEMORY section now looks like:
Total memory allocated 1107296256; in additional pool allocated 0
Dictionary memory allocated 80360
Buffer pool size
65535
Free buffers
0
Database pages
63920
Old database pages 23600
Modified db pages 34969
Pending reads 32
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 414946, not young 2930673
1274.75 youngs/s, 16521.90 non-youngs/s
Pages read 486005, created 3178, written 160585
2132.37 reads/s, 3.40 creates/s, 323.74 writes/s
Buffer pool hit rate 950 / 1000, young-making rate 30 / 1000 not 392 / 1000
Pages read ahead 1510.10/s, evicted without access 0.00/s
LRU len: 63920, unzip_LRU len: 0
I/O sum[43690]:cur[221], unzip sum[0]:cur[0]
• Old database pages is the number of pages in the “old” segment of the LRU list.
• Pages made young and not young is the total number of “old” pages that have been made
young or not respectively.
• youngs/s and non-young/s is the rate at which page accesses to the “old” pages have resulted in
making such pages young or otherwise respectively since the last invocation of the command.
• young-making rate and not provides the same rate but in terms of overall buffer cache
accesses instead of accesses just to the “old” pages.
Note
Per second averages provided in InnoDB Monitor output are based on the
elapsed time between the current time and the last time InnoDB Monitor output
was printed.
7.14.1 Guidelines for innodb_old_blocks_pct and
innodb_old_blocks_time
The default values of both parameters leave the original behavior as of InnoDB Plugin 1.0.4 intact. To
take advantage of this feature, you must set different values.
Because the effects of these parameters can vary widely based on your hardware configuration, your
data, and the details of your workload, always benchmark to verify the effectiveness before changing
these settings in any performance-critical or production environment.
45
Improvements to Crash Recovery Performance
In mixed workloads where most of the activity is OLTP type with periodic batch reporting queries which
result in large scans, setting the value of innodb_old_blocks_time during the batch runs can help
keep the working set of the normal workload in the buffer cache.
When scanning large tables that cannot fit entirely in the buffer pool, setting
innodb_old_blocks_pct to a small value keeps the data that is only read once from consuming a
significant portion of the buffer pool. For example, setting innodb_old_blocks_pct=5 restricts this
data that is only read once to 5% of the buffer pool.
When scanning small tables that do fit into memory, there is less overhead for moving pages around
within the buffer pool, so you can leave innodb_old_blocks_pct at its default value, or even
higher, such as innodb_old_blocks_pct=50.
The effect of the innodb_old_blocks_time parameter is harder to predict than the
innodb_old_blocks_pct parameter, is relatively small, and varies more with the workload. To
arrive at an optimal value, conduct your own benchmarks if the performance improvement from
adjusting innodb_old_blocks_pct is not sufficient.
7.15 Improvements to Crash Recovery Performance
Starting with InnoDB Plugin 1.0.7, a number of optimizations speed up certain steps of the recovery
that happens on the next startup after a crash. In particular, scanning the redo log and applying
the redo log are faster. You do not need to take any actions to take advantage of this performance
enhancement. If you kept the size of your logfiles artificially low because recovery took a long time, you
can consider increasing the logfile size.
46
Chapter 8 Changes for Flexibility, Ease of Use and Reliability
Table of Contents
8.1 Overview ............................................................................................................................... 47
8.2 Enabling New File Formats .................................................................................................... 47
8.3 Dynamic Control of System Configuration Parameters ............................................................ 48
8.3.1 Dynamically Changing innodb_file_per_table ..................................................... 48
8.3.2 Dynamically Changing innodb_stats_on_metadata ............................................... 48
8.3.3 Dynamically Changing innodb_lock_wait_timeout ............................................... 49
8.3.4 Dynamically Changing innodb_adaptive_hash_index ............................................ 49
8.4 TRUNCATE TABLE Reclaims Space ...................................................................................... 49
8.5 InnoDB Strict Mode ............................................................................................................... 49
8.6 Controlling Optimizer Statistics Estimation .............................................................................. 50
8.7 Better Error Handling when Dropping Indexes ........................................................................ 51
8.8 More Compact Output of SHOW ENGINE INNODB MUTEX ..................................................... 52
8.9 More Read Ahead Statistics .................................................................................................. 52
8.1 Overview
This chapter describes several changes in the InnoDB Plugin that offer new flexibility and improve ease
of use, reliability and performance:
• Section 8.2, “Enabling New File Formats”
• Section 8.3, “Dynamic Control of System Configuration Parameters”
• Section 8.4, “TRUNCATE TABLE Reclaims Space”
• Section 8.5, “InnoDB Strict Mode”
• Section 8.6, “Controlling Optimizer Statistics Estimation”
• Section 8.7, “Better Error Handling when Dropping Indexes”
• Section 8.8, “More Compact Output of SHOW ENGINE INNODB MUTEX”
• Section 8.9, “More Read Ahead Statistics”
8.2 Enabling New File Formats
The InnoDB Plugin introduces named file formats to improve compatibility between database file
formats and various InnoDB versions.
To create new tables that require a new file format, you must enable the new “Barracuda” file format,
using the configuration parameter innodb_file_format. The value of this parameter will determine
whether it will be possible to create a table or index using compression or the new DYNAMIC row
format. If you omit innodb_file_format or set it to “Antelope”, you preclude the use of new features
that would make your database inaccessible to the built-in InnoDB in MySQL 5.1 and prior releases.
You can set the value of innodb_file_format on the command line when you start mysqld, or
in the option file my.cnf (Unix operating systems) or my.ini (Windows). You can also change it
dynamically with the SET GLOBAL command.
Further information about managing file formats is presented in Chapter 4, InnoDB File-Format
Management.
47
Dynamic Control of System Configuration Parameters
8.3 Dynamic Control of System Configuration Parameters
With the InnoDB Plugin it now is possible to change certain system configuration parameters
dynamically, without shutting down and restarting the server as was previously necessary. This
increases up-time and facilitates testing of various options. You can now set these parameters
dynamically:
• innodb_file_per_table
• innodb_stats_on_metadata
• innodb_lock_wait_timeout
• innodb_adaptive_hash_index
8.3.1 Dynamically Changing innodb_file_per_table
Since MySQL version 4.1, InnoDB has provided two options for how tables are stored on disk. You can
choose to create a new table and its indexes in the shared system tablespace (corresponding to the set
of files named ibdata files), along with other internal InnoDB system information. Or, you can choose
to use a separate file (an .ibd file) to store a new table and its indexes.
The tablespace style used for new tables is determined by the setting of the configuration parameter
innodb_file_per_table at the time a table is created. Previously, the only way to set this
parameter was in the MySQL option file (my.cnf or my.ini), and changing it required shutting
down and restarting the server. Beginning with the InnoDB Plugin, the configuration parameter
innodb_file_per_table is dynamic, and can be set ON or OFF using the SET GLOBAL command.
The default setting is OFF, so new tables and indexes are created in the system tablespace.
Dynamically changing the value of this parameter requires the SUPER privilege and immediately affects
the operation of all connections.
Tables created when innodb_file_per_table is disabled cannot use the new compression
capability, or use the new row format DYNAMIC. Tables created when innodb_file_per_table is
enabled can use those new features, and each table and its indexes will be stored in a new .ibd file.
The ability to change the setting of innodb_file_per_table dynamically is useful for testing. As
noted above, the parameter innodb_file_format is also dynamic, and must be set to “Barracuda”
to create new compressed tables, or tables that use the new row format DYNAMIC. Since both
parameters are dynamic, it is easy to experiment with these table formats and the downgrade
procedure described in Chapter 11, Downgrading from the InnoDB Plugin without a system shutdown
and restart.
Note that the InnoDB Plugin can add and drop a table's secondary indexes without re-creating the
table, but must recreate the table when you change the clustered (primary key) index (see Chapter 2,
Fast Index Creation in the InnoDB Storage Engine). When a table is recreated as a result of creating
or dropping an index, the table and its indexes will be stored in the shared system tablespace or in its
own .ibd file just as if it were created using a CREATE TABLE command (and depending on the setting
of innodb_file_per_table). When an index is created without rebuilding the table, the index is
stored in the same file as the clustered index, regardless of the setting of innodb_file_per_table.
8.3.2 Dynamically Changing innodb_stats_on_metadata
As noted in Section 8.6, “Controlling Optimizer Statistics Estimation”, the InnoDB Plugin allows you to
control the way in which InnoDB gathers information about the number of distinct values in an index
key. A related parameter, innodb_stats_on_metadata, has existed since MySQL release 5.1.17 to
control whether or not InnoDB performs statistics gathering when metadata statements are executed.
See the MySQL manual on InnoDB Startup Options and System Variables for details.
Beginning with release 1.0.2 of the InnoDB Plugin, it is possible to change the setting of
innodb_stats_on_metadata dynamically at runtime with the command SET GLOBAL
48
Dynamically Changing innodb_lock_wait_timeout
innodb_stats_on_metadata=mode, where mode is either ON or OFF (or 1 or 0). Changing this
setting requires the SUPER privilege and immediately affects the operation of all connections.
8.3.3 Dynamically Changing innodb_lock_wait_timeout
When a transaction is waiting for a resource, it will wait for the resource to become free, or stop waiting
and return with the error
ERROR HY000: Lock wait timeout exceeded; try restarting transaction
The length of time a transaction will wait for a resource before “giving up” is determined by the value
of the configuration parameter innodb_lock_wait_timeout. The default setting for this parameter
is 50 seconds. The minimum setting is 1 second, and values above 100,000,000 disable the timeout,
so a transaction will wait “forever”. Following a timeout, the SQL statement that was executing will be
rolled back. (In MySQL 5.0.12 and earlier, the transaction rolled back.) The user application may try the
statement again (usually after waiting for a while), or rollback the entire transaction and restart.
Before InnoDB Plugin 1.0.2, the only way to set this parameter was in the MySQL option file (my.cnf
or my.ini), and changing it required shutting down and restarting the server. Beginning with the
InnoDB Plugin 1.0.2, the configuration parameter innodb_lock_wait_timeout can be set at
runtime with the SET GLOBAL or SET SESSION commands. Changing the GLOBAL setting requires
the SUPER privilege and affects the operation of all clients that subsequently connect. Any client can
change the SESSION setting for innodb_lock_wait_timeout, which affects only that client.
8.3.4 Dynamically Changing innodb_adaptive_hash_index
As described in Section 7.5, “Controlling Adaptive Hash Indexing”, it may be desirable, depending on
your workload, to dynamically enable or disable the adaptive hash indexing scheme InnoDB uses to
improve query performance.
Version 5.1.24 of MySQL introduced the start-up option innodb_adaptive_hash_index that allows
the adaptive hash index to be disabled. It is enabled by default. Starting with InnoDB Plugin 1.0.3, the
parameter can be modified by the SET GLOBAL command, without restarting the server. Changing the
setting requires the SUPER privilege.
Disabling the adaptive hash index will empty the hash table immediately. Normal operations can
continue while the hash table is emptied, and executing queries that have been using the hash table
will access the index B-trees directly instead of attempting to utilize the hash index. When the adaptive
hash index is enabled, the hash table will be populated during normal operation.
8.4 TRUNCATE TABLE Reclaims Space
Starting with the InnoDB Plugin, when the user requests to TRUNCATE a table that is stored in an .ibd
file of its own (because innodb_file_per_table was enabled when the table was created), and
if the table is not referenced in a FOREIGN KEY constraint, the InnoDB Plugin will drop and re-create
the table in a new .ibd file. This operation is much faster than deleting the rows one by one, and will
return disk space to the operating system and reduce the size of page-level backups.
Previous versions of InnoDB would re-use the existing .ibd file, thus releasing the space only
to InnoDB for storage management, but not to the operating system. Note that when the table is
truncated, the count of rows affected by the TRUNCATE command is an arbitrary number.
Note: if there are referential constraints between the table being truncated and other tables, MySQL
instead automatically converts the TRUNCATE command to a DELETE command that operates row-byrow, so that ON DELETE operations can occur on “child” tables.
8.5 InnoDB Strict Mode
To guard against ignored typos and syntax errors in SQL, or other unintended consequences of various
combinations of operational modes and SQL commands, the InnoDB Plugin provides a “strict mode” of
49
Controlling Optimizer Statistics Estimation
operations. In this mode, InnoDB will raise error conditions in certain cases, rather than issue a warning
and process the specified command (perhaps with some unintended defaults). This is analogous to
MySQL's sql_mode, which controls what SQL syntax MySQL will accept, and determines whether
it will silently ignore errors, or validate input syntax and data values. Note that there is no strict mode
with the built-in InnoDB, so some commands that execute without errors with the built-in InnoDB will
generate errors with the InnoDB Plugin, unless you disable strict mode.
In the InnoDB Plugin, the setting of InnoDB strict mode affects the handling of syntax errors on the
CREATE TABLE, ALTER TABLE and CREATE INDEX commands. Starting with InnoDB Plugin version
1.0.2, the strict mode also enables a record size check, so that an INSERT or UPDATE will never fail
due to the record being too large for the selected page size.
Using the new clauses and settings for ROW_FORMAT and KEY_BLOCK_SIZE on CREATE TABLE
and ALTER TABLE commands and the CREATE INDEX can be confusing when not running in strict
mode. Unless you run in strict mode, InnoDB will ignore certain syntax errors and will create the table
or index, with only a warning in the message log. However if InnoDB strict mode is on, such errors will
generate an immediate error and the table or index will not be created, thus saving time by catching the
error at the time the command is issued.
The default for strict mode is off, but in the future, the default may be changed. It is best to start using
strict mode with the InnoDB Plugin, and make sure your SQL scripts use commands that do not
generate warnings or unintended effects.
InnoDB strict mode is set with the configuration parameter innodb_strict_mode, which can
be specified as on or off. You can set the value on the command line when you start mysqld,
or in the configuration file my.cnf (Unix operating systems) or my.ini (Windows). You can also
enable or disable InnoDB strict mode at runtime with the command SET [GLOBAL|SESSION]
innodb_strict_mode=mode, where mode is either ON or OFF. Changing the GLOBAL setting
requires the SUPER privilege and affects the operation of all clients that subsequently connect. Any
client can change the SESSION setting for innodb_strict_mode, which affects only that client.
8.6 Controlling Optimizer Statistics Estimation
The MySQL query optimizer uses estimated statistics about key distributions to select or avoid using an
index in an execution plan, based on the relative selectivity of the index. Previously, InnoDB sampled 8
random pages from an index to get an estimate of the cardinality of (i.e., the number of distinct values
in) the index. (This page sampling technique is frequently described as “index dives”.) This small
number of page samples frequently was insufficient, and could give inaccurate estimates of an index's
selectivity and thus lead to poor choices by the query optimizer.
To give users control over the quality of the statistics estimate (and thus better information for
the query optimizer), the number of sampled pages now can be changed using the parameter
innodb_stats_sample_pages.
This feature addresses user requests such as that as expressed in MySQL Bug #25640: InnoDB
Analyze Table Should Allow User Selection of Index Dives.
You can change the number of sampled pages using the global parameter
innodb_stats_sample_pages, which can be set at runtime (i.e., it is a dynamic parameter). The
default value for this parameter is 8, preserving the same behavior as in past releases.
Note that the value of innodb_stats_sample_pages affects the index sampling for all tables and
indexes. You should also be aware that there are the following potentially significant impacts when you
change the index sample size:
• small values like 1 or 2 can result in very inaccurate estimates of cardinality
• values much larger than 8 (say, 100), can cause a big slowdown in the time it takes to open a table
or execute SHOW TABLE STATUS.
50
Better Error Handling when Dropping Indexes
• the optimizer may choose very different query plans based on different estimates of index selectivity
Note that the cardinality estimation can be disabled for metadata commands such as SHOW TABLE
STATUS by executing the command SET GLOBAL innodb_stats_on_metadata=OFF (or 0). Before
InnoDB Plugin 1.0.2, this variable could only be set in the MySQL option file (my.cnf or my.ini), and
changing it required shutting down and restarting the server.
The cardinality (the number of different key values) in every index of a table is calculated when a table
is opened, at SHOW TABLE STATUS and ANALYZE TABLE and on other circumstances (like when
the table has changed too much). Note that all tables are opened, and the statistics are re-estimated,
when the mysql client starts if the auto-rehash setting is set on (the default). The auto-rehash feature
enables automatic name completion of database, table, and column names for interactive users. You
may prefer setting auto-rehash off to improve the start up time of the mysql client.
You should note that it does not make sense to increase the index sample size, then run ANALYZE
TABLE and decrease sample size to attempt to obtain better statistics. This is because the statistics
are not persistent. They are automatically recalculated at various times other than on execution of
ANALYZE TABLE. Sooner or later the “better” statistics calculated by ANALYZE running with a high
value of innodb_stats_sample_pages will be wiped away.
The estimated cardinality for an index will be more accurate with a larger number of samples, but each
sample might require a disk read, so you do not want to make the sample size too large. You should
choose a value for innodb_stats_sample_pages that results in reasonably accurate estimates for
all tables in your database without requiring excessive I/O.
Although it is not possible to specify the sample size on a per-table basis, smaller tables generally
would require fewer index samples than larger tables require. If your database has many large tables,
you may want to consider using a higher value for innodb_stats_sample_pages than if you have
mostly smaller tables.
8.7 Better Error Handling when Dropping Indexes
For efficiency, InnoDB requires an index to exist on foreign key columns so that UPDATE and DELETE
operations on a “parent” table can easily check for the existence or non-existence of corresponding
rows in the “child” table. To ensure that there is an appropriate index for such checks, MySQL will
sometimes implicitly create or drop such indexes as a side-effect of CREATE TABLE, CREATE INDEX,
and ALTER TABLE statements.
When you explicitly DROP an index, InnoDB will check that an index suitable for referential integrity
checking will still exist following the DROP of the index. InnoDB will prevent you from dropping the last
usable index for enforcing any given referential constraint. Users have been confused by this behavior,
as reported in MySQL Bug #21395.
In releases prior to InnoDB Plugin 1.0.2, attempts to drop the only usable index would result in an error
message such as
ERROR 1025 (HY000): Error on rename of './db2/#sql-18eb_3'
to './db2/foo'(errno: 150)
Beginning with InnoDB Plugin 1.0.2, this error condition is reported with a more friendly message:
ERROR 1553 (HY000): Cannot drop index 'fooIdx':
needed in a foreign key constraint
As a related matter, because all user data in InnoDB is maintained in the so-called “clustered index” (or
primary key index), InnoDB ensures that there is such an index for every table, even if the user does
not declare an explicit PRIMARY KEY. In such cases, InnoDB will create an implicit clustered index
using the first columns of the table that have been declared UNIQUE and NOT NULL.
When the InnoDB Plugin is used with a MySQL version earlier than 5.1.29, an attempt to drop an
implicit clustered index (the first UNIQUE NOT NULL index) will fail if the table does not contain a
51
More Compact Output of SHOW ENGINE INNODB MUTEX
PRIMARY KEY. This has been reported as MySQL Bug #31233. Attempts to use the DROP INDEX or
ALTER TABLE command to drop such an index will generate this error:
ERROR 42000: This table type requires a primary key
Beginning with MySQL 5.1.29 when using the InnoDB Plugin, attempts to drop such an index will
copy the table, rebuilding the index using a different UNIQUE NOT NULL group of columns or a
system-generated key. Note that all indexes will be re-created by copying the table, as described in
Section 2.3, “Implementation”.
In those versions of MySQL that are affected by this bug, one way to change an index of this type is
to create a new table and copy the data into it using INSERT INTO newtable SELECT * FROM
oldtable, and then DROP the old table and rename the new table.
However, if there are existing tables with references to the table whose index you are dropping, you
will first need to use the ALTER TABLE command to remove foreign key references from or to other
tables. Unfortunately, MySQL does not support dropping or creating FOREIGN KEY constraints, even
though dropping a constraint would be trivial. Therefore, if you use ALTER TABLE to add or remove a
REFERENCES constraint, the child table will be copied, rather than using “Fast Index Creation”.
8.8 More Compact Output of SHOW ENGINE INNODB MUTEX
The command SHOW ENGINE INNODB MUTEX displays information about InnoDB mutexes and rwlocks. It can be a useful tuning aid on multi-core systems. However, with a big buffer pool, the size
of the output may be overwhelming. There is a mutex and rw-lock in each 16K buffer pool block. It is
highly improbable that an individual block mutex or rw-lock could become a performance bottleneck,
and there are 65,536 blocks per gigabyte.
Starting with InnoDB Plugin 1.0.4, SHOW ENGINE INNODB MUTEX will skip the mutexes and rw-locks
of buffer pool blocks. Furthermore, it will not list any mutexes or rw-locks that have never been waited
on (os_waits=0). Therefore, SHOW ENGINE INNODB MUTEX only displays information about such
mutexes and rw-locks that does not belong to the buffer pool blocks and for whom there have been at
least one OS level wait.
8.9 More Read Ahead Statistics
As described in Section 7.7, “Changes in the Read Ahead Algorithm” a read ahead request is an
asynchronous IO request issued in anticipation that the page being read in will be used in near future.
It can be very useful if a DBA has the information about how many pages are read in as part of read
ahead and how many of them are evicted from the buffer pool without ever being accessed. Based on
this information a DBA can then fine tune the degree of aggressiveness of the read ahead using the
parameter innodb_read_ahead_threshold.
Starting from InnoDB Plugin 1.0.5 two new status variables are added to the SHOW
STATUS output. These global status variables Innodb_buffer_pool_read_ahead and
Innodb_buffer_pool_read_ahead_evicted indicate the number of pages read in as part of
read ahead and the number of such pages evicted without ever being accessed respectively. These
counters provide global values since the start of the server. Please also note that the status variables
Innodb_buffer_pool_read_ahead_rnd and Innodb_buffer_pool_read_ahead_seq have
been removed from the SHOW STATUS output.
In addition to the two counters mentioned above SHOW INNODB STATUS will also show the rate at
which the read ahead pages are being read in and the rate at which such pages are being evicted
without being accessed. The per second averages are based on the statistics collected since the last
invocation of SHOW INNODB STATUS and are displayed in the BUFFER POOL AND MEMORY section of
the output.
52
Chapter 9 Installing the InnoDB Plugin
Table of Contents
9.1 Overview of Installing the InnoDB Plugin ................................................................................ 53
9.2 Checking for Compatible Version Levels ................................................................................ 54
9.3 Installing the Precompiled InnoDB Plugin as a Shared Library ................................................. 54
9.3.1 Installing the InnoDB Plugin as a Shared Library on Unix or Linux ................................. 55
9.3.2 Installing the Binary InnoDB Plugin as a Shared Library on Microsoft Windows ............... 58
9.3.3 Errors When Installing the InnoDB Plugin on Microsoft Windows ................................... 60
9.4 Building the InnoDB Plugin from Source Code ........................................................................ 61
9.4.1 Building the InnoDB Plugin on Linux or Unix ................................................................ 61
9.4.2 Building the InnoDB Plugin on Microsoft Windows ........................................................ 62
9.5 Configuring the InnoDB Plugin ............................................................................................... 64
9.6 Frequently Asked Questions about Plugin Installation ............................................................. 64
9.6.1 Should I use the InnoDB-supplied plugin or the one that is included with MySQL 5.1.38
or higher? ........................................................................................................................... 64
9.6.2 Why doesn't the MySQL service on Windows start after the replacement? ..................... 64
9.6.3 The Plugin is installed... now what? ............................................................................ 65
9.6.4 Once the Plugin is installed, is it permanent? ............................................................... 65
9.1 Overview of Installing the InnoDB Plugin
You can acquire the plugin in these formats:
• As a platform-specific executable binary file that is dynamically linked or “plugged in” to the MySQL
server.
• In source code form, available under the GNU General Public License (GPL), version 2.
Notes:
While it is possible to use the source code to build a special version of MySQL
containing the InnoDB Plugin, we recommend you install the binary shared
library for the InnoDB Plugin instead, without building from source. Replacing
the shared library is simpler and much less error prone than building from
source.
The InnoDB Plugin is included in the MySQL distribution, starting from MySQL
5.1.38. From MySQL 5.1.46 and up, this is the only download location for the
InnoDB Plugin; it is not available from the InnoDB web site. If you used any
scripts or configuration files with the earlier InnoDB Plugin from the InnoDB web
site, be aware that the filename of the shared library as supplied by MySQL
is ha_innodb_plugin.so or ha_innodb_plugin.dll, as opposed to
ha_innodb.so or ha_innodb.dll in the older Plugin downloaded from the
InnoDB web site. You might need to change the applicable file names in your
startup or configuration scripts.
This discussion pertains to using the InnoDB Plugin with the MySQL Community
Edition, whether source or binary. Except for download locations for MySQL
software, the procedures documented here should work without change when
you use MySQL Enterprise.
Whether you dynamically install the binary InnoDB Plugin or build from source,
configure MySQL by editing the configuration file to use InnoDB as the default
engine (if desired) and set appropriate configuration parameters to enable use
53
Checking for Compatible Version Levels
of new InnoDB Plugin features, as described in Section 9.5, “Configuring the
InnoDB Plugin”.
At this time, the InnoDB Plugin has not been compiled or tested with the Intel C
Compiler (icc), so you should use a version of MySQL compiled with the GNU
Compiler Collection (gcc).
9.2 Checking for Compatible Version Levels
Use the following table to confirm that the version of the InnoDB Plugin (whether source or binary)
is compatible with your platform, hardware type (including 32-bit vs 64-bit) and with your version of
MySQL. In general, a specific release of the InnoDB Plugin is designed to be compatible with a range
of MySQL versions, but this may vary, so check the information on the download page.
When building MySQL from source, you can generally use the source for the InnoDB Plugin in place of
the source for the built-in InnoDB. However, due to limitations of MySQL, a given binary version of the
InnoDB Plugin is compatible only with a specific version of MySQL, as follows.
Table 9.1 InnoDB Plugin Compatibility
InnoDB Plugin Release
MySQL Release
(Binary Compatibility)
MySQL Release
(Source Compatibility)
1.0.0
5.1.23
5.1.23 or newer
1.0.1
5.1.24
5.1.24 or newer
1.0.2
5.1.30
5.1.24 or newer
1.0.3
5.1.30 (not 5.1.31)
5.1.24 or newer
1.0.4
5.1.37
5.1.24 or newer
1.0.5
5.1.41
5.1.24 or newer
1.0.6
5.1.41
5.1.24 or newer
1.0.7 and higher
Incorporated into the applicable
5.1.x MySQL release; not
separately downloadable.
N/A
Note
MySQL Bug #42610 prevents using the binary InnoDB Plugin with MySQL
5.1.31 or 5.1.32. There is no binary InnoDB Plugin for MySQL 5.1.33. The only
way to use InnoDB Plugin with MySQL 5.1.31 through 5.1.35 is by building from
source. This issue was resolved in MySQL 5.1.37 and InnoDB Plugin 1.0.4.
9.3 Installing the Precompiled InnoDB Plugin as a Shared Library
The simplest way to install the InnoDB Plugin is to use a precompiled (binary) shared library file,
when one is available. The procedures are similar for installing the InnoDB Plugin using the binary
on different hardware and operating systems platforms, but the specific details differ between Unix or
Linux and Microsoft Windows. See below for notes specific to your platform.
Note that due to MySQL Bug #42610, the procedure of installing the binary InnoDB Plugin changed in
MySQL 5.1.33. If your version of MySQL is older than 5.1.33, refer to Appendix B, Using the InnoDB
Plugin with MySQL 5.1.30 or Earlier.
The steps for installing the InnoDB Plugin as a shared library are as follows:
• Download, extract and install the suitable MySQL executable for your platform.
• Make sure the MySQL server is not running. If you have to shut down the database server, you use a
special “slow” shutdown procedure, described later.
54
Installing the InnoDB Plugin as a Shared Library on Unix or Linux
• On database startup, make MySQL ignore the builtin InnoDB, and load the InnoDB Plugin and all
new InnoDB Information Schema tables implemented in the InnoDB Plugin, using one of these
alternatives:
• Edit the option file (my.cnf, or my.ini) to contain the necessary options.
• Specify equivalent options on the MySQL command line.
• Edit the option file to disable InnoDB, then use INSTALL statements on the MySQL command line
after startup.
These procedures are described in detail in the following sections.
• Set appropriate configuration parameters to enable new InnoDB Plugin features.
• Start MySQL, and verify the installation of the plugins.
The following sections detail these steps for Unix or Linux systems, and for Microsoft Windows.
9.3.1 Installing the InnoDB Plugin as a Shared Library on Unix or Linux
For Unix and Linux systems, use the following procedure to install the InnoDB Plugin as a shared
library:
1. Download, extract and install the suitable MySQL executable for your server platform and operating
system from the MySQL download section for MySQL Database Server 5.1. Be sure to use a 32-bit
or 64-bit version as appropriate for your hardware and operating system.
2. Make sure the MySQL server is not running. If the server is running, do a “slow” shutdown by
issuing the following command before performing the shutdown:
SET GLOBAL innodb_fast_shutdown=0;
Then finish the shutdown process, as described in The Shutdown Process in the MySQL
documentation. This option setting performs a full purge and an insert buffer merge before the
shutdown, which can typically take minutes, or even hours for very large and busy databases.
3. The InnoDB Plugin shared library is already installed in the directory lib/plugin as part of the
MySQL installation.
4. Edit the option file (my.cnf) to ignore the builtin InnoDB, and load the InnoDB Plugin and all
Information Schema tables implemented in the InnoDB Plugin when the server starts:
ignore_builtin_innodb
plugin-load=innodb=ha_innodb_plugin.so;innodb_trx=ha_innodb_plugin.so;
innodb_locks=ha_innodb_plugin.so;innodb_lock_waits=ha_innodb_plugin.so;
innodb_cmp=ha_innodb_plugin.so;innodb_cmp_reset=ha_innodb_plugin.so;
innodb_cmpmem=ha_innodb_plugin.so;innodb_cmpmem_reset=ha_innodb_plugin.so
Note that all plugins for plugin-load should be on the same line in the option file.
Alternatively, you can use the equivalent options on the MySQL command line:
mysqld --ignore-builtin-innodb --plugin-load=innodb=ha_innodb_plugin.so;
innodb_trx=ha_innodb_plugin.so;innodb_locks=ha_innodb_plugin.so;
innodb_lock_waits=ha_innodb_plugin.so;innodb_cmp=ha_innodb_plugin.so;
innodb_cmp_reset=ha_innodb_plugin.so;innodb_cmpmem=ha_innodb_plugin.so;
innodb_cmpmem_reset=ha_innodb_plugin.so
You can also install the InnoDB Plugin and the new InnoDB Information Schema tables
implemented in ha_innodb_plugin.so with INSTALL commands:
INSTALL PLUGIN INNODB SONAME 'ha_innodb_plugin.so';
55
Installing the InnoDB Plugin as a Shared Library on Unix or Linux
INSTALL
INSTALL
INSTALL
INSTALL
INSTALL
INSTALL
INSTALL
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
INNODB_TRX SONAME 'ha_innodb_plugin.so';
INNODB_LOCKS SONAME 'ha_innodb_plugin.so';
INNODB_LOCK_WAITS SONAME 'ha_innodb_plugin.so';
INNODB_CMP SONAME 'ha_innodb_plugin.so';
INNODB_CMP_RESET SONAME 'ha_innodb_plugin.so';
INNODB_CMPMEM SONAME 'ha_innodb_plugin.so';
INNODB_CMPMEM_RESET SONAME 'ha_innodb_plugin.so';
If you use INSTALL PLUGIN statement to install the InnoDB Plugin and the Information Schema
tables, ensure the following conditions are set up:
• In the mysqld command line or my.cnf option file, prepend each InnoDB option with
loose_, so that MySQL will start even when InnoDB is unavailable. For example, write
loose_innodb_file_per_table instead of innodb_file_per_table.
• Start the MySQL server while it is configured to skip loading the built-in InnoDB and to make
MyISAM the default storage engine. This can be done by editing the option file my.cnf to contain
these two lines:
ignore_builtin_innodb
default_storage_engine=MyISAM
Or, you can use the equivalent options on the MySQL command line:
mysqld --ignore-builtin-innodb --default-storage-engine=MyISAM …
See the MySQL Manual section on INSTALL PLUGIN Syntax for information on how these
commands work.
5. Edit the option file my.cnf to use InnoDB as the default engine (if desired) and set appropriate
configuration parameters to enable use of new InnoDB Plugin features, as described in Section 9.5,
“Configuring the InnoDB Plugin”. In particular, we recommend that you set the following specific
parameters as follows:
default-storage-engine=InnoDB
innodb_file_per_table=1
innodb_file_format=barracuda
innodb_strict_mode=1
IMPORTANT:
The MySQL server always must be started with the option
ignore_builtin_innodb, as long as you want to use the InnoDB
Plugin as a shared library. Also, remember that the startup option
skip_grant_tables prevents MySQL from loading any plugins.
6. Verify the installation of the plugins with the MySQL command SHOW PLUGINS, which should
produce the following output:
Name
Status
binlog
Type
Library
License
ACTIVE STORAGE ENGINE
NULL
GPL
CSV
ACTIVE STORAGE ENGINE
NULL
GPL
MEMORY
ACTIVE STORAGE ENGINE
NULL
GPL
InnoDB
ACTIVE STORAGE ENGINE
ha_innodb_plugin.so
GPL
INNODB_TRX
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.so
GPL
INNODB_LOCKS
ACTIVE INFORMATION
SCHEMA
56
ha_innodb_plugin.so
GPL
Installing the InnoDB Plugin as a Shared Library on Unix or Linux
Name
Status
INNODB_LOCK_WAITS
Type
Library
License
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.so
GPL
INNODB_CMP
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.so
GPL
INNODB_CMP_RESET
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.so
GPL
INNODB_CMPMEM
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.so
GPL
INNODB_CMPMEM_RESETACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.so
GPL
MRG_MYISAM
ACTIVE STORAGE ENGINE
NULL
GPL
MyISAM
ACTIVE STORAGE ENGINE
NULL
GPL
If the plugins fail to load properly, see Section 9.3.1.1, “Errors When Installing the InnoDB Plugin on
Unix or Linux” for possible causes and corrections.
After verifying that the Plugin is recognized by MySQL, create an InnoDB table as another
confirmation of success.
9.3.1.1 Errors When Installing the InnoDB Plugin on Unix or Linux
If MySQL or its associated daemon process cannot start, or a post-startup INSTALL PLUGIN
statement fails, look at the MySQL error log (usually named machine_name.err and located in the
MySQL data directory) for the detailed error message. The log is in chronological order, so look at the
end of the file. Try to resolve the problem based on other information in the message.
The following table outlines installation-related InnoDB Plugin error conditions or messages and
possible solutions.
Error Condition or Message
Possible Solution
Diagnose the cause from the following message
details.
Can't open shared library
library_name
API version for STORAGE ENGINE plugin The version of the Plugin is not compatible with
is too different
the version of the MySQL server. Consult the
compatibility chart.
No such file or directory
Check that the file ha_innodb_plugin.so
or .dll was copied to the correct location.
Confirm that you specified the right file
name (ha_innodb_plugin.so or .dll
for the library from the InnoDB web site;
ha_innodb_plugin.so or .dll for the library
supplied along with MySQL 5.1.38 and up.)
Permission denied
Check that the directory and file access
permissions are set properly, or change them
using chmod on Unix-like systems . The mysqld
process must have permission to read (r) the file
ha_innodb_plugin.so and to access files (x)
in the plugin directory.
wrong ELF class or any other message
Ensure that ha_innodb_plugin.so is for the
same system platform as mysqld. In particular,
note that a 32-bit mysqld is unable to load a 64bit plugin, and vice versa. Be sure to download
57
Installing the Binary InnoDB Plugin as a Shared Library on Microsoft Windows
Error Condition or Message
Possible Solution
an InnoDB Plugin that is compatible with your
platform.
Note
The Information Schema tables are themselves plugins to the MySQL server,
but they depend on having the InnoDB storage engine plugin installed as well.
These tables will appear to be empty if the storage engine is not installed.
9.3.2 Installing the Binary InnoDB Plugin as a Shared Library on Microsoft
Windows
The InnoDB Plugin is supported on any of the Windows operating system versions supported by
MySQL. In particular, this includes Microsoft Windows 2008 Server, Windows Vista, Windows 2003
Server and Windows XP. Note that on Vista certain special procedures must be followed that are not
documented here.
Use the following procedure to dynamically install the InnoDB Plugin on Microsoft Windows.
1. Download, extract and install the suitable MySQL executable for your server platform and operating
system from the MySQL download section for MySQL Database Server 5.1. Be sure to use a 32-bit
or 64-bit version as appropriate for your hardware and Windows version.
2. Make sure the MySQL server is not running. You do a “slow” shutdown by issuing the following
command before performing the shutdown:
SET GLOBAL innodb_fast_shutdown=0;
Then finish the shutdown process, as described in The Shutdown Process in the MySQL
documentation. This option setting performs a full purge and an insert buffer merge before the
shutdown, which can typically take minutes, or even hours for very large and busy databases.
3. The InnoDB Plugin shared library is already installed in the directory lib\plugin as part of the
MySQL installation.
4. Edit the option file (my.ini) to ignore the builtin InnoDB, and load the InnoDB Plugin and all
Information Schema tables implemented in the InnoDB Plugin when the server starts:
ignore_builtin_innodb
plugin-load=innodb=ha_innodb_plugin.dll;innodb_trx=ha_innodb_plugin.dll;
innodb_locks=ha_innodb_plugin.dll;innodb_lock_waits=ha_innodb_plugin.dll;
innodb_cmp=ha_innodb_plugin.dll;innodb_cmp_reset=ha_innodb_plugin.dll;
innodb_cmpmem=ha_innodb_plugin.dll;innodb_cmpmem_reset=ha_innodb_plugin.dll
Note
All plugins for plugin-load should be on the same line in the option file.
Be careful when copying and pasting that the line does not split.
Alternatively, you can use the equivalent options on the MySQL command line:
mysqld --ignore-builtin-innodb --plugin-load=
innodb=ha_innodb_plugin.dll;
innodb_trx=ha_innodb_plugin.dll;innodb_locks=ha_innodb_plugin.dll;
innodb_lock_waits=ha_innodb_plugin.dll;innodb_cmp=ha_innodb_plugin.dll;
innodb_cmp_reset=ha_innodb_plugin.dll;innodb_cmpmem=ha_innodb_plugin.dll;
innodb_cmpmem_reset=ha_innodb_plugin.dll
You can also install the InnoDB Plugin and the new InnoDB Information Schema tables
implemented in ha_innodb_plugin.so with INSTALL commands, as follows:
58
Installing the Binary InnoDB Plugin as a Shared Library on Microsoft Windows
INSTALL
INSTALL
INSTALL
INSTALL
INSTALL
INSTALL
INSTALL
INSTALL
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
INNODB SONAME 'ha_innodb_plugin.dll';
INNODB_TRX SONAME 'ha_innodb_plugin.dll';
INNODB_LOCKS SONAME 'ha_innodb_plugin.dll';
INNODB_LOCK_WAITS SONAME 'ha_innodb_plugin.dll';
INNODB_CMP SONAME 'ha_innodb_plugin.dll';
INNODB_CMP_RESET SONAME 'ha_innodb_plugin.dll';
INNODB_CMPMEM SONAME 'ha_innodb_plugin.dll';
INNODB_CMPMEM_RESET SONAME 'ha_innodb_plugin.dll';
If you use INSTALL PLUGIN statements to install the InnoDB Plugin and the Information Schema
tables, ensure the following conditions are set up:
• In the mysqld command line or my.ini option file, prepend each InnoDB option with
loose_, so that MySQL will start even when InnoDB is unavailable. For example, write
loose_innodb_file_per_table instead of innodb_file_per_table.
• Start the MySQL server while it is configured to skip loading the built-in InnoDB and to make
MyISAM the default storage engine. This can be done by editing the option file my.cnf to contain
these two lines:
ignore_builtin_innodb
default_storage_engine=MyISAM
Or, you can use the equivalent options on the MySQL command line:
mysqld --ignore-builtin-innodb --default-storage-engine=MyISAM …
See the MySQL Manual section on INSTALL PLUGIN Syntax for information on how these
commands work.
5. Edit the option file my.ini to use InnoDB as the default engine (if desired) and set appropriate
configuration parameters to enable use of new InnoDB Plugin features, as described in Section 9.5,
“Configuring the InnoDB Plugin”. In particular, we recommend that you set the following specific
parameters as follows:
default-storage-engine=InnoDB
innodb_file_per_table=1
innodb_file_format=barracuda
innodb_strict_mode=1
IMPORTANT: The MySQL server always must be started with the option
ignore_builtin_innodb, as long as you want to use the dynamic InnoDB Plugin. Also,
remember that the startup option skip_grant_tables prevents MySQL from loading any plugins.
6. Verify the installation of the plugins with the MySQL command SHOW PLUGINS, which should
produce the following output:
Name
Status
binlog
Type
Library
License
ACTIVE STORAGE ENGINE
NULL
GPL
CSV
ACTIVE STORAGE ENGINE
NULL
GPL
MEMORY
ACTIVE STORAGE ENGINE
NULL
GPL
InnoDB
ACTIVE STORAGE ENGINE
ha_innodb_plugin.dll
GPL
INNODB_TRX
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.dll
GPL
INNODB_LOCKS
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.dll
GPL
59
Errors When Installing the InnoDB Plugin on Microsoft Windows
Name
Status
INNODB_LOCK_WAITS
Type
Library
License
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.dll
GPL
INNODB_CMP
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.dll
GPL
INNODB_CMP_RESET
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.dll
GPL
INNODB_CMPMEM
ACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.dll
GPL
INNODB_CMPMEM_RESETACTIVE INFORMATION
SCHEMA
ha_innodb_plugin.dll
GPL
MRG_MYISAM
ACTIVE STORAGE ENGINE
NULL
GPL
MyISAM
ACTIVE STORAGE ENGINE
NULL
GPL
If the plugins fail to load properly, see Section 9.3.3, “Errors When Installing the InnoDB Plugin on
Microsoft Windows” for possible causes and corrections.
After verifying that the Plugin is recognized by MySQL, create an InnoDB table as another
confirmation of success.
9.3.3 Errors When Installing the InnoDB Plugin on Microsoft Windows
If MySQL or the associated Windows service can not start, or a post-startup INSTALL PLUGIN
statement fails, look at the MySQL error log (usually named machine_name.err and located in the
MySQL data directory) for the detailed error message. The log is in chronological order, so look at the
end of the file. Try to resolve the problem based on other information in the message.
The following table outlines installation-related InnoDB Plugin error conditions or messages and
possible solutions.
Error Condition or Message
Possible Solution
Diagnose the cause from the following message
details.
Can't open shared library
library_name
API version for STORAGE ENGINE plugin The version of the Plugin is not compatible with
is too different
the version of the MySQL server. Consult the
compatibility chart.
No such file or directory
Check that the file ha_innodb_plugin.so
or .dll was copied to the correct location.
Confirm that you specified the right file
name (ha_innodb_plugin.so or .dll
for the library from the InnoDB web site;
ha_innodb_plugin.so or .dll for the library
supplied along with MySQL 5.1.38 and up.)
Permission denied
Check that the folder and file access
permissions are set properly. The mysqld
process must have permission to read the file
ha_innodb_plugin.dll and to read files in the
plugin folder. On Windows XP, file permissions
can be seen or changed by right-clicking a file and
pressing Properties, and then the Security Tab.
To see the Security Tab, you may need to adjust
the Folder Options on the Control Panel to turn off
“Use Simple File Sharing”.
60
Building the InnoDB Plugin from Source Code
Error Condition or Message
Possible Solution
Can't open shared library
'ha_innodb_plugin.dll' (errno: 0)
Ensure that ha_innodb_plugin.dll is for the
same system platform as mysqld. In particular,
note that a 32-bit mysqld is unable to load a 64bit plugin, and vice versa.
Note: The Information Schema tables are themselves plugins to the MySQL server, but they depend on
having the InnoDB storage engine plugin installed as well. These tables will appear to be empty if the
storage engine is not installed.
9.4 Building the InnoDB Plugin from Source Code
Sometimes, you may wish to build the plugin from the source code using special compilation options,
or there might be no binary plugin available for your server platform. With the resulting special version
of MySQL containing the new InnoDB functionality, it is not necessary to INSTALL any plugins or be
concerned about startup parameters that preclude loading plugins.
To build the InnoDB Plugin from the source code, you also need the MySQL source code and some
software tools. You should become familiar with the MySQL manual section on Installing MySQL from
Source.
The general steps for building MySQL from source, containing the InnoDB Plugin in place of the
standard built-in InnoDB, are as follows:
• Download the MySQL source code.
• Download the InnoDB Plugin source code.
• Replace the source code for the built-in InnoDB with the InnoDB Plugin source tree.
• Compile MySQL as usual, generating a new mysqld executable file.
• Configure the MySQL server by editing the configuration file to use InnoDB as the default engine (if
desired) and set appropriate configuration parameters to enable use of new InnoDB Plugin features.
The following sections detail these steps for Linux or Unix systems, and for Microsoft Windows.
9.4.1 Building the InnoDB Plugin on Linux or Unix
1. Download the MySQL source code, version 5.1.24 or later from http://dev.mysql.com/downloads/
mysql/5.1.html#source and extract it.
2. Download the InnoDB Plugin source code from http://dev.mysql.com/downloads/.
3. Replace the contents of the storage/innobase directory in the MySQL source tree with the
InnoDB Plugin source tree.
Note
In MySQL 5.1.38 and up, the MySQL source tree also contains a storage/
innodb_plugin directory, but that does not affect this procedure. The
source that you download from the InnoDB web site may contain additional
changes and fixes.
4. Compile and build MySQL. Instead of building a dynamic InnoDB Plugin, it is advisable to build a
version of MySQL that contains the InnoDB Plugin. This is because a dynamic InnoDB Plugin
must be built with exactly the same tools and options as the mysqld executable, or spurious
errors may occur. Example:
% wget ftp://ftp.easynet.be/mysql/Downloads/MySQL-5.1/mysql-5.1.37.tar.gz
61
Building the InnoDB Plugin on Microsoft Windows
%
%
%
%
%
%
%
%
%
tar -zxf mysql-5.1.37.tar.gz
cd mysql-5.1.37/storage
wget http://dev.mysql.com/downloads/innodb_plugin/innodb_plugin-1.0.8.tar.gz
tar -zxf innodb-1.0.8.tar.gz
rm -fr innobase
mv innodb-1.0.8 innobase
cd ..
./configure --with-plugins=innobase
make
5. Reconfigure the MySQL server by editing the my.cnf option file to use InnoDB as the default
engine (if desired) and set appropriate configuration parameters to enable use of new InnoDB
Plugin features, as described in section Section 9.5, “Configuring the InnoDB Plugin”. In particular,
we recommend that you set the following specific parameters as follows:
default_storage_engine=InnoDB
innodb_file_per_table=1
innodb_file_format=barracuda
innodb_strict_mode=1
6. If you build a version of MySQL that contains the InnoDB Plugin (--with-plugins=innobase),
you do not have to tell MySQL to specify ignore_builtin_innodb or specify plugin-load, or
issue any INSTALL PLUGIN statements. The mysqld executable that you compiled will contain
the new InnoDB Plugin features.
Note: To fully exploit the performance improvements discussed in Section 7.2, “Faster Locking for
Improved Scalability”, the InnoDB Plugin source code and build process makes some compiletime tests of platform capabilities to automatically use instructions for atomic memory access where
available. If this logic fails, you may need to contact MySQL support.
9.4.2 Building the InnoDB Plugin on Microsoft Windows
Installing MySQL from Source on Windows includes some information about building from source on
Windows. The following discussion is specifically focused on building a version of MySQL containing
the InnoDB Plugin.
You need the following tools:
• A compiler environment, one of the following:
• Microsoft Visual C++ 2003
• Microsoft Visual C++ 2005
• Microsoft Visual C++ 2008 (Note: for building MySQL 5.1.31 or later)
• Microsoft Visual C++ 2005 Express Edition (free of charge)
• Download and install the Microsoft Visual C++ 2005 Express Edition.
• Download and install the Windows Platform SDK .
• Configure the Visual Studio Express Edition to use the Platform SDK according to the
instruction.
• Microsoft Visual C++ 2008 Express Edition (free of charge, for building MySQL 5.1.31 or later)
• Download and install the Microsoft Visual C++ 2008 Express Edition. The Visual C++ 2008
Express Edition has already been integrated with the Windows SDK.
• GNU Bison for Windows, a general-purpose parser generator that is largely compatible with Berkeley
Yacc. This tool is used automatically as part of compiling and building MySQL. For most users, it is
sufficient to download and run the “complete package” to install GNU Bison.
62
Building the InnoDB Plugin on Microsoft Windows
• CMake 2.6.0 or later, a cross-platform make system that can generate MSVC project files.
In addition to installing these tools, you must also set CScript as the default Windows script host by
executing the following command in the Command Prompt:
cscript //H:CScript
After you have installed and configured all the required tools, you may proceed with the compilation.
1. Download the MySQL source code, version 5.1.24 or later from the MySQL website and extract the
source files.
2. Download the InnoDB plugin source code from the MySQL download site.
3. Extract the files from the source code archives.
4. Replace the contents of the storage\innobase folder in the MySQL source tree with the InnoDB
plugin source tree.
In MySQL 5.1.38 and up, the MySQL source tree also contains a storage\innodb_plugin
directory, but that does not affect this procedure. The source that you download from the InnoDB
web site may contain additional changes and fixes.
5. Compile and build MySQL under the Microsoft Visual Studio Command Prompt as follows:
Visual Studio 2003:
win\configure WITH_INNOBASE_STORAGE_ENGINE __NT__
win\build-vs7.bat
devenv mysql /build release /project ALL_BUILD
Visual Studio 2005:
win\configure WITH_INNOBASE_STORAGE_ENGINE __NT__
win\build-vs8.bat
devenv mysql /build release /project ALL_BUILD
Visual Studio 2008:
win\configure WITH_INNOBASE_STORAGE_ENGINE __NT__
win\build-vs9.bat
vcbuild mysql.sln "Release"
For the 64-bit version, use win\build-vsN_x64.bat instead of win\build-vsN.bat.
6. Install the compiled mysqld.exe from the sql\release folder of the source tree by doing one of
the following:
a. Copy the mysqld.exe to the bin folder of an earlier MySQL 5.1 installation.
b. Make a distribution package and unpack it to the folder where MySQL will be installed. See
the MySQL manual section on make_win_bin_dist — Package MySQL Distribution as Zip
Archive. Note that scripts\make_win_bin_dist requires the Cygwin environment.
7. Reconfigure the MySQL server by editing the my.cnf or my.ini option file to use InnoDB as
the default engine (if desired) and set appropriate configuration parameters to enable use of new
InnoDB Plugin features, as described in section Section 9.5, “Configuring the InnoDB Plugin”. In
particular, we recommend that you set the following specific parameters as follows:
default_storage_engine=InnoDB
innodb_file_per_table=1
innodb_file_format=barracuda
innodb_strict_mode=1
8. Since you built a version of MySQL that contains the InnoDB Plugin, you do not have to specify
ignore_builtin_innodb or specify plugin-load, or issue any INSTALL PLUGIN statements.
The mysqld.exe that you compiled contains the new InnoDB Plugin features.
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Configuring the InnoDB Plugin
9.5 Configuring the InnoDB Plugin
Because the MySQL server as distributed by MySQL includes a built-in copy of InnoDB, if you are
using the dynamic InnoDB Plugin and have INSTALLed it into the MySQL server, you must always
start the server with the option ignore_builtin_innodb, either in the option file or on the mysqld
command line. Also, remember that the startup option skip_grant_tables prevents MySQL from
loading any plugins. Neither of these options is needed when using a specialized version of MySQL
that you build from source.
By default, the InnoDB Plugin does not create tables in a format that is incompatible with the built-in
InnoDB in MySQL. Tables in the new format may be compressed, and they may store portions of long
columns off-page, outside the B-tree nodes. You may wish to enable the creation of tables in the new
format, using one of these techniques:
• Include innodb_file_per_table=1 and innodb_file_format=barracuda in the [mysqld]
section of the MySQL option file.
• Add --innodb_file_per_table=1 and --innodb_file_format=barracuda to the mysqld
command line.
• Issue the statements:
SET GLOBAL innodb_file_format=barracuda;
SET GLOBAL innodb_file_per_table=ON;
in the MySQL client when running with SUPER privileges.
You may also want to enable the new InnoDB strict mode, which guards SQL or certain operational
errors that otherwise generate warnings and possible unintended consequences of ignored or incorrect
SQL commands or parameters. As described in Section 8.5, “InnoDB Strict Mode”, the GLOBAL
parameter innodb_strict_mode can be set ON or OFF in the same way as the parameters just
mentioned. You can also use the command SET SESSION innodb_strict_mode=mode (where
mode is ON or OFF) to enable or disable InnoDB strict mode on a per-session basis.
Take care when using new InnoDB configuration parameters or values that apply only when using
the InnoDB Plugin. When the MySQL server encounters an unknown option, it fails to start and
returns an error: unknown variable. This happens, for example, if you include the new parameter
innodb_file_format when you start the MySQL server with the built-in InnoDB rather than the
plugin. This can cause a problem if you accidentally use the built-in InnoDB after a system crash,
because InnoDB crash recovery runs before MySQL checks the startup parameters. See Section 11.4,
“Possible Problems” why this can be a problem. One safeguard is to specify the prefix loose_ before
the names of new options, so that if they are not recognized on startup, the server gives a warning
instead of a fatal error.
9.6 Frequently Asked Questions about Plugin Installation
9.6.1 Should I use the InnoDB-supplied plugin or the one that is included
with MySQL 5.1.38 or higher?
The Plugin that you download from the InnoDB web site should always be at the same level or newer
than the shared library that is included with the MySQL distribution starting with version 5.1.38. To pick
up the very latest fixes, download from the InnoDB site.
9.6.2 Why doesn't the MySQL service on Windows start after the
replacement?
For the types of errors and how to diagnose them, see Section 9.3.3, “Errors When Installing the
InnoDB Plugin on Microsoft Windows”. Be especially careful that the plugin-load line in the option
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The Plugin is installed... now what?
file does not get split across lines when you copy and paste from the README or this manual, which
can produce an “unrecognized option” error in the error log.
9.6.3 The Plugin is installed... now what?
You automatically benefit from the “fast index creation” feature for every index you create on a large
InnoDB table. If you switch to the “Barracuda” file format using the innodb_file_format option in
combination with the innodb_file_per_table option, you can take advantage of other features
such as table compression. For the full list of features, refer to Section 1.2, “Features of the InnoDB
Plugin”.
9.6.4 Once the Plugin is installed, is it permanent?
The Plugin must loaded whenever the MySQL database server is started. As we saw earlier, there are
several ways to configure MySQL to use the Plugin rather than the built-in InnoDB: in the option file,
with mysqld command-line options, or with INSTALL statements after the server starts.
To ensure that you do not accidentally revert to the older InnoDB, be careful to carry any configuration
file, command-line options, or post-startup commands forward in the future, such as when transitioning
from a development system to a test system, setting up a replication slave, or when writing new
mysqld startup scripts.
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Chapter 10 Upgrading the InnoDB Plugin
Table of Contents
10.1 Upgrading the Dynamic InnoDB Plugin ................................................................................. 67
10.2 Upgrading a Statically Built InnoDB Plugin ........................................................................... 67
10.3 Converting Compressed Tables Created Before Version 1.0.2 ............................................... 68
Thanks to the pluggable storage engine architecture of MySQL, upgrading the InnoDB Plugin should
be a simple matter of shutting down MySQL, replacing a platform-specific executable file, and
restarting the server. If you wish to upgrade and use your existing database, it is essential to perform
a “slow” shutdown, or the new plugin may fail when merging buffered inserts or purging deleted
records. If your database does not contain any compressed tables, you should be able to use your
database with the newest InnoDB Plugin without problems after a slow shutdown.
However, if your database contains compressed tables, it may not be compatible with InnoDB
Plugin 1.0.8. Because of an incompatible change introduced in InnoDB Plugin version 1.0.2, some
compressed tables may need to be rebuilt, as noted in Section 10.3, “Converting Compressed Tables
Created Before Version 1.0.2”. Please follow these steps carefully.
You may, of course, rebuild your database using mysqldump or other methods. This may be a
preferable approach if your database is small or there are many referential constraints among tables.
Note that once you have accessed your database with InnoDB Plugin 1.0.8, you should not try to use it
with the Plugin prior to 1.0.2.
10.1 Upgrading the Dynamic InnoDB Plugin
Before shutting down the MySQL server containing the InnoDB Plugin, you must enable “slow”
shutdown:
SET GLOBAL innodb_fast_shutdown=0;
For the details of the shutdown procedure, see the MySQL manual on The Shutdown Process.
In the directory where the MySQL server looks for plugins, rename the executable file of the old
InnoDB Plugin (ha_innodb_plugin.so or ha_innodb_plugin.dll), so that you can restore it
later if needed. You may remove the file later. The plugin directory is specified by the system variable
plugin_dir. The default location is usually the lib/plugin subdirectory of the directory specified
by basedir.
Download a suitable package for your server platform, operating system and MySQL version.
Extract the contents of the archive using tar or a similar tool for Linux and Unix, or Windows
Explorer or WinZip or similar utility for Windows. Copy the file ha_innodb_plugin.so or
ha_innodb_plugin.dll to the directory where the MySQL server looks for plugins.
Start the MySQL server. Follow the procedure in Section 10.3, “Converting Compressed Tables
Created Before Version 1.0.2” to convert any compressed tables if needed.
10.2 Upgrading a Statically Built InnoDB Plugin
As with a dynamically installed InnoDB Plugin, you must perform a “slow” shutdown of the MySQL
server. If you have built MySQL from source code and replaced the built-in InnoDB in MySQL with the
InnoDB Plugin in the source tree as discussed in Section 9.4, “Building the InnoDB Plugin from Source
Code”, you will have a special version of the mysqld executable that contains the InnoDB Plugin.
If you intend to upgrade to a dynamically linked InnoDB Plugin, you can follow the advice of
Section 11.3.4, “Uninstalling a Statically Built InnoDB Plugin” and Section 9.3, “Installing the
Precompiled InnoDB Plugin as a Shared Library”.
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Converting Compressed Tables Created Before Version 1.0.2
If you intend to upgrade a statically built InnoDB Plugin to another statically built plugin, you will have
to rebuild the mysqld executable, shut down the server, and replace the mysqld executable before
starting the server.
Either way, please be sure to follow the instructions of Section 10.3, “Converting Compressed Tables
Created Before Version 1.0.2” if any compressed tables were created.
10.3 Converting Compressed Tables Created Before Version 1.0.2
The InnoDB Plugin version 1.0.2 introduces an incompatible change to the format of compressed
tables. This means that some compressed tables that were created with an earlier version of the
InnoDB Plugin may need to be rebuilt with a bigger KEY_BLOCK_SIZE before they can be used.
If you must keep your existing database when you upgrade to InnoDB Plugin 1.0.2 or newer, you
will need to perform a “slow” shutdown of MySQL running the previous version of the InnoDB Plugin.
Following such a shutdown, and using the newer release of the InnoDB Plugin, you will need to
determine which compressed tables need conversion and then follow a procedure to upgrade these
tables. Because most users will not have tables where this process is required, this manual does not
detail the procedures required. If you have created compressed tables with the InnoDB Plugin prior to
release 1.0.2, you may want to contact MySQL support.
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Chapter 11 Downgrading from the InnoDB Plugin
Table of Contents
11.1 Overview .............................................................................................................................
11.2 The Built-in InnoDB, the Plugin and File Formats ..................................................................
11.3 How to Downgrade ..............................................................................................................
11.3.1 Converting Tables ....................................................................................................
11.3.2 Adjusting the Configuration .......................................................................................
11.3.3 Uninstalling a Dynamic Library ..................................................................................
11.3.4 Uninstalling a Statically Built InnoDB Plugin ...............................................................
11.4 Possible Problems ...............................................................................................................
11.4.1 Accessing COMPRESSED or DYNAMIC Tables .............................................................
11.4.2 Issues with UNDO and REDO ...................................................................................
11.4.3 Issues with the Doublewrite Buffer .............................................................................
11.4.4 Issues with the Insert Buffer ......................................................................................
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11.1 Overview
There are times when you might want to use the InnoDB Plugin with a given database, and then
downgrade to the built-in InnoDB in MySQL. One reason to do this is because you want to take
advantage of a new InnoDB Plugin feature (such as “Fast Index Creation”), but revert to the standard
built-in InnoDB in MySQL for production operation.
If you have created new tables using the InnoDB Plugin, you may need to convert them to a
format that the built-in InnoDB in MySQL can read. Specifically, if you have created tables that use
ROW_FORMAT=COMPRESSED or ROW_FORMAT=DYNAMIC you must convert them to a different format,
if you plan to access these tables with the built-in InnoDB in MySQL. If you do not do so, anomalous
results may occur.
Although InnoDB checks the format of tables and database files (specifically *.ibd files) for
compatibility, it is unable to start if there are buffered changes for “too new format” tables in the
redo log or in the system tablespace. Thus it is important to carefully follow these procedures when
downgrading from the InnoDB Plugin to the built-in InnoDB in MySQL, version 5.1.
This chapter describes the downgrade scenario, and the steps you should follow to ensure correct
processing of your database.
11.2 The Built-in InnoDB, the Plugin and File Formats
Starting with version 5.0.21, the built-in InnoDB in MySQL checks the table type before opening a table.
Until now, all InnoDB tables have been tagged with the same type, although some changes to the
format have been introduced in MySQL versions 4.0, 4.1, and 5.0.
One of the important new features introduced with the InnoDB Plugin is support for identified
file formats. This allows the InnoDB Plugin and versions of InnoDB since 5.0.21 to check for file
compatibility. It also allows the user to preclude the use of features that would generate downward
incompatibilities. By paying attention to the file format used, you can protect your database from
corruptions, and ensure a smooth downgrade process.
In general, before using a database file created with the InnoDB Plugin with the built-in InnoDB in
MySQL you should verify that the tablespace files (the *.ibd files) are compatible with the built-in
InnoDB in MySQL. The InnoDB Plugin can read and write tablespaces in both the formats “Antelope”
and “Barracuda”. The built-in InnoDB can only read and write tablespaces in “Antelope” format. To
make all tablespaces “legible” to the built-in InnoDB in MySQL, you should follow the instructions in
Section 11.3, “How to Downgrade” to reformat all tablespaces to be in the “Antelope” format.
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How to Downgrade
Generally, after a “slow” shutdown of the InnoDB Plugin (innodb_fast_shutdown=0), it should be
safe to open the data files with the built-in InnoDB in MySQL. See Section 11.4, “Possible Problems”
for a discussion of possible problems that can arise in this scenario and workarounds for them.
11.3 How to Downgrade
11.3.1 Converting Tables
The built-in InnoDB in MySQL can access only tables in the “Antelope” file format, that is, in the
REDUNDANT or COMPACT row format. If you have created tables in COMPRESSED or DYNAMIC format,
the corresponding tablespaces in the new “Barracuda” file format, and it is necessary to downgrade
these tables.
First, identify the tables that require conversion, by executing this command:
SELECT table_schema, table_name, row_format
FROM information_schema.tables
WHERE engine='innodb'
AND row_format NOT IN ('Redundant', 'Compact');
Next, for each table that requires conversion, run the following command:
ALTER TABLE table_name ROW_FORMAT=COMPACT;
This command copies the table and its indexes to a new tablespace in the “Antelope” format. See
Chapter 2, Fast Index Creation in the InnoDB Storage Engine for a discussion of exactly how such
index creation operations are performed.
11.3.2 Adjusting the Configuration
Before you shut down the InnoDB Plugin and start the basic built-in InnoDB in MySQL, review the
configuration files. Changes to the startup options do not take effect until the server is restarted, or the
InnoDB Plugin is uninstalled and reinstalled.
The InnoDB Plugin introduces several configuration parameters that are not recognized by the builtin InnoDB in MySQL, including: innodb_file_format, innodb_file_format_check, and
innodb_strict_mode. See Section C.1, “New Parameters” for a complete list of new configuration
parameters in the InnoDB Plugin. You can include these parameters in the configuration file, only if you
use the loose_ form of the parameter names, so that the built-in InnoDB in MySQL can start.
If the InnoDB Plugin was installed as a dynamic plugin, the startup option ignore_builtin_innodb
or skip_innodb must have been set to disable the built-in InnoDB in MySQL. These options must be
removed, so that the built-in InnoDB in MySQL is enabled the next time the server is started.
If the InnoDB Plugin was loaded using plugin-load option. This option has to be removed, too.
In MySQL, configuration options can be specified in the mysqld command line or the option file
(my.cnf or my.ini). See the MySQL manual on Using Option Files for more information.
11.3.3 Uninstalling a Dynamic Library
The following applies if the InnoDB Plugin was installed as a dynamic library with the INSTALL
PLUGIN command, as described in Section 9.3, “Installing the Precompiled InnoDB Plugin as a Shared
Library”.
Issue the command UNINSTALL PLUGIN for every “plugin” supplied by the library
ha_innodb_plugin.so (or ha_innodb_plugin.dll on Windows). Note that the following
commands initiate a shutdown of the InnoDB Plugin:
SET GLOBAL innodb_fast_shutdown=0;
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Uninstalling a Statically Built InnoDB Plugin
UNINSTALL
UNINSTALL
UNINSTALL
UNINSTALL
UNINSTALL
UNINSTALL
UNINSTALL
UNINSTALL
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
PLUGIN
INNODB;
INNODB_CMP;
INNODB_CMP_RESET;
INNODB_CMPMEM;
INNODB_CMPMEM_RESET;
INNODB_TRX;
INNODB_LOCKS;
INNODB_LOCK_WAITS;
Due to MySQL Bug #33731, please ensure that the plugin definitions are actually deleted from the
database, so that they are not loaded again:
SELECT * FROM mysql.plugin;
DELETE FROM mysql.plugin WHERE name='…';
Restart the server. For the details of the shutdown procedure, see the MySQL manual on The
Shutdown Process.
11.3.4 Uninstalling a Statically Built InnoDB Plugin
If you have built MySQL from source code and replaced the built-in InnoDB in MySQL with the InnoDB
Plugin in the source tree as discussed in Section 9.4, “Building the InnoDB Plugin from Source Code”,
you have a special version of the mysqld executable that contains the InnoDB Plugin. To “uninstall”
the InnoDB Plugin, you replace this executable with something that is built from an unmodified MySQL
source code distribution.
Before shutting down the version of the MySQL server with built-in InnoDB Plugin, you must enable
“slow” shutdown:
SET GLOBAL innodb_fast_shutdown=0;
For the details of the shutdown procedure, see the MySQL manual on The Shutdown Process.
11.4 Possible Problems
Failure to follow the downgrading procedure described in Section 11.3, “How to Downgrade” may lead
to compatibility issues when files written by the InnoDB Plugin are accessed by the built-in InnoDB in
MySQL. This section describes some internal recovery algorithms, to help explain why it is important to
follow the downgrade procedure described above. It discusses the issues that may arise, and covers
possible ways to fix them.
A general fix is to install the plugin as described in Chapter 9, Installing the InnoDB Plugin and then
follow the downgrading procedure described in Section 11.3, “How to Downgrade”.
In the future, the file format management features described in Chapter 4, InnoDB File-Format
Management will guard against the types of problems described in this section.
11.4.1 Accessing COMPRESSED or DYNAMIC Tables
The built-in InnoDB in MySQL can only open tables that were created in REDUNDANT or COMPACT
format. Starting with MySQL version 5.0.21, an attempt to open a table in some other format results in
ERROR 1146 (42S02): Table 'test.t' doesn't exist. Furthermore, a message “unknown
table type” appears in the error log.
In the InnoDB Plugin, you may rebuild an incompatible table by issuing a statement ALTER TABLE
table_name ROW_FORMAT=COMPACT.
11.4.2 Issues with UNDO and REDO
As noted in Section 11.3, “How to Downgrade”, you should ensure a “slow” shutdown is done with the
InnoDB Plugin, before running with the built-in InnoDB in MySQL, to clean up all buffers. To initiate a
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Issues with the Doublewrite Buffer
slow shutdown, execute the command SET GLOBAL innodb_fast_shutdown=0 before initiating
the shutdown of the InnoDB Plugin.
We recommend “slow” shutdown (innodb_fast_shutdown=0) because the InnoDB Plugin
may write special records to the transaction undo log that cause problems if the built-in InnoDB in
MySQL attempts to read the log. Specifically, these special records are written when a record in a
COMPRESSED or DYNAMIC table is updated or deleted and the record contains columns stored offpage. The built-in InnoDB in MySQL cannot read these undo log records. Also, the built-in InnoDB in
MySQL cannot roll back incomplete transactions that affect tables that it is unable to read (tables in
COMPRESSED or DYNAMIC format).
Note that a “normal” shutdown does not necessarily empty the undo log. A normal shutdown occurs
when innodb_fast_shutdown=1, the default. When InnoDB is shut down, some active transactions
may have uncommitted modifications, or they may be holding a read view that prevents the purging of
some version information from the undo log. The next time InnoDB is started after a normal shutdown
(innodb_fast_shutdown=1), it rolls back any incomplete transactions and purge old version
information. Therefore, it is important to perform a “slow” shutdown (innodb_fast_shutdown=0) as
part of the downgrade process.
In case it is not possible to have the InnoDB Plugin clear the undo log, you can prevent the built-in
InnoDB in MySQL from accessing the undo log by setting innodb_force_recovery=3. However,
this is not a recommended approach, since in addition to preventing the purge of old versions, this
recovery mode prevents the rollback of uncommitted transactions. For more information, see the
MySQL manual on Forcing InnoDB Recovery.
When it comes to downgrading, there are also considerations with respect to redo log information. For
the purpose of crash recovery, InnoDB writes to the log files information about every modification to the
data files. When recording changes to tables that were created in DYNAMIC or COMPRESSED format,
the InnoDB Plugin writes redo log entries that cannot be recognized by the built-in InnoDB in MySQL.
The built-in InnoDB in MySQL refuses to start if it sees any unknown entries in the redo log.
When InnoDB is shut down cleanly, it flushes all unwritten changes from the buffer pool to the data
files and makes a checkpoint in the redo log. When InnoDB is subsequently restarted, it scans the
redo log starting from the last checkpoint. After a clean shutdown, InnoDB crash recovery only then
sees the end-of-log marker in the redo log. In this case, the built-in InnoDB in MySQL would not see
any unrecognizable redo log entries. This is a second reason why you should ensure a clean, slow
shutdown of MySQL (innodb_fast_shutdown=0) before you attempt a downgrade.
In an emergency, you may prevent the redo log scan and the crash recovery from the redo log by
setting the parameter innodb_force_recovery=6. However, this is strongly discouraged,
because may lead into severe corruption. See the MySQL manual on Forcing InnoDB Recovery for
more information.
11.4.3 Issues with the Doublewrite Buffer
InnoDB uses a novel file flush technique called doublewrite. Before writing pages to a data file, InnoDB
first writes them to a contiguous area called the doublewrite buffer. Only after the write and the flush
to the doublewrite buffer have completed does InnoDB write the pages to their proper positions in the
data file. If the operating system crashes in the middle of a page write, InnoDB can later find a good
copy of the page from the doublewrite buffer during recovery.
The doublewrite buffer may also contain compressed pages. However, the built-in InnoDB in MySQL
cannot recognize such pages, and it assumes that compressed pages in the doublewrite buffer are
corrupted. It also wrongly assumes that the tablespace (the .ibd file) consists of 16K byte pages.
Thus, you may find InnoDB warnings in the error log of the form “a page in the doublewrite buffer is not
within space bounds”.
The doublewrite buffer is not scanned after a clean shutdown. In an emergency, you may prevent crash
recovery by setting innodb_force_recovery=6. However, this is strongly discouraged, because
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Issues with the Insert Buffer
it may lead into severe corruption. For more information, see the MySQL manual on Forcing InnoDB
Recovery
11.4.4 Issues with the Insert Buffer
Secondary indexes are usually nonunique, and insertions into secondary indexes happen in a relatively
random order. This would cause a lot of random disk I/O operations without a special mechanism used
in InnoDB called the insert buffer.
When a record is inserted into a nonunique secondary index page that is not in the buffer pool, InnoDB
inserts the record into a special B-tree: the insert buffer. Periodically, the insert buffer is merged into
the secondary index trees in the database. A merge also occurs whenever a secondary index page is
loaded to the buffer pool.
A “normal” shutdown does not clear the insert buffer. A normal shutdown occurs when
innodb_fast_shutdown=1, the default. If the insert buffer is not empty when the InnoDB Plugin is
shut down, it may contain changes for tables in DYNAMIC or COMPRESSED format. Thus, starting the
built-in InnoDB in MySQL on the data files may lead into a crash if the insert buffer is not empty.
A “slow” shutdown merges all changes from the insert buffer. To initiate a slow shutdown, execute the
command SET GLOBAL innodb_fast_shutdown=0 before initiating the shutdown of the InnoDB
Plugin.
To disable insert buffer merges, you may set innodb_force_recovery=4 so that you can back up
the uncompressed tables with the built-in InnoDB in MySQL. Be sure not to use any WHERE conditions
that would require access to secondary indexes. For more information, see the MySQL manual on
Forcing InnoDB Recovery
In the InnoDB Plugin 1.0.3 and later, you can disable the buffering of new operations by setting the
parameter innodb_change_buffering. See Section 7.4, “Controlling InnoDB Insert Buffering” for
details.
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Chapter 12 InnoDB Plugin Change History
Table of Contents
12.1 Changes in InnoDB Plugin 1.0.9 and Higher .........................................................................
12.2 Changes in InnoDB Plugin 1.0.8 (May, 2010) .......................................................................
12.3 Changes in InnoDB Plugin 1.0.7 (April, 2010) .......................................................................
12.4 Changes in InnoDB Plugin 1.0.6 (November 27, 2009) ..........................................................
12.5 Changes in InnoDB Plugin 1.0.5 (November 18, 2009) ..........................................................
12.6 Changes in InnoDB Plugin 1.0.4 (August 11, 2009) ...............................................................
12.7 Changes in InnoDB Plugin 1.0.3 (March 11, 2009) ................................................................
12.8 Changes in InnoDB Plugin 1.0.2 (December 1, 2008) ...........................................................
12.9 Changes in InnoDB Plugin 1.0.1 (May 8, 2008) ....................................................................
12.10 Changes in InnoDB Plugin 1.0.0 (April 15, 2008) ................................................................
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The complete change history of the InnoDB Plugin can be viewed in the file ChangeLog that is
included in the source and binary distributions.
12.1 Changes in InnoDB Plugin 1.0.9 and Higher
With the tighter integration of InnoDB into the MySQL server starting in MySQL 5.1, you can find recent
InnoDB change log entries in the MySQL 5.1 Release Notes.
12.2 Changes in InnoDB Plugin 1.0.8 (May, 2010)
12.3 Changes in InnoDB Plugin 1.0.7 (April, 2010)
Improved crash recovery performance.
Fixed MySQL Bug #52102: InnoDB Plugin shows performance drop comparing to builtin InnoDB on
Windows only. Disabled Windows atomics by default.
Fixed MySQL Bug #51378: Init 'ref_length' to correct value, in case of an out of bound MySQL
primary_key.
Made SHOW ENGINE INNODB MUTEX STATUS display SUM(os_waits) for the buffer pool block
mutexes and locks.
Fixed ALTER TABLE ... IMPORT TABLESPACE of compressed tables.
Fixed MySQL Bug #49535: Available memory check slows down crash recovery tens of times.
Let the master thread sleep if the amount of work to be done is calibrated as taking less than a second.
Fixed MySQL Bug #49001: SHOW INNODB STATUS deadlock info incorrect when deadlock detection
aborts.
Fixed MySQL Bug #35077: Very slow DROP TABLE (ALTER TABLE, OPTIMIZE TABLE) on
compressed tables.
Fixed MySQL Bug #49497: Error 1467 (ER_AUTOINC_READ_FAILED) on inserting a negative value.
Do not merge buffered inserts to compressed pages before the redo log has been applied in crash
recovery.
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Changes in InnoDB Plugin 1.0.6 (November 27, 2009)
Do not attempt to access a clustered index record that has been marked for deletion, On the READ
UNCOMMITTED isolation level. In previous versions, the InnoDB would attempt to retrieve a previous
version of the record in this case.
Fixed an uninitialized access to block->is_hashed, when disabling the adaptive hash index.
Fixed MySQL Bug #46193>: Crash when accessing tables after enabling innodb_force_recovery
option.
Fixed MySQL Bug #49238: Creating / Dropping a temporary table while at 1023 transactions will cause
assert.
Display the zlib version number at startup.
12.4 Changes in InnoDB Plugin 1.0.6 (November 27, 2009)
Fixed MySQL Bug #48782: On lock wait timeout, CREATE INDEX attempts DROP TABLE.
Report duplicate table names to the client connection, not to the error log.
Allow CREATE INDEX to be interrupted.
Fixed MySQL Bug #47167: InnoDB Plugin "set global innodb_file_format_check" cannot
set value by User-Defined Variable.
Fixed MySQL Bug #45992: InnoDB memory not freed after shutdown; and MySQL Bug #46656:
InnoDB Plugin memory leaks (Valgrind).
12.5 Changes in InnoDB Plugin 1.0.5 (November 18, 2009)
Clean up after a crash during DROP INDEX. When InnoDB crashes while dropping an index, ensure
that the index will be completely dropped during crash recovery.
When a secondary index exists in the MySQL .frm file but not in the InnoDB data dictionary, return an
error instead of letting an assertion fail in index_read.
Prevent the reuse of tablespace identifiers after InnoDB has crashed during table creation. Also, refuse
to start if files with duplicate tablespace identifiers are encountered.
Fixed MySQL Bug #47055: Unconditional exit on ERROR_WORKING_SET_QUOTA 1453 (0x5AD) for
InnoDB backend.
Fixed MySQL Bug #37232: InnoDB might get too many read locks for DML with repeatable-read.
Fixed MySQL Bug #31183: Tablespace full problems not reported in error log; error message unclear.
Modified innodb-zip.test so that the test will pass with zlib 1.2.3.3. Apparently, the zlib function
compressBound() has been slightly changed, and the maximum record size of a table with 1K
compressed page size has been reduced by one byte.
Fixed a regression introduced by the fix for MySQL Bug #26316.
Fixed MySQL Bug #44571: InnoDB Plugin crashes on ADD INDEX.
Fixed a bug in the merge sort that can corrupt indexes in fast index creation.
Introduced the settable global variables innodb_old_blocks_pct and innodb_old_blocks_time
for controlling the buffer pool eviction policy, making it possible to tune the buffer pool LRU eviction
policy to be more resistant against index scans. See Section 7.14, “Making Buffer Cache Scan
Resistant”.
76
Changes in InnoDB Plugin 1.0.4 (August 11, 2009)
Fixed MySQL Bug #42885: buf_read_ahead_random, buf_read_ahead_linear counters, thread
wakeups. See Section 8.9, “More Read Ahead Statistics”.
Fixed MySQL Bug #46650: InnoDB assertion autoinc_lock == lock in
lock_table_remove_low on INSERT SELECT.
Fixed MySQL Bug #46657: InnoDB Plugin: invalid read in index_merge_innodb test (Valgrind).
Fixed MySQL Bug #42829: binlogging enabled for all schemas regardless of binlog-db-db / binlogignore-db.
12.6 Changes in InnoDB Plugin 1.0.4 (August 11, 2009)
Enabled inlining of functions and prefetch with Sun Studio.
Changed the defaults for innodb_sync_spin_loops from 20 to 30 and
innodb_spin_wait_delay from 5 to 6.
Implemented adaptive flushing of dirty pages, which uses heuristics to avoid I/O bursts at checkpoint.
A new parameter innodb_adaptive_flushing is added to control whether the new flushing
algorithm should be used. See Section 7.11, “Controlling the Flushing Rate of Dirty Pages”.
Implemented I/O capacity tuning. A new parameter innodb_io_capacity is added to control the
master threads I/O rate. (To preserve the former behavior, set this parameter to a value of 100.) The
ibuf merge is also changed from synchronous to asynchronous. See Section 7.10, “Controlling the
Master Thread I/O Rate”.
Introduced the PAUSE instruction inside spin-loop where available. See Section 7.12, “Using the
PAUSE instruction in InnoDB spin loops”.
Fixed a crash on SET GLOBAL innodb_file_format=DEFAULT or SET GLOBAL
innodb_file_format_check=DEFAULT.
Changed the default values for innodb_max_dirty_pages_pct,
innodb_additional_mem_pool_size, innodb_buffer_pool_size, and
innodb_log_buffer_size.
Enabled group commit functionality that was broken in 5.0 when distributed transactions were
introduced. See Section 7.9, “Group Commit”.
Enabled the functionality of having multiple background threads, with two new configuration
parameters, innodb_read_io_threads and innodb_write_io_threads. The Windows only
parameter innodb_file_io_threads has been removed. See Section 7.8, “Multiple Background I/
O Threads”.
Changed the linear read ahead algorithm and disabled random read ahead. Also introduced a new
configuration parameter innodb_read_ahead_threshold to control the sensitivity of the linear read
ahead. See Section 7.7, “Changes in the Read Ahead Algorithm”.
Standardized comments that allow the extraction of documentation from code base with the Doxygen
tool.
Fixed a bug that could cause failures in secondary index lookups in consistent reads right after crash
recovery.
Corrected the estimation of space needed on a compressed page when performing an update by
delete-and-insert.
Removed the statically linked copies of the zlib and strings libraries from the binary Windows
plugin. Invoke the copies of these libraries in the mysqld executable, like the binary InnoDB Plugin
does on other platforms.
77
Changes in InnoDB Plugin 1.0.3 (March 11, 2009)
Trimmed the output of SHOW ENGINE INNODB MUTEX. See Section 8.8, “More Compact Output of
SHOW ENGINE INNODB MUTEX”.
On Microsoft Windows, make use of atomic memory access to implement mutexes and rw-locks more
efficiently. On Sun Solaris 10, if GCC built-in functions for atomic memory access are unavailable, use
library functions instead. See Section 7.2, “Faster Locking for Improved Scalability”.
Fixed MySQL Bug #44032: in ROW_FORMAT=REDUNDANT, update UTF-8 CHAR to/from NULL is not inplace.
Fixed MySQL Bug #43660: SHOW INDEXES/ANALYZE does not update cardinality for indexes of
InnoDB table.
Made the parameter innodb_change_buffering settable by mysqld start-up option. Due to a
programming mistake, it was only possible to set this parameter by the SET GLOBAL command in
InnoDB Plugin 1.0.3.
Added a parameter innodb_spin_wait_delay for controlling the polling of mutexes and rw-locks.
See Section 7.13, “Control of Spin Lock Polling”.
In consistent reads, issue an error message on attempts to use newly created indexes that may lack
required history. See Section 2.6, “Limitations”.
12.7 Changes in InnoDB Plugin 1.0.3 (March 11, 2009)
Improved the scalability of InnoDB on multi-core CPUs. See Section 7.2, “Faster Locking for Improved
Scalability”.
Added a parameter innodb_change_buffering for controlling the insert buffering. See Section 7.4,
“Controlling InnoDB Insert Buffering”.
Added a parameter innodb_use_sys_malloc for using an operating system memory allocation
rather than the InnoDB internal memory allocator. See Section 7.3, “Using Operating System Memory
Allocators”.
Made it possible to dynamically enable or disable adaptive hash indexing. See Section 7.5, “Controlling
Adaptive Hash Indexing”.
Changed the default value of innodb_thread_concurrency from 8 to 0, for unlimited concurrency
by default. See Section 7.6, “Changes Regarding Thread Concurrency”.
Fixed an issue that the InnoDB Plugin fails if innodb_buffer_pool_size is defined bigger than
4095M on 64-bit Windows.
Fixed MySQL Bug #41676: Table names are case insensitive in locking.
Fixed MySQL Bug #41904: Create unique index problem.
Fixed MySQL Bug #43043: Crash on BLOB delete operation.
Fixed a bug in recovery when dropping incomplete indexes left behind by fast index creation.
Fixed a crash bug when all rows of a compressed table are deleted.
Fixed a corruption bug when a table is dropped on a busy system that contains compressed tables.
Fixed an assertion failure involving the variable ut_total_allocated_memory that was caused by
unprotected access during fast index creation.
12.8 Changes in InnoDB Plugin 1.0.2 (December 1, 2008)
Implemented the dynamic plugin (ha_innodb.dll) on Windows.
78
Changes in InnoDB Plugin 1.0.1 (May 8, 2008)
Added a parameter innodb_stats_sample_pages for controlling the index cardinality estimates.
Made innodb_stats_on_metadata a settable global parameter. (MySQL Bug #38189)
Made innodb_lock_wait_timeout a settable session parameter. (MySQL Bug #36285)
Fixed bugs related to off-page columns (see Section 5.3, “DYNAMIC Row Format”).
Fixed various bugs related to compressed tables. This includes MySQL Bug #36172, a possible but
rare corruption, and an incompatible file format change relating to very long rows in compressed tables,
and to off-page storage of long column values.
Fixed a bug in crash recovery which was a side effect of incorrect implementation of the system
tablespace tagging.
Fixed MySQL bugs related to auto_increment columns: Bug #26316, Bug #35498, Bug #35602,
Bug #36411, Bug #37531, Bug #37788, Bug #38839, Bug #39830, Bug #40224.
Fixed some race conditions, hangs or crashes related to INFORMATION_SCHEMA tables, fast index
creation, and to the recovery of PREPARED transactions.
Fixed crashes on DROP TABLE or CREATE TABLE when there are FOREIGN KEY constraints. (MySQL
Bug #38786)
Fixed a crash caused by a conflict between TRUNCATE TABLE and LOCK TABLES. (MySQL Bug
#38231)
Fixed MySQL Bug #39939: DROP TABLE or DISCARD TABLESPACE takes a long time.
Fixed MySQL Bug #40359: InnoDB plugin error/warning message during shutdown.
Fixed MySQL Bug #40360: Binlog related errors with binlog off.
Applied all changes from MySQL through version 5.1.30.
12.9 Changes in InnoDB Plugin 1.0.1 (May 8, 2008)
Fixed bugs related to the packaging of the InnoDB Plugin: MySQL Bug #36222, Bug #36434.
Fixed crash bugs related to the new features of the InnoDB Plugin: MySQL Bug #36169, Bug #36310.
Implemented the system tablespace tagging discussed in Section 4.4.1, “Startup File Format
Compatibility Checking”.
Applied all changes from MySQL through version 5.1.25.
12.10 Changes in InnoDB Plugin 1.0.0 (April 15, 2008)
The initial release of the InnoDB Plugin is based on the built-in InnoDB in MySQL version 5.1. See
Section 1.2, “Features of the InnoDB Plugin” for the main features.
79
80
Appendix A Third-Party Software
Table of Contents
A.1 Performance Patches from Google ........................................................................................ 81
A.2 Multiple Background I/O Threads Patch from Percona ............................................................ 82
A.3 Performance Patches from Sun Microsystems ........................................................................ 82
Innobase Oy acknowledges that certain Third Party and Open Source software has been used to
develop or is incorporated in InnoDB (including the InnoDB Plugin). This appendix includes required
third-party license information.
A.1 Performance Patches from Google
Innobase Oy gratefully acknowledges the following contributions from Google, Inc. to improve InnoDB
performance:
• Replacing InnoDB's use of Pthreads mutexes with calls to GCC atomic builtins, as discussed in
Section 7.2, “Faster Locking for Improved Scalability”. This change means that InnoDB mutex and
rw-lock operations take less CPU time, and improves throughput on those platforms where the
atomic operations are available.
• Controlling master thread I/O rate, as discussed in Section 7.10, “Controlling the Master Thread I/
O Rate”. The master thread in InnoDB is a thread that performs various tasks in the background.
Historically, InnoDB has used a hard coded value as the total I/O capacity of the server. With this
change, user can control the number of I/O operations that can be performed per second based on
their own workload.
Changes from the Google contributions were incorporated in the following source code files:
btr0cur.c, btr0sea.c, buf0buf.c, buf0buf.ic, ha_innodb.cc, log0log.c, log0log.h,
os0sync.h, row0sel.c, srv0srv.c, srv0srv.h, srv0start.c, sync0arr.c, sync0rw.c,
sync0rw.h, sync0rw.ic, sync0sync.c, sync0sync.h, sync0sync.ic, and univ.i.
These contributions are incorporated subject to the conditions contained in the file COPYING.Google,
which are reproduced here.
Copyright (c) 2008, 2009, Google Inc.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following
disclaimer in the documentation and/or other materials
provided with the distribution.
* Neither the name of the Google Inc. nor the names of its
contributors may be used to endorse or promote products
derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
81
Multiple Background I/O Threads Patch from Percona
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
A.2 Multiple Background I/O Threads Patch from Percona
Innobase Oy gratefully acknowledges the contribution of Percona, Inc. to improve InnoDB performance
by implementing configurable background threads, as discussed in Section 7.8, “Multiple Background
I/O Threads”. InnoDB uses background threads to service various types of I/O requests. The change
provides another way to make InnoDB more scalable on high end systems.
Changes from the Percona, Inc. contribution were incorporated in the following source code files:
ha_innodb.cc, os0file.c, os0file.h, srv0srv.c, srv0srv.h, and srv0start.c.
This contribution is incorporated subject to the conditions contained in the file COPYING.Percona,
which are reproduced here.
Copyright (c) 2008, 2009, Percona Inc.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following
disclaimer in the documentation and/or other materials
provided with the distribution.
* Neither the name of the Percona Inc. nor the names of its
contributors may be used to endorse or promote products
derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
A.3 Performance Patches from Sun Microsystems
Innobase Oy gratefully acknowledges the following contributions from Sun Microsystems, Inc. to
improve InnoDB performance:
• Introducing the PAUSE instruction inside spin loops, as discussed in Section 7.12, “Using the
PAUSE instruction in InnoDB spin loops”. This change increases performance in high concurrency,
CPU-bound workloads.
• Enabling inlining of functions and prefetch with Sun Studio.
Changes from the Sun Microsystems, Inc. contribution were incorporated in the following source code
files: univ.i, ut0ut.c, and ut0ut.h.
This contribution is incorporated subject to the conditions contained in the file
COPYING.Sun_Microsystems, which are reproduced here.
82
Performance Patches from Sun Microsystems
Copyright (c) 2009, Sun Microsystems, Inc.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following
disclaimer in the documentation and/or other materials
provided with the distribution.
* Neither the name of Sun Microsystems, Inc. nor the names of its
contributors may be used to endorse or promote products
derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
83
84
Appendix B Using the InnoDB Plugin with MySQL 5.1.30 or
Earlier
Up to MySQL 5.1.30, the InnoDB Plugin replaced the built-in InnoDB in MySQL when the server was
started with the option skip_innodb. Due to MySQL Bug #42610, it was impossible to replace the
built-in InnoDB in MySQL with a plugin in MySQL 5.1.31 and 5.1.32. MySQL 5.1.33 introduced the
option ignore_builtin_innodb to allow InnoDB Plugin installation in the binary release.
Up to MySQL 5.1.30, installing the binary InnoDB Plugin requires that MySQL be shut down and
restarted after issuing the INSTALL PLUGIN statements. This is because the INSTALL PLUGIN
statement started the plugin with default options. The options would only be read from the option file
(my.cnf or my.ini) after restarting the server. The InnoDB Plugin worked around this limitation by
copying parameters from the internal data structures of the built-in InnoDB in MySQL. Beginning with
MySQL 5.1.33, the INSTALL PLUGIN statement re-reads the option file and passes all options to the
plugin, even those that are not recognized by the built-in InnoDB in MySQL.
To use the binary InnoDB Plugin with MySQL 5.1.30 or earlier, you may follow the instructions given
in Section 9.3, “Installing the Precompiled InnoDB Plugin as a Shared Library”, with one change:
Replace the option ignore_builtin_innodb with skip_innodb. The general steps for dynamically
installing the binary InnoDB Plugin are thus as follows:
• Make sure the MySQL server is not running, using a “slow” shutdown.
• Prepend each InnoDB option with loose_, e.g., loose_innodb_file_per_table instead of
innodb_file_per_table, so that MySQL starts even when InnoDB is unavailable.
• Add skip_innodb and default_storage_engine=MyISAM to the options, to prevent the built-in
InnoDB from starting.
• Start the MySQL server.
• INSTALL the InnoDB Plugin and the Information Schema tables, using the supplied script or
equivalent commands.
• Verify the installation of the plugins.
• Shut down and reconfigure the MySQL server by editing the appropriate configuration file to use
InnoDB as the default engine (if desired), and set appropriate configuration parameters to enable use
of new InnoDB Plugin features.
This change only affects the binary distributions of MySQL and InnoDB Plugin. The procedure for
building from source code is unchanged.
85
86
Appendix C List of Parameters Changed in the InnoDB Plugin
1.0
Table of Contents
C.1 New Parameters ................................................................................................................... 87
C.2 Deprecated Parameters ........................................................................................................ 89
C.3 Parameters with New Defaults .............................................................................................. 89
C.1 New Parameters
Throughout the course of development, the InnoDB Plugin has introduced new configuration
parameters. The following table summarizes those parameters:
Table C.1 InnoDB Plugin New Parameter Summary
Name
CmdLine
Option System Scope
File
Var
Dynamic Default
innodb_adaptive_flushing
YES
YES
YES
GLOBAL
YES
TRUE
innodb_change_buffering
YES
YES
YES
GLOBAL
YES
inserts
innodb_file_format
YES
YES
YES
GLOBAL
YES
Antelope
innodb_file_format_check
YES
YES
YES
GLOBAL
YES
ON
innodb_io_capacity
YES
YES
YES
GLOBAL
YES
200
innodb_old_blocks_pct
YES
YES
YES
GLOBAL
YES
37
innodb_old_blocks_time
YES
YES
YES
GLOBAL
YES
0
innodb_read_ahead_threshold
YES
YES
YES
GLOBAL
YES
56
innodb_read_io_threads
YES
YES
YES
GLOBAL
NO
4
innodb_spin_wait_delay
YES
YES
YES
GLOBAL
YES
6
innodb_stats_sample_pages
YES
YES
YES
GLOBAL
YES
8
innodb_strict_mode
YES
YES
YES
GLOBAL| YES
SESSION
FALSE
innodb_use_sys_malloc
YES
YES
YES
GLOBAL
NO
TRUE
innodb_write_io_threads
YES
YES
YES
GLOBAL
NO
4
innodb_adaptive_flushing
Whether InnoDB uses a new algorithm to estimate the
required rate of flushing. The default value is TRUE.
This parameter was added in InnoDB Plugin 1.0.4. See
Section 7.11, “Controlling the Flushing Rate of Dirty Pages”
for more information.
innodb_change_buffering
Whether InnoDB performs insert buffering. The default
value is "inserts" (buffer insert operations). This
parameter was added in InnoDB Plugin 1.0.3. See
Section 7.4, “Controlling InnoDB Insert Buffering” for more
information.
innodb_file_format
Whether to enable the new “Barracuda” file format. The
default value is “Antelope”. This parameter was added
87
New Parameters
in InnoDB Plugin 1.0.1. See Section 4.3, “Enabling File
Formats” for more information.
innodb_file_format_check
Whether InnoDB performs file format compatibility checking
when opening a database. The default value is ON.
This parameter was added in InnoDB Plugin 1.0.1. See
Section 4.4.1, “Startup File Format Compatibility Checking”
for more information.
innodb_io_capacity
The number of I/O operations that can be performed per
second. The allowable value range is any number 100 or
greater, and the default value is 200. This parameter was
added in InnoDB Plugin 1.0.4. To reproduce the earlier
behavior, use a value of 100. See Section 7.10, “Controlling
the Master Thread I/O Rate” for more information.
innodb_old_blocks_pct
Controls the desired percentage of “old” blocks in the
LRU list of the buffer pool. The default value is 37 and the
allowable value range is 5 to 95. This parameter was
added in InnoDB Plugin 1.0.5. See Section 7.14, “Making
Buffer Cache Scan Resistant” for more information.
innodb_old_blocks_time
The time in milliseconds since the first access to a block
during which it can be accessed again without being made
“young”. The default value is 0 which means that blocks
are moved to the “young” end of the LRU list at the first
access. This parameter was added in InnoDB Plugin 1.0.5.
See Section 7.14, “Making Buffer Cache Scan Resistant” for
more information.
innodb_read_ahead_threshold
Control the sensitivity of the linear read ahead. The
allowable value range is 0 to 64 and the default value is
56. This parameter was added in InnoDB Plugin 1.0.4. See
Section 7.7, “Changes in the Read Ahead Algorithm” for
more information.
innodb_read_io_threads
The number of background I/O threads used for reads. The
allowable value range is 1 to 64 and the default value is
4. This parameter was added in InnoDB Plugin 1.0.4. See
Section 7.8, “Multiple Background I/O Threads” for more
information.
innodb_spin_wait_delay
Maximum delay between polling for a spin lock. The
allowable value range is 0 (meaning unlimited) or positive
integers and the default value is 6. This parameter was
added in InnoDB Plugin 1.0.4. See Section 7.13, “Control of
Spin Lock Polling” for more information.
innodb_stats_sample_pages
The number of index pages to sample when calculating
statistics. The allowable value range is 1-unlimited and
the default value is 8. This parameter was added in InnoDB
Plugin 1.0.2. See Section 8.6, “Controlling Optimizer
Statistics Estimation” for more information.
innodb_strict_mode
Whether InnoDB raises error conditions in certain cases,
rather than issuing a warning. The default value is OFF.
This parameter was added in InnoDB Plugin 1.0.2. See
Section 8.5, “InnoDB Strict Mode” for more information.
innodb_use_sys_malloc
Whether InnoDB uses its own memory allocator or an
allocator of the operating system. The default value is ON
88
Deprecated Parameters
(use an allocator of the underlying system). This parameter
was added in InnoDB Plugin 1.0.3. See Section 7.3, “Using
Operating System Memory Allocators” for more information.
innodb_write_io_threads
The number of background I/O threads used for writes.
The allowable value range is 1 to 64 and the default value
is 4. This parameter was added in InnoDB Plugin 1.0.4. See
Section 7.8, “Multiple Background I/O Threads” for more
information.
C.2 Deprecated Parameters
Beginning in InnoDB Plugin 1.0.4 the following configuration parameter has been removed:
innodb_file_io_threads
This parameter has been replaced by two new
parameters innodb_read_io_threads and
innodb_write_io_threads. See Section 7.8, “Multiple
Background I/O Threads” for more information.
C.3 Parameters with New Defaults
For better out-of-the-box performance, InnoDB Plugin 1.0.4 changes the default values for the following
configuration parameters:
Table C.2 InnoDB Plugin Parameters with New Defaults
Name
Old Default
New Default
innodb_additional_mem_pool_size
1MB
8MB
innodb_buffer_pool_size
8MB
128MB
innodb_log_buffer_size
1MB
8MB
innodb_max_dirty_pages_pct
90
75
innodb_sync_spin_loops
20
30
8
0
innodb_thread_concurrency
89
90
InnoDB Glossary
These terms are commonly used in information about the InnoDB storage engine.
A
.ARM file
Metadata for ARCHIVE tables. Contrast with .ARZ file. Files with this extension are always included in
backups produced by the mysqlbackup command of the MySQL Enterprise Backup product.
See Also .ARZ file, MySQL Enterprise Backup, mysqlbackup command.
.ARZ file
Data for ARCHIVE tables. Contrast with .ARM file. Files with this extension are always included in backups
produced by the mysqlbackup command of the MySQL Enterprise Backup product.
See Also .ARM file, MySQL Enterprise Backup, mysqlbackup command.
ACID
An acronym standing for atomicity, consistency, isolation, and durability. These properties are all desirable
in a database system, and are all closely tied to the notion of a transaction. The transactional features of
InnoDB adhere to the ACID principles.
Transactions are atomic units of work that can be committed or rolled back. When a transaction makes
multiple changes to the database, either all the changes succeed when the transaction is committed, or all the
changes are undone when the transaction is rolled back.
The database remains in a consistent state at all times -- after each commit or rollback, and while transactions
are in progress. If related data is being updated across multiple tables, queries see either all old values or all
new values, not a mix of old and new values.
Transactions are protected (isolated) from each other while they are in progress; they cannot interfere with
each other or see each other's uncommitted data. This isolation is achieved through the locking mechanism.
Experienced users can adjust the isolation level, trading off less protection in favor of increased performance
and concurrency, when they can be sure that the transactions really do not interfere with each other.
The results of transactions are durable: once a commit operation succeeds, the changes made by that
transaction are safe from power failures, system crashes, race conditions, or other potential dangers that
many non-database applications are vulnerable to. Durability typically involves writing to disk storage, with a
certain amount of redundancy to protect against power failures or software crashes during write operations.
(In InnoDB, the doublewrite buffer assists with durability.)
See Also atomic, commit, concurrency, doublewrite buffer, isolation level, locking, rollback, transaction.
adaptive flushing
An algorithm for InnoDB tables that smooths out the I/O overhead introduced by checkpoints. Instead of
flushing all modified pages from the buffer pool to the data files at once, MySQL periodically flushes small
sets of modified pages. The adaptive flushing algorithm extends this process by estimating the optimal rate to
perform these periodic flushes, based on the rate of flushing and how fast redo information is generated. First
introduced in MySQL 5.1, in the InnoDB Plugin.
See Also buffer pool, checkpoint, data files, flush, InnoDB, page, redo log.
adaptive hash index
An optimization for InnoDB tables that can speed up lookups using = and IN operators, by constructing
a hash index in memory. MySQL monitors index searches for InnoDB tables, and if queries could
benefit from a hash index, it builds one automatically for index pages that are frequently accessed.
In a sense, the adaptive hash index configures MySQL at runtime to take advantage of ample main
memory, coming closer to the architecture of main-memory databases. This feature is controlled by the
innodb_adaptive_hash_index configuration option. Because this feature benefits some workloads
and not others, and the memory used for the hash index is reserved in the buffer pool, typically you should
benchmark with this feature both enabled and disabled.
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The hash index is always built based on an existing InnoDB secondary index, which is organized as a B-tree
structure. MySQL can build a hash index on a prefix of any length of the key defined for the B-tree, depending
on the pattern of searches against the index. A hash index can be partial; the whole B-tree index does not
need to be cached in the buffer pool.
In MySQL 5.6 and higher, another way to take advantage of fast single-value lookups with InnoDB tables is to
use the memcached interface to InnoDB. See InnoDB Integration with memcached for details.
See Also B-tree, buffer pool, hash index, memcached, page, secondary index.
AHI
Acronym for adaptive hash index.
See Also adaptive hash index.
AIO
Acronym for asynchronous I/O. You might see this acronym in InnoDB messages or keywords.
See Also asynchronous I/O.
Antelope
The code name for the original InnoDB file format. It supports the redundant and compact row formats, but
not the newer dynamic and compressed row formats available in the Barracuda file format.
If your application could benefit from InnoDB table compression, or uses BLOBs or large text columns that
could benefit from the dynamic row format, you might switch some tables to Barracuda format. You select the
file format to use by setting the innodb_file_format option before creating the table.
See Also Barracuda, compact row format, compressed row format, dynamic row format, file format,
innodb_file_format, redundant row format.
application programming interface (API)
A set of functions or procedures. An API provides a stable set of names and types for functions, procedures,
parameters, and return values.
apply
When a backup produced by the MySQL Enterprise Backup product does not include the most recent
changes that occurred while the backup was underway, the process of updating the backup files to include
those changes is known as the apply step. It is specified by the apply-log option of the mysqlbackup
command.
Before the changes are applied, we refer to the files as a raw backup. After the changes are applied, we refer
to the files as a prepared backup. The changes are recorded in the ibbackup_logfile file; once the apply
step is finished, this file is no longer necessary.
See Also hot backup, ibbackup_logfile, MySQL Enterprise Backup, prepared backup, raw backup.
asynchronous I/O
A type of I/O operation that allows other processing to proceed before the I/O is completed. Also known as
non-blocking I/O and abbreviated as AIO. InnoDB uses this type of I/O for certain operations that can run in
parallel without affecting the reliability of the database, such as reading pages into the buffer pool that have
not actually been requested, but might be needed soon.
Historically, InnoDB has used asynchronous I/O on Windows systems only. Starting with the InnoDB Plugin
1.1, InnoDB uses asynchronous I/O on Linux systems. This change introduces a dependency on libaio. On
other Unix-like systems, InnoDB uses synchronous I/O only.
See Also buffer pool, non-blocking I/O.
atomic
In the SQL context, transactions are units of work that either succeed entirely (when committed) or have
no effect at all (when rolled back). The indivisible ("atomic") property of transactions is the "A" in the acronym
ACID.
See Also ACID, commit, rollback, transaction.
atomic instruction
Special instructions provided by the CPU, to ensure that critical low-level operations cannot be interrupted.
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auto-increment
A property of a table column (specified by the AUTO_INCREMENT keyword) that automatically adds an
ascending sequence of values in the column. InnoDB supports auto-increment only for primary key columns.
It saves work for the developer, not to have to produce new unique values when inserting new rows. It
provides useful information for the query optimizer, because the column is known to be not null and with
unique values. The values from such a column can be used as lookup keys in various contexts, and because
they are auto-generated there is no reason to ever change them; for this reason, primary key columns are
often specified as auto-incrementing.
Auto-increment columns can be problematic with statement-based replication, because replaying the
statements on a slave might not produce the same set of column values as on the master, due to timing
issues. When you have an auto-incrementing primary key, you can use statement-based replication only
with the setting innodb_autoinc_lock_mode=1. If you have innodb_autoinc_lock_mode=2, which
allows higher concurrency for insert operations, use row-based replication rather than statement-based
replication. The setting innodb_autoinc_lock_mode=0 is the previous (traditional) default setting and
should not be used except for compatibility purposes.
See Also auto-increment locking, innodb_autoinc_lock_mode, primary key, row-based replication, statementbased replication.
auto-increment locking
The convenience of an auto-increment primary key involves some tradeoff with concurrency. In the simplest
case, if one transaction is inserting values into the table, any other transactions must wait to do their own
inserts into that table, so that rows inserted by the first transaction receive consecutive primary key values.
InnoDB includes optimizations, and the innodb_autoinc_lock_mode option, so that you can choose how
to trade off between predictable sequences of auto-increment values and maximum concurrency for insert
operations.
See Also auto-increment, concurrency, innodb_autoinc_lock_mode.
autocommit
A setting that causes a commit operation after each SQL statement. This mode is not recommended for
working with InnoDB tables with transactions that span several statements. It can help performance for
read-only transactions on InnoDB tables, where it minimizes overhead from locking and generation of
undo data, especially in MySQL 5.6.4 and up. It is also appropriate for working with MyISAM tables, where
transactions are not applicable.
See Also commit, locking, read-only transaction, SQL, transaction, undo.
availability
The ability to cope with, and if necessary recover from, failures on the host, including failures of MySQL, the
operating system, or the hardware and maintenance activity that may otherwise cause downtime. Often paired
with scalability as critical aspects of a large-scale deployment.
See Also scalability.
B
B-tree
A tree data structure that is popular for use in database indexes. The structure is kept sorted at all times,
enabling fast lookup for exact matches (equals operator) and ranges (for example, greater than, less than,
and BETWEEN operators). This type of index is available for most storage engines, such as InnoDB and
MyISAM.
Because B-tree nodes can have many children, a B-tree is not the same as a binary tree, which is limited to 2
children per node.
Contrast with hash index, which is only available in the MEMORY storage engine. The MEMORY storage
engine can also use B-tree indexes, and you should choose B-tree indexes for MEMORY tables if some
queries use range operators.
See Also hash index.
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backticks
Identifiers within MySQL SQL statements must be quoted using the backtick character (`) if they contain
special characters or reserved words. For example, to refer to a table named FOO#BAR or a column named
SELECT, you would specify the identifiers as `FOO#BAR` and `SELECT`. Since the backticks provide an
extra level of safety, they are used extensively in program-generated SQL statements, where the identifier
names might not be known in advance.
Many other database systems use double quotation marks (") around such special names. For portability, you
can enable ANSI_QUOTES mode in MySQL and use double quotation marks instead of backticks to qualify
identifier names.
See Also SQL.
backup
The process of copying some or all table data and metadata from a MySQL instance, for safekeeping. Can
also refer to the set of copied files. This is a crucial task for DBAs. The reverse of this process is the restore
operation.
With MySQL, physical backups are performed by the MySQL Enterprise Backup product, and logical
backups are performed by the mysqldump command. These techniques have different characteristics in
terms of size and representation of the backup data, and speed (especially speed of the restore operation).
Backups are further classified as hot, warm, or cold depending on how much they interfere with normal
database operation. (Hot backups have the least interference, cold backups the most.)
See Also cold backup, hot backup, logical backup, MySQL Enterprise Backup, mysqldump, physical backup,
warm backup.
Barracuda
The code name for an InnoDB file format that supports compression for table data. This file format was
first introduced in the InnoDB Plugin. It supports the compressed row format that enables InnoDB table
compression, and the dynamic row format that improves the storage layout for BLOB and large text columns.
You can select it through the innodb_file_format option.
Because the InnoDB system tablespace is stored in the original Antelope file format, to use the Barracuda
file format you must also enable the file-per-table setting, which puts newly created tables in their own
tablespaces separate from the system tablespace.
The MySQL Enterprise Backup product version 3.5 and above supports backing up tablespaces that use the
Barracuda file format.
See Also Antelope, compact row format, compressed row format, dynamic row format, file format, file-pertable, innodb_file_format, MySQL Enterprise Backup, row format, system tablespace.
beta
An early stage in the life of a software product, when it is available only for evaluation, typically without a
definite release number or a number less than 1. InnoDB does not use the beta designation, preferring an
early adopter phase that can extend over several point releases, leading to a GA release.
See Also early adopter, GA.
binary log
A file containing a record of all statements that attempt to change table data. These statements can be
replayed to bring slave servers up to date in a replication scenario, or to bring a database up to date after
restoring table data from a backup. The binary logging feature can be turned on and off, although Oracle
recommends always enabling it if you use replication or perform backups.
You can examine the contents of the binary log, or replay those statements during replication or recovery, by
using the mysqlbinlog command. For full information about the binary log, see The Binary Log. For MySQL
configuration options related to the binary log, see Binary Log Options and Variables.
For the MySQL Enterprise Backup product, the file name of the binary log and the current position within the
file are important details. To record this information for the master server when taking a backup in a replication
context, you can specify the --slave-info option.
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Prior to MySQL 5.0, a similar capability was available, known as the update log. In MySQL 5.0 and higher, the
binary log replaces the update log.
See Also binlog, MySQL Enterprise Backup, replication.
binlog
An informal name for the binary log file. For example, you might see this abbreviation used in e-mail
messages or forum discussions.
See Also binary log.
blind query expansion
A special mode of full-text search enabled by the WITH QUERY EXPANSION clause. It performs the search
twice, where the search phrase for the second search is the original search phrase concatenated with the
few most highly relevant documents from the first search. This technique is mainly applicable for short search
phrases, perhaps only a single word. It can uncover relevant matches where the precise search term does not
occur in the document.
See Also full-text search.
bottleneck
A portion of a system that is constrained in size or capacity, that has the effect of limiting overall throughput.
For example, a memory area might be smaller than necessary; access to a single required resource might
prevent multiple CPU cores from running simultaneously; or waiting for disk I/O to complete might prevent the
CPU from running at full capacity. Removing bottlenecks tends to improve concurrency. For example, the
ability to have multiple InnoDB buffer pool instances reduces contention when multiple sessions read from
and write to the buffer pool simultaneously.
See Also buffer pool, concurrency.
bounce
A shutdown operation immediately followed by a restart. Ideally with a relatively short warmup period so
that performance and throughput quickly return to a high level.
See Also shutdown.
buddy allocator
A mechanism for managing different-sized pages in the InnoDB buffer pool.
See Also buffer pool, page, page size.
buffer
A memory or disk area used for temporary storage. Data is buffered in memory so that it can be written
to disk efficiently, with a few large I/O operations rather than many small ones. Data is buffered on disk for
greater reliability, so that it can be recovered even when a crash or other failure occurs at the worst possible
time. The main types of buffers used by InnoDB are the buffer pool, the doublewrite buffer, and the insert
buffer.
See Also buffer pool, crash, doublewrite buffer, insert buffer.
buffer pool
The memory area that holds cached InnoDB data for both tables and indexes. For efficiency of high-volume
read operations, the buffer pool is divided into pages that can potentially hold multiple rows. For efficiency of
cache management, the buffer pool is implemented as a linked list of pages; data that is rarely used is aged
out of the cache, using a variation of the LRU algorithm. On systems with large memory, you can improve
concurrency by dividing the buffer pool into multiple buffer pool instances.
Several InnoDB status variables, information_schema tables, and performance_schema tables help
to monitor the internal workings of the buffer pool. Starting in MySQL 5.6, you can also dump and restore
the contents of the buffer pool, either automatically during shutdown and restart, or manually at any time,
through a set of InnoDB configuration variables such as innodb_buffer_pool_dump_at_shutdown and
innodb_buffer_pool_load_at_startup.
See Also buffer pool instance, LRU, page, warm up.
buffer pool instance
Any of the multiple regions into which the buffer pool can be divided, controlled by the
innodb_buffer_pool_instances configuration option. The total memory size specified by the
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innodb_buffer_pool_size is divided among all the instances. Typically, multiple buffer pool instances are
appropriate for systems devoting multiple gigabytes to the InnoDB buffer pool, with each instance 1 gigabyte
or larger. On systems loading or looking up large amounts of data in the buffer pool from many concurrent
sessions, having multiple instances reduces the contention for exclusive access to the data structures that
manage the buffer pool.
See Also buffer pool.
built-in
The built-in InnoDB storage engine within MySQL is the original form of distribution for the storage engine.
Contrast with the InnoDB Plugin. Starting with MySQL 5.5, the InnoDB Plugin is merged back into the
MySQL code base as the built-in InnoDB storage engine (known as InnoDB 1.1).
This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin
but not the built-in InnoDB, or vice versa.
See Also InnoDB, plugin.
business rules
The relationships and sequences of actions that form the basis of business software, used to run a
commercial company. Sometimes these rules are dictated by law, other times by company policy. Careful
planning ensures that the relationships encoded and enforced by the database, and the actions performed
through application logic, accurately reflect the real policies of the company and can handle real-life situations.
For example, an employee leaving a company might trigger a sequence of actions from the human resources
department. The human resources database might also need the flexibility to represent data about a person
who has been hired, but not yet started work. Closing an account at an online service might result in data
being removed from a database, or the data might be moved or flagged so that it could be recovered if the
account is re-opened. A company might establish policies regarding salary maximums, minimums, and
adjustments, in addition to basic sanity checks such as the salary not being a negative number. A retail
database might not allow a purchase with the same serial number to be returned more than once, or might not
allow credit card purchases above a certain value, while a database used to detect fraud might allow these
kinds of things.
See Also relational.
C
.cfg file
A metadata file used with the InnoDB transportable tablespace feature. It is produced by the command
FLUSH TABLES ... FOR EXPORT, puts one or more tables in a consistent state that can be copied to
another server. The .cfg file is copied along with the corresponding .ibd file, and used to adjust the internal
values of the .ibd file, such as the space ID, during the ALTER TABLE ... IMPORT TABLESPACE step.
See Also .ibd file, space ID, transportable tablespace.
cache
The general term for any memory area that stores copies of data for frequent or high-speed retrieval. In
InnoDB, the primary kind of cache structure is the buffer pool.
See Also buffer, buffer pool.
cardinality
The number of different values in a table column. When queries refer to columns that have an associated
index, the cardinality of each column influences which access method is most efficient. For example, for a
column with a unique constraint, the number of different values is equal to the number of rows in the table. If
a table has a million rows but only 10 different values for a particular column, each value occurs (on average)
100,000 times. A query such as SELECT c1 FROM t1 WHERE c1 = 50; thus might return 1 row or a huge
number of rows, and the database server might process the query differently depending on the cardinality of
c1.
If the values in a column have a very uneven distribution, the cardinality might not be a good way to determine
the best query plan. For example, SELECT c1 FROM t1 WHERE c1 = x; might return 1 row when x=50
and a million rows when x=30. In such a case, you might need to use index hints to pass along advice about
which lookup method is more efficient for a particular query.
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Cardinality can also apply to the number of distinct values present in multiple columns, as in a composite
index.
For InnoDB, the process of estimating cardinality for indexes is influenced by the
innodb_stats_sample_pages and the innodb_stats_on_metadata configuration options. The
estimated values are more stable when the persistent statistics feature is enabled (in MySQL 5.6 and
higher).
See Also column, composite index, index, index hint, persistent statistics, random dive, selectivity, unique
constraint.
change buffer
A special data structure that records changes to pages in secondary indexes. These values could result
from SQL INSERT, UPDATE, or DELETE statements (DML). The set of features involving the change buffer
is known collectively as change buffering, consisting of insert buffering, delete buffering, and purge
buffering.
Changes are only recorded in the change buffer when the relevant page from the secondary index is not in
the buffer pool. When the relevant index page is brought into the buffer pool while associated changes are
still in the change buffer, the changes for that page are applied in the buffer pool (merged) using the data
from the change buffer. Periodically, the purge operation that runs during times when the system is mostly
idle, or during a slow shutdown, writes the new index pages to disk. The purge operation can write the disk
blocks for a series of index values more efficiently than if each value were written to disk immediately.
Physically, the change buffer is part of the system tablespace, so that the index changes remain buffered
across database restarts. The changes are only applied (merged) when the pages are brought into the buffer
pool due to some other read operation.
The kinds and amount of data stored in the change buffer are governed by the innodb_change_buffering
and innodb_change_buffer_max_size configuration options. To see information about the current data
in the change buffer, issue the SHOW ENGINE INNODB STATUS command.
Formerly known as the insert buffer.
See Also buffer pool, change buffering, delete buffering, DML, insert buffer, insert buffering, merge, page,
purge, purge buffering, secondary index, system tablespace.
change buffering
The general term for the features involving the change buffer, consisting of insert buffering, delete
buffering, and purge buffering. Index changes resulting from SQL statements, which could normally
involve random I/O operations, are held back and performed periodically by a background thread.
This sequence of operations can write the disk blocks for a series of index values more efficiently than
if each value were written to disk immediately. Controlled by the innodb_change_buffering and
innodb_change_buffer_max_size configuration options.
See Also change buffer, delete buffering, insert buffering, purge buffering.
checkpoint
As changes are made to data pages that are cached in the buffer pool, those changes are written to the
data files sometime later, a process known as flushing. The checkpoint is a record of the latest changes
(represented by an LSN value) that have been successfully written to the data files.
See Also buffer pool, data files, flush, fuzzy checkpointing, LSN.
checksum
In InnoDB, a validation mechanism to detect corruption when a page in a tablespace is read from disk
into the InnoDB buffer pool. This feature is turned on and off by the innodb_checksums configuration
option. In MySQL 5.6, you can enable a faster checksum algorithm by also specifying the configuration option
innodb_checksum_algorithm=crc32.
The innochecksum command helps to diagnose corruption problems by testing the checksum values for a
specified tablespace file while the MySQL server is shut down.
MySQL also uses checksums for replication purposes. For details, see the configuration options
binlog_checksum, master_verify_checksum, and slave_sql_verify_checksum.
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See Also buffer pool, page, tablespace.
child table
In a foreign key relationship, a child table is one whose rows refer (or point) to rows in another table with an
identical value for a specific column. This is the table that contains the FOREIGN KEY ... REFERENCES
clause and optionally ON UPDATE and ON DELETE clauses. The corresponding row in the parent table
must exist before the row can be created in the child table. The values in the child table can prevent delete or
update operations on the parent table, or can cause automatic deletion or updates in the child table, based on
the ON CASCADE option used when creating the foreign key.
See Also foreign key, parent table.
clean page
A page in the InnoDB buffer pool where all changes made in memory have also been written (flushed) to
the data files. The opposite of a dirty page.
See Also buffer pool, data files, dirty page, flush, page.
clean shutdown
A shutdown that completes without errors and applies all changes to InnoDB tables before finishing, as
opposed to a crash or a fast shutdown. Synonym for slow shutdown.
See Also crash, fast shutdown, shutdown, slow shutdown.
client
A type of program that sends requests to a server, and interprets or processes the results. The client
software might run only some of the time (such as a mail or chat program), and might run interactively (such
as the mysql command processor).
See Also mysql, server.
clustered index
The InnoDB term for a primary key index. InnoDB table storage is organized based on the values of the
primary key columns, to speed up queries and sorts involving the primary key columns. For best performance,
choose the primary key columns carefully based on the most performance-critical queries. Because modifying
the columns of the clustered index is an expensive operation, choose primary columns that are rarely or never
updated.
In the Oracle Database product, this type of table is known as an index-organized table.
See Also index, primary key, secondary index.
cold backup
A backup taken while the database is shut down. For busy applications and web sites, this might not be
practical, and you might prefer a warm backup or a hot backup.
See Also backup, hot backup, warm backup.
column
A data item within a row, whose storage and semantics are defined by a data type. Each table and index is
largely defined by the set of columns it contains.
Each column has a cardinality value. A column can be the primary key for its table, or part of the primary
key. A column can be subject to a unique constraint, a NOT NULL constraint, or both. Values in different
columns, even across different tables, can be linked by a foreign key relationship.
In discussions of MySQL internal operations, sometimes field is used as a synonym.
See Also cardinality, foreign key, index, primary key, row, SQL, table, unique constraint.
column index
An index on a single column.
See Also composite index, index.
column prefix
When an index is created with a length specification, such as CREATE INDEX idx ON t1 (c1(N)), only
the first N characters of the column value are stored in the index. Keeping the index prefix small makes the
index compact, and the memory and disk I/O savings help performance. (Although making the index prefix too
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small can hinder query optimization by making rows with different values appear to the query optimizer to be
duplicates.)
For columns containing binary values or long text strings, where sorting is not a major consideration and
storing the entire value in the index would waste space, the index automatically uses the first N (typically 768)
characters of the value to do lookups and sorts.
See Also index.
commit
A SQL statement that ends a transaction, making permanent any changes made by the transaction. It is the
opposite of rollback, which undoes any changes made in the transaction.
InnoDB uses an optimistic mechanism for commits, so that changes can be written to the data files before
the commit actually occurs. This technique makes the commit itself faster, with the tradeoff that more work is
required in case of a rollback.
By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL
statement.
See Also autocommit, optimistic, rollback, SQL, transaction.
compact row format
The default InnoDB row format since MySQL 5.0.3. Available for tables that use the Antelope file format. It
has a more compact representation for nulls and variable-length fields than the prior default (redundant row
format).
Because of the B-tree indexes that make row lookups so fast in InnoDB, there is little if any performance
benefit to keeping all rows the same size.
For additional information about InnoDB COMPACT row format, see Section 5.2, “COMPACT and REDUNDANT
Row Formats”.
See Also Antelope, file format, redundant row format, row format.
composite index
An index that includes multiple columns.
See Also index, index prefix.
compressed backup
The compression feature of the MySQL Enterprise Backup product makes a compressed copy of each
tablespace, changing the extension from .ibd to .ibz. Compressing the backup data allows you to
keep more backups on hand, and reduces the time to transfer backups to a different server. The data is
uncompressed during the restore operation. When a compressed backup operation processes a table that is
already compressed, it skips the compression step for that table, because compressing again would result in
little or no space savings.
A set of files produced by the MySQL Enterprise Backup product, where each tablespace is compressed.
The compressed files are renamed with a .ibz file extension.
Applying compression right at the start of the backup process helps to avoid storage overhead during the
compression process, and to avoid network overhead when transferring the backup files to another server.
The process of applying the binary log takes longer, and requires uncompressing the backup files.
See Also apply, binary log, compression, hot backup, MySQL Enterprise Backup, tablespace.
compressed row format
A row format that enables data and index compression for InnoDB tables. It was introduced in the InnoDB
Plugin, available as part of the Barracuda file format. Large fields are stored away from the page that holds
the rest of the row data, as in dynamic row format. Both index pages and the large fields are compressed,
yielding memory and disk savings. Depending on the structure of the data, the decrease in memory and disk
usage might or might not outweigh the performance overhead of uncompressing the data as it is used. See
Chapter 3, InnoDB Data Compression for usage details.
For additional information about InnoDB COMPRESSED row format, see Section 5.3, “DYNAMIC Row Format”.
See Also Barracuda, compression, dynamic row format, row format.
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compression
A feature with wide-ranging benefits from using less disk space, performing less I/O, and using less memory
for caching. InnoDB table and index data can be kept in a compressed format during database operation.
The data is uncompressed when needed for queries, and re-compressed when changes are made by DML
operations. After you enable compression for a table, this processing is transparent to users and application
developers. DBAs can consult information_schema tables to monitor how efficiently the compression
parameters work for the MySQL instance and for particular compressed tables.
When InnoDB table data is compressed, the compression applies to the table itself, any associated index
data, and the pages loaded into the buffer pool. Compression does not apply to pages in the undo buffer.
The table compression feature requires using MySQL 5.5 or higher, or the InnoDB Plugin in
MySQL 5.1 or earlier, and creating the table using the Barracuda file format and compressed
row format, with the innodb_file_per_table setting turned on. The compression for each table is
influenced by the KEY_BLOCK_SIZE clause of the CREATE TABLE and ALTER TABLE statements.
In MySQL 5.6 and higher, compression is also affected by the server-wide configuration options
innodb_compression_failure_threshold_pct, innodb_compression_level, and
innodb_compression_pad_pct_max. See Chapter 3, InnoDB Data Compression for usage details.
Another type of compression is the compressed backup feature of the MySQL Enterprise Backup product.
See Also Barracuda, buffer pool, compressed row format, DML, hot backup, index,
INFORMATION_SCHEMA, innodb_file_per_table, plugin, table, undo buffer.
compression failure
Not actually an error, rather an expensive operation that can occur when using compression in combination
with DML operations. It occurs when: updates to a compressed page overflow the area on the page
reserved for recording modifications; the page is compressed again, with all changes applied to the table
data; the re-compressed data does not fit on the original page, requiring MySQL to split the data into
two new pages and compress each one separately. To check the frequency of this condition, query the
table INFORMATION_SCHEMA.INNODB_CMP and check how much the value of the COMPRESS_OPS
column exceeds the value of the COMPRESS_OPS_OK column. Ideally, compression failures do not
occur often; when they do, you can adjust the configuration options innodb_compression_level,
innodb_compression_failure_threshold_pct, and innodb_compression_pad_pct_max.
See Also compression, DML, page.
concatenated index
See composite index.
concurrency
The ability of multiple operations (in database terminology, transactions) to run simultaneously, without
interfering with each other. Concurrency is also involved with performance, because ideally the protection
for multiple simultaneous transactions works with a minimum of performance overhead, using efficient
mechanisms for locking.
See Also ACID, locking, transaction.
configuration file
The file that holds the option values used by MySQL at startup. Traditionally, on Linux and UNIX this file is
named my.cnf, and on Windows it is named my.ini. You can set a number of options related to InnoDB
under the [mysqld] section of the file.
Typically, this file is searched for in the locations /etc/my.cnf /etc/mysql/my.cnf /usr/local/
mysql/etc/my.cnf and ~/.my.cnf. See Using Option Files for details about the search path for this file.
When you use the MySQL Enterprise Backup product, you typically use two configuration files: one that
specifies where the data comes from and how it is structured (which could be the original configuration file
for your real server), and a stripped-down one containing only a small set of options that specify where the
backup data goes and how it is structured. The configuration files used with the MySQL Enterprise Backup
product must contain certain options that are typically left out of regular configuration files, so you might need
to add some options to your existing configuration file for use with MySQL Enterprise Backup.
See Also my.cnf, option file.
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consistent read
A read operation that uses snapshot information to present query results based on a point in time, regardless
of changes performed by other transactions running at the same time. If queried data has been changed by
another transaction, the original data is reconstructed based on the contents of the undo log. This technique
avoids some of the locking issues that can reduce concurrency by forcing transactions to wait for other
transactions to finish.
With the repeatable read isolation level, the snapshot is based on the time when the first read operation is
performed. With the read committed isolation level, the snapshot is reset to the time of each consistent read
operation.
Consistent read is the default mode in which InnoDB processes SELECT statements in READ COMMITTED
and REPEATABLE READ isolation levels. Because a consistent read does not set any locks on the tables
it accesses, other sessions are free to modify those tables while a consistent read is being performed on the
table.
For technical details about the applicable isolation levels, see Consistent Nonlocking Reads.
See Also ACID, concurrency, isolation level, locking, MVCC, READ COMMITTED, READ UNCOMMITTED,
REPEATABLE READ, SERIALIZABLE, transaction, undo log.
constraint
An automatic test that can block database changes to prevent data from becoming inconsistent. (In computer
science terms, a kind of assertion related to an invariant condition.) Constraints are a crucial component of
the ACID philosophy, to maintain data consistency. Constraints supported by MySQL include FOREIGN KEY
constraints and unique constraints.
See Also ACID, foreign key, relational, unique constraint.
counter
A value that is incremented by a particular kind of InnoDB operation. Useful for measuring how busy a
server is, troubleshooting the sources of performance issues, and testing whether changes (for example,
to configuration settings or indexes used by queries) have the desired low-level effects. Different kinds of
counters are available through performance_schema tables and information_schema tables, particularly
information_schema.innodb_metrics.
See Also INFORMATION_SCHEMA, metrics counter, Performance Schema.
covering index
An index that includes all the columns retrieved by a query. Instead of using the index values as pointers to
find the full table rows, the query returns values from the index structure, saving disk I/O. InnoDB can apply
this optimization technique to more indexes than MyISAM can, because InnoDB secondary indexes also
include the primary key columns. InnoDB cannot apply this technique for queries against tables modified by a
transaction, until that transaction ends.
Any column index or composite index could act as a covering index, given the right query. Design your
indexes and queries to take advantage of this optimization technique wherever possible.
See Also column index, composite index, index, secondary index.
crash
MySQL uses the term "crash" to refer generally to any unexpected shutdown operation where the server
cannot do its normal cleanup. For example, a crash could happen due to a hardware fault on the database
server machine or storage device; a power failure; a potential data mismatch that causes the MySQL server to
halt; a fast shutdown initiated by the DBA; or many other reasons. The robust, automatic crash recovery for
InnoDB tables ensures that data is made consistent when the server is restarted, without any extra work for
the DBA.
See Also crash recovery, fast shutdown, InnoDB, redo log, shutdown.
crash recovery
The cleanup activities that occur when MySQL is started again after a crash. For InnoDB tables, changes
from incomplete transactions are replayed using data from the redo log. Changes that were committed
before the crash, but not yet written into the data files, are reconstructed from the doublewrite buffer. When
the database is shut down normally, this type of activity is performed during shutdown by the purge operation.
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During normal operation, committed data can be stored in the change buffer for a period of time before being
written to the data files. There is always a tradeoff between keeping the data files up-to-date, which introduces
performance overhead during normal operation, and buffering the data, which can make shutdown and crash
recovery take longer.
See Also change buffer, commit, crash, data files, doublewrite buffer, InnoDB, purge, redo log.
CRUD
Acronym for "create, read, update, delete", a common sequence of operations in database applications.
Often denotes a class of applications with relatively simple database usage (basic DDL, DML and query
statements in SQL) that can be implemented quickly in any language.
See Also DDL, DML, query, SQL.
cursor
An internal data structure that is used to represent the result set of a query, or other operation that performs
a search using an SQL WHERE clause. It works like an iterator in other high-level languages, producing each
value from the result set as requested.
Although usually SQL handles the processing of cursors for you, you might delve into the inner workings when
dealing with performance-critical code.
See Also query.
D
data definition language
See DDL.
data dictionary
Metadata that keeps track of InnoDB-related objects such as tables, indexes, and table columns. This
metadata is physically located in the InnoDB system tablespace. For historical reasons, it overlaps to some
degree with information stored in the .frm files.
Because the MySQL Enterprise Backup product always backs up the system tablespace, all backups
include the contents of the data dictionary.
See Also column, .frm file, hot backup, index, MySQL Enterprise Backup, system tablespace, table.
data directory
The directory under which each MySQL instance keeps the data files for InnoDB and the directories
representing individual databases. Controlled by the datadir configuration option.
See Also data files, instance.
data files
The files that physically contain the InnoDB table and index data. There can be a one-to-many relationship
between data files and tables, as in the case of the system tablespace, which can hold multiple InnoDB
tables as well as the data dictionary. There can also be a one-to-one relationship between data files and
tables, as when the file-per-table setting is enabled, causing each newly created table to be stored in a
separate tablespace.
See Also data dictionary, file-per-table, index, system tablespace, table, tablespace.
data manipulation language
See DML.
data warehouse
A database system or application that primarily runs large queries. The read-only or read-mostly data might
be organized in denormalized form for query efficiency. Can benefit from the optimizations for read-only
transactions in MySQL 5.6 and higher. Contrast with OLTP.
See Also denormalized, OLTP, query, read-only transaction.
database
Within the MySQL data directory, each database is represented by a separate directory. The InnoDB
system tablespace, which can hold table data from multiple databases within a MySQL instance, is kept in
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its data files that reside outside the individual database directories. When file-per-table mode is enabled, the
.ibd files representing individual InnoDB tables are stored inside the database directories.
For long-time MySQL users, a database is a familiar notion. Users coming from an Oracle Database
background will find that the MySQL meaning of a database is closer to what Oracle Database calls a
schema.
See Also data files, file-per-table, .ibd file, instance, schema, system tablespace.
DCL
Data control language, a set of SQL statements for managing privileges. In MySQL, consists of the GRANT
and REVOKE statements. Contrast with DDL and DML.
See Also DDL, DML, SQL.
DDL
Data definition language, a set of SQL statements for manipulating the database itself rather than individual
table rows. Includes all forms of the CREATE, ALTER, and DROP statements. Also includes the TRUNCATE
statement, because it works differently than a DELETE FROM table_name statement, even though the
ultimate effect is similar.
DDL statements automatically commit the current transaction; they cannot be rolled back.
The InnoDB-related aspects of DDL include speed improvements for CREATE INDEX and DROP INDEX
statements, and the way the file-per-table setting affects the behavior of the TRUNCATE TABLE statement.
Contrast with DML and DCL.
See Also commit, DCL, DML, file-per-table, rollback, SQL, transaction.
deadlock
A situation where different transactions are unable to proceed, because each holds a lock that the other
needs. Because both transactions are waiting for a resource to become available, neither will ever release the
locks it holds.
A deadlock can occur when the transactions lock rows in multiple tables (through statements such as UPDATE
or SELECT ... FOR UPDATE), but in the opposite order. A deadlock can also occur when such statements
lock ranges of index records and gaps, with each transaction acquiring some locks but not others due to a
timing issue.
To reduce the possibility of deadlocks, use transactions rather than LOCK TABLE statements; keep
transactions that insert or update data small enough that they do not stay open for long periods of time; when
different transactions update multiple tables or large ranges of rows, use the same order of operations (such
as SELECT ... FOR UPDATE) in each transaction; create indexes on the columns used in SELECT ...
FOR UPDATE and UPDATE ... WHERE statements. The possibility of deadlocks is not affected by the
isolation level, because the isolation level changes the behavior of read operations, while deadlocks occur
because of write operations.
If a deadlock does occur, InnoDB detects the condition and rolls back one of the transactions (the victim).
Thus, even if your application logic is perfectly correct, you must still handle the case where a transaction
must be retried. To see the last deadlock in an InnoDB user transaction, use the command SHOW ENGINE
INNODB STATUS. If frequent deadlocks highlight a problem with transaction structure or application error
handling, run with the innodb_print_all_deadlocks setting enabled to print information about all
deadlocks to the mysqld error log.
For background information on how deadlocks are automatically detected and handled, see Deadlock
Detection and Rollback. For tips on avoiding and recovering from deadlock conditions, see How to Cope with
Deadlocks.
See Also concurrency, gap, isolation level, lock, locking, rollback, transaction, victim.
deadlock detection
A mechanism that automatically detects when a deadlock occurs, and automatically rolls back one of the
transactions involved (the victim).
See Also deadlock, rollback, transaction, victim.
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delete
When InnoDB processes a DELETE statement, the rows are immediately marked for deletion and no longer
are returned by queries. The storage is reclaimed sometime later, during the periodic garbage collection
known as the purge operation, performed by a separate thread. For removing large quantities of data, related
operations with their own performance characteristics are truncate and drop.
See Also drop, purge, truncate.
delete buffering
The technique of storing index changes due to DELETE operations in the insert buffer rather than writing
them immediately, so that the physical writes can be performed to minimize random I/O. (Because delete
operations are a two-step process, this operation buffers the write that normally marks an index record for
deletion.) It is one of the types of change buffering; the others are insert buffering and purge buffering.
See Also change buffer, change buffering, insert buffer, insert buffering, purge buffering.
denormalized
A data storage strategy that duplicates data across different tables, rather than linking the tables with foreign
keys and join queries. Typically used in data warehouse applications, where the data is not updated
after loading. In such applications, query performance is more important than making it simple to maintain
consistent data during updates. Contrast with normalized.
See Also data warehouse, normalized.
descending index
A type of index available with some database systems, where index storage is optimized to process ORDER
BY column DESC clauses. Currently, although MySQL allows the DESC keyword in the CREATE TABLE
statement, it does not use any special storage layout for the resulting index.
See Also index.
dirty page
A page in the InnoDB buffer pool that has been updated in memory, where the changes are not yet written
(flushed) to the data files. The opposite of a clean page.
See Also buffer pool, clean page, data files, flush, page.
dirty read
An operation that retrieves unreliable data, data that was updated by another transaction but not yet
committed. It is only possible with the isolation level known as read uncommitted.
This kind of operation does not adhere to the ACID principle of database design. It is considered very risky,
because the data could be rolled back, or updated further before being committed; then, the transaction
doing the dirty read would be using data that was never confirmed as accurate.
Its polar opposite is consistent read, where InnoDB goes to great lengths to ensure that a transaction does
not read information updated by another transaction, even if the other transaction commits in the meantime.
See Also ACID, commit, consistent read, isolation level, READ COMMITTED, READ UNCOMMITTED,
rollback.
disk-based
A kind of database that primarily organizes data on disk storage (hard drives or equivalent). Data is brought
back and forth between disk and memory to be operated upon. It is the opposite of an in-memory database.
Although InnoDB is disk-based, it also contains features such as the buffer pool, multiple buffer pool
instances, and the adaptive hash index that allow certain kinds of workloads to work primarily from memory.
See Also adaptive hash index, buffer pool, in-memory database.
disk-bound
A type of workload where the primary bottleneck is CPU operations in memory. Typically involves readintensive operations where the results can all be cached in the buffer pool.
See Also bottleneck, buffer pool, disk-bound, workload.
disk-bound
A type of workload where the primary bottleneck is disk I/O. (Also known as I/O-bound.) Typically involves
frequent writes to disk, or random reads of more data than can fit into the buffer pool.
See Also bottleneck, buffer pool, disk-bound, workload.
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DML
Data manipulation language, a set of SQL statements for performing insert, update, and delete operations.
The SELECT statement is sometimes considered as a DML statement, because the SELECT ... FOR
UPDATE form is subject to the same considerations for locking as INSERT, UPDATE, and DELETE.
DML statements for an InnoDB table operate in the context of a transaction, so their effects can be
committed or rolled back as a single unit.
Contrast with DDL and DCL.
See Also commit, DCL, DDL, locking, rollback, SQL, transaction.
document id
In the InnoDB full-text search feature, a special column in the table containing the FULLTEXT index,
to uniquely identify the document associated with each ilist value. Its name is FTS_DOC_ID (uppercase
required). The column itself must be of BIGINT UNSIGNED NOT NULL type, with a unique index named
FTS_DOC_ID_INDEX. Preferably, you define this column when creating the table. If InnoDB must add the
column to the table while creating a FULLTEXT index, the indexing operation is considerably more expensive.
See Also full-text search, FULLTEXT index, ilist.
doublewrite buffer
InnoDB uses a novel file flush technique called doublewrite. Before writing pages to the data files, InnoDB
first writes them to a contiguous area called the doublewrite buffer. Only after the write and the flush to the
doublewrite buffer have completed, does InnoDB write the pages to their proper positions in the data file. If the
operating system crashes in the middle of a page write, InnoDB can later find a good copy of the page from
the doublewrite buffer during crash recovery.
Although data is always written twice, the doublewrite buffer does not require twice as much I/O overhead
or twice as many I/O operations. Data is written to the buffer itself as a large sequential chunk, with a single
fsync() call to the operating system.
To turn off the doublewrite buffer, specify the option innodb_doublewrite=0.
See Also crash recovery, data files, page, purge.
drop
A kind of DDL operation that removes a schema object, through a statement such as DROP TABLE
or DROP INDEX. It maps internally to an ALTER TABLE statement. From an InnoDB perspective, the
performance considerations of such operations involve the time that the data dictionary is locked to ensure
that interrelated objects are all updated, and the time to update memory structures such as the buffer pool.
For a table, the drop operation has somewhat different characteristics than a truncate operation (TRUNCATE
TABLE statement).
See Also buffer pool, data dictionary, DDL, table, truncate.
dynamic row format
A row format introduced in the InnoDB Plugin, available as part of the Barracuda file format. Because TEXT
and BLOB fields are stored outside of the rest of the page that holds the row data, it is very efficient for rows
that include large objects. Since the large fields are typically not accessed to evaluate query conditions, they
are not brought into the buffer pool as often, resulting in fewer I/O operations and better utilization of cache
memory.
For additional information about InnoDB DYNAMIC row format, see Section 5.3, “DYNAMIC Row Format”.
See Also Barracuda, buffer pool, file format, row format.
E
early adopter
A stage similar to beta, when a software product is typically evaluated for performance, functionality, and
compatibility in a non-mission-critical setting. InnoDB uses the early adopter designation rather than beta,
through a succession of point releases leading up to a GA release.
See Also beta, GA.
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error log
A type of log showing information about MySQL startup and critical runtime errors and crash information. For
details, see The Error Log.
See Also crash, log.
eviction
The process of removing an item from a cache or other temporary storage area, such as the InnoDB buffer
pool. Often, but not always, uses the LRU algorithm to determine which item to remove. When a dirty page
is evicted, its contents are flushed to disk, and any dirty neighbor pages might be flushed also.
See Also buffer pool, dirty page, flush, LRU.
exclusive lock
A kind of lock that prevents any other transaction from locking the same row. Depending on the transaction
isolation level, this kind of lock might block other transactions from writing to the same row, or might also
block other transactions from reading the same row. The default InnoDB isolation level, REPEATABLE
READ, enables higher concurrency by allowing transactions to read rows that have exclusive locks, a
technique known as consistent read.
See Also concurrency, consistent read, isolation level, lock, REPEATABLE READ, shared lock, transaction.
extent
A group of pages within a tablespace totaling 1 megabyte. With the default page size of 16KB, an extent
contains 64 pages. In MySQL 5.6, the page size can also be 4KB or 8KB, in which case an extent contains
more pages, still adding up to 1MB.
InnoDB features such as segments, read-ahead requests and the doublewrite buffer use I/O operations
that read, write, allocate, or free data one extent at a time.
See Also doublewrite buffer, neighbor page, page, page size, read-ahead, segment, tablespace.
F
.frm file
A file containing the metadata, such as the table definition, of a MySQL table.
For backups, you must always keep the full set of .frm files along with the backup data to be able to restore
tables that are altered or dropped after the backup.
Although each InnoDB table has a .frm file, InnoDB maintains its own table metadata in the system
tablespace; the .frm files are not needed for InnoDB to operate on InnoDB tables.
These files are backed up by the MySQL Enterprise Backup product. These files must not be modified by
an ALTER TABLE operation while the backup is taking place, which is why backups that include non-InnoDB
tables perform a FLUSH TABLES WITH READ LOCK operation to freeze such activity while backing up the
.frm files. Restoring a backup can result in .frm files being created, changed, or removed to match the state
of the database at the time of the backup.
See Also MySQL Enterprise Backup.
Fast Index Creation
A capability first introduced in the InnoDB Plugin, now part of the MySQL server in 5.5 and higher, that
speeds up creation of InnoDB secondary indexes by avoiding the need to completely rewrite the associated
table. The speedup applies to dropping secondary indexes also.
Because index maintenance can add performance overhead to many data transfer operations, consider doing
operations such as ALTER TABLE ... ENGINE=INNODB or INSERT INTO ... SELECT * FROM ...
without any secondary indexes in place, and creating the indexes afterward.
In MySQL 5.6, this feature becomes more general: you can read and write to tables while an index is being
created, and many more kinds of ALTER TABLE operations can be performed without copying the table,
without blocking DML operations, or both. Thus in MySQL 5.6 and higher, we typically refer to this set of
features as online DDL rather than Fast Index Creation.
See Also DML, index, online DDL, secondary index.
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fast shutdown
The default shutdown procedure for InnoDB, based on the configuration setting
innodb_fast_shutdown=1. To save time, certain flush operations are skipped. This type of shutdown
is safe during normal usage, because the flush operations are performed during the next startup, using the
same mechanism as in crash recovery. In cases where the database is being shut down for an upgrade or
downgrade, do a slow shutdown instead to ensure that all relevant changes are applied to the data files
during the shutdown.
See Also crash recovery, data files, flush, shutdown, slow shutdown.
file format
The format used by InnoDB for each table, typically with the file-per-table setting enabled so that each table
is stored in a separate .ibd file. Currently, the file formats available in InnoDB are known as Antelope and
Barracuda. Each file format supports one or more row formats. The row formats available for Barracuda
tables, COMPRESSED and DYNAMIC, enable important new storage features for InnoDB tables.
See Also Antelope, Barracuda, file-per-table, .ibd file, ibdata file, row format.
file-per-table
A general name for the setting controlled by the innodb_file_per_table option. That is a very important
configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O
characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by
default.
For each table created while this setting is in effect, the data is stored in a separate .ibd file rather than
in the ibdata files of the system tablespace. When table data is stored in individual files, you have more
flexibility to choose nondefault file formats and row formats, which are required for features such as data
compression. The TRUNCATE TABLE operation is also much faster, and the reclaimed space can be used
by the operating system rather than remaining reserved for InnoDB.
The MySQL Enterprise Backup product is more flexible for tables that are in their own files. For example,
tables can be excluded from a backup, but only if they are in separate files. Thus, this setting is suitable for
tables that are backed up less frequently or on a different schedule.
See Also compressed row format, compression, file format, .ibd file, ibdata file, innodb_file_per_table, row
format, system tablespace.
fill factor
In an InnoDB index, the proportion of a page that is taken up by index data before the page is split. The
unused space when index data is first divided between pages allows for rows to be updated with longer
string values without requiring expensive index maintenance operations. If the fill factor is too low, the index
consumes more space than needed, causing extra I/O overhead when reading the index. If the fill factor
is too high, any update that increases the length of column values can cause extra I/O overhead for index
maintenance. See Physical Structure of an InnoDB Index for more information.
See Also index, page.
fixed row format
This row format is used by the MyISAM storage engine, not by InnoDB. If you create an InnoDB table with the
option row_format=fixed, InnoDB translates this option to use the compact row format instead, although
the fixed value might still show up in output such as SHOW TABLE STATUS reports.
See Also compact row format, row format.
flush
To write changes to the database files, that had been buffered in a memory area or a temporary disk storage
area. The InnoDB storage structures that are periodically flushed include the redo log, the undo log, and the
buffer pool.
Flushing can happen because a memory area becomes full and the system needs to free some space,
because a commit operation means the changes from a transaction can be finalized, or because a slow
shutdown operation means that all outstanding work should be finalized. When it is not critical to flush all
the buffered data at once, InnoDB can use a technique called fuzzy checkpointing to flush small batches of
pages to spread out the I/O overhead.
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See Also buffer pool, commit, fuzzy checkpointing, neighbor page, redo log, slow shutdown, undo log.
flush list
An internal InnoDB data structure that tracks dirty pages in the buffer pool: that is, pages that have been
changed and need to be written back out to disk. This data structure is updated frequently by InnoDB's
internal mini-transactions, and so is protected by its own mutex to allow concurrent access to the buffer
pool.
See Also buffer pool, dirty page, LRU, mini-transaction, mutex, page, page cleaner.
foreign key
A type of pointer relationship, between rows in separate InnoDB tables. The foreign key relationship is
defined on one column in both the parent table and the child table.
In addition to enabling fast lookup of related information, foreign keys help to enforce referential integrity,
by preventing any of these pointers from becoming invalid as data is inserted, updated, and deleted. This
enforcement mechanism is a type of constraint. A row that points to another table cannot be inserted if
the associated foreign key value does not exist in the other table. If a row is deleted or its foreign key value
changed, and rows in another table point to that foreign key value, the foreign key can be set up to prevent
the deletion, cause the corresponding column values in the other table to become null, or automatically delete
the corresponding rows in the other table.
One of the stages in designing a normalized database is to identify data that is duplicated, separate that data
into a new table, and set up a foreign key relationship so that the multiple tables can be queried like a single
table, using a join operation.
See Also child table, FOREIGN KEY constraint, join, normalized, NULL, parent table, referential integrity,
relational.
FOREIGN KEY constraint
The type of constraint that maintains database consistency through a foreign key relationship. Like other
kinds of constraints, it can prevent data from being inserted or updated if data would become inconsistent; in
this case, the inconsistency being prevented is between data in multiple tables. Alternatively, when a DML
operation is performed, FOREIGN KEY constraints can cause data in child rows to be deleted, changed to
different values, or set to null, based on the ON CASCADE option specified when creating the foreign key.
See Also child table, constraint, DML, foreign key, NULL.
FTS
In most contexts, an acronym for full-text search. Sometimes in performance discussions, an acronym for
full table scan.
See Also full table scan, full-text search.
full backup
A backup that includes all the tables in each MySQL database, and all the databases in a MySQL instance.
Contrast with partial backup.
See Also backup, database, instance, partial backup, table.
full table scan
An operation that requires reading the entire contents of a table, rather than just selected portions using an
index. Typically performed either with small lookup tables, or in data warehousing situations with large tables
where all available data is aggregated and analyzed. How frequently these operations occur, and the sizes of
the tables relative to available memory, have implications for the algorithms used in query optimization and
managing the buffer pool.
The purpose of indexes is to allow lookups for specific values or ranges of values within a large table, thus
avoiding full table scans when practical.
See Also buffer pool, index, LRU.
full-text search
The MySQL feature for finding words, phrases, Boolean combinations of words, and so on within table data,
in a faster, more convenient, and more flexible way than using the SQL LIKE operator or writing your own
application-level search algorithm. It uses the SQL function MATCH() and FULLTEXT indexes.
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See Also FULLTEXT index.
FULLTEXT index
The special kind of index that holds the search index in the MySQL full-text search mechanism.
Represents the words from values of a column, omitting any that are specified as stopwords. Originally, only
available for MyISAM tables. Starting in MySQL 5.6.4, it is also available for InnoDB tables.
See Also full-text search, index, InnoDB, search index, stopword.
fuzzy checkpointing
A technique that flushes small batches of dirty pages from the buffer pool, rather than flushing all dirty
pages at once which would disrupt database processing.
See Also buffer pool, dirty page, flush.
G
GA
"Generally available", the stage when a software product leaves beta and is available for sale, official
support, and production use.
See Also beta, early adopter.
gap
A place in an InnoDB index data structure where new values could be inserted. When you lock a set of rows
with a statement such as SELECT ... FOR UPDATE, InnoDB can create locks that apply to the gaps as
well as the actual values in the index. For example, if you select all values greater than 10 for update, a gap
lock prevents another transaction from inserting a new value that is greater than 10. The supremum record
and infimum record represent the gaps containing all values greater than or less than all the current index
values.
See Also concurrency, gap lock, index, infimum record, isolation level, supremum record.
gap lock
A lock on a gap between index records, or a lock on the gap before the first or after the last index record.
For example, SELECT c1 FOR UPDATE FROM t WHERE c1 BETWEEN 10 and 20; prevents other
transactions from inserting a value of 15 into the column t.c1, whether or not there was already any such
value in the column, because the gaps between all existing values in the range are locked. Contrast with
record lock and next-key lock.
Gap locks are part of the tradeoff between performance and concurrency, and are used in some transaction
isolation levels and not others.
See Also gap, infimum record, lock, next-key lock, record lock, supremum record.
general log
See general query log.
general query log
A type of log used for diagnosis and troubleshooting of SQL statements processed by the MySQL server.
Can be stored in a file or in a database table. You must enable this feature through the general_log
configuration option to use it. You can disable it for a specific connection through the sql_log_off
configuration option.
Records a broader range of queries than the slow query log. Unlike the binary log, which is used for
replication, the general query log contains SELECT statements and does not maintain strict ordering. For more
information, see The General Query Log.
See Also binary log, general query log, log.
global_transaction
A type of transaction involved in XA operations. It consists of several actions that are transactional in
themselves, but that all must either complete successfully as a group, or all be rolled back as a group. In
essence, this extends ACID properties "up a level" so that multiple ACID transactions can be executed
in concert as components of a global operation that also has ACID properties. For this type of distributed
transaction, you must use the SERIALIZABLE isolation level to achieve ACID properties.
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See Also ACID, SERIALIZABLE, transaction, XA.
group commit
An InnoDB optimization that performs some low-level I/O operations (log write) once for a set of commit
operations, rather than flushing and syncing separately for each commit.
When the binlog is enabled, you typically also set the configuration option sync_binlog=0, because group
commit for the binary log is only supported if it is set to 0.
See Also commit, plugin, XA.
H
hash index
A type of index intended for queries that use equality operators, rather than range operators such as greaterthan or BETWEEN. It is available for MEMORY tables. Although hash indexes are the default for MEMORY
tables for historic reasons, that storage engine also supports B-tree indexes, which are often a better choice
for general-purpose queries.
MySQL includes a variant of this index type, the adaptive hash index, that is constructed automatically for
InnoDB tables if needed based on runtime conditions.
See Also adaptive hash index, B-tree, index, InnoDB.
HDD
Acronym for "hard disk drive". Refers to storage media using spinning platters, usually when comparing and
contrasting with SSD. Its performance characteristics can influence the throughput of a disk-based workload.
See Also disk-based, SSD.
heartbeat
A periodic message that is sent to indicate that a system is functioning properly. In a replication context, if
the master stops sending such messages, one of the slaves can take its place. Similar techniques can be
used between the servers in a cluster environment, to confirm that all of them are operating properly.
See Also replication.
high-water mark
A value representing an upper limit, either a hard limit that should not be exceeded at runtime, or a record of
the maximum value that was actually reached. Contrast with low-water mark.
See Also low-water mark.
history list
A list of transactions with delete-marked records scheduled to be processed by the InnoDB purge
operation. Recorded in the undo log. The length of the history list is reported by the command SHOW
ENGINE INNODB STATUS. If the history list grows longer than the value of the innodb_max_purge_lag
configuration option, each DML operation is delayed slightly to allow the purge operation to finish flushing the
deleted records.
Also known as purge lag.
See Also flush, purge, purge lag, rollback segment, transaction, undo log.
hot
A condition where a row, table, or internal data structure is accessed so frequently, requiring some form of
locking or mutual exclusion, that it results in a performance or scalability issue.
Although "hot" typically indicates an undesirable condition, a hot backup is the preferred type of backup.
See Also hot backup.
hot backup
A backup taken while the database and is running and applications are reading and writing to it. The backup
involves more than simply copying data files: it must include any data that was inserted or updated while the
backup was in process; it must exclude any data that was deleted while the backup was in process; and it
must ignore any changes that were not committed.
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The Oracle product that performs hot backups, of InnoDB tables especially but also tables from MyISAM and
other storage engines, is known as MySQL Enterprise Backup.
The hot backup process consists of two stages. The initial copying of the data files produces a raw backup.
The apply step incorporates any changes to the database that happened while the backup was running.
Applying the changes produces a prepared backup; these files are ready to be restored whenever necessary.
See Also apply, MySQL Enterprise Backup, prepared backup, raw backup.
I
.ibd file
Each InnoDB table created using the file-per-table mode goes into its own tablespace file, with a .ibd
extension, inside the database directory. This file contains the table data and any indexes for the table. Fileper-table mode, controlled by the innodb_file_per_table option, affects many aspects of InnoDB storage
usage and performance, and is enabled by default in MySQL 5.6.7 and higher.
This extension does not apply to the system tablespace, which consists of the ibdata files.
When a .ibd file is included in a compressed backup by the MySQL Enterprise Backup product, the
compressed equivalent is a .ibz file.
If a table is create with the DATA DIRECTORY = clause in MySQL 5.6 and higher, the .ibd file is located
outside the normal database directory, and is pointed to by a .isl file.
See Also database, file-per-table, ibdata file, .ibz file, index, innodb_file_per_table, .isl file, MySQL Enterprise
Backup, system tablespace, table, tablespace.
.ibz file
When the MySQL Enterprise Backup product performs a compressed backup, it transforms each
tablespace file that is created using the file-per-table setting from a .ibd extension to a .ibz extension.
The compression applied during backup is distinct from the compressed row format that keeps table data
compressed during normal operation. A compressed backup operation skips the compression step for a
tablespace that is already in compressed row format, as compressing a second time would slow down the
backup but produce little or no space savings.
See Also compressed backup, compressed row format, file-per-table, .ibd file, MySQL Enterprise Backup,
tablespace.
.isl file
A file that specifies the location of a .ibd file for an InnoDB table created with the DATA DIRECTORY =
clause in MySQL 5.6 and higher. It functions like a symbolic link, without the platform restrictions of the actual
symbolic link mechanism. You can store InnoDB tablespaces outside the database directory, for example,
on an especially large or fast storage device depending on the usage of the table. For details, see Specifying
the Location of a Tablespace.
See Also database, .ibd file, table, tablespace.
I/O-bound
See disk-bound.
ib-file set
The set of files managed by InnoDB within a MySQL database: the system tablespace, any file-per-table
tablespaces, and the (typically 2) redo log files. Used sometimes in detailed discussions of InnoDB file
structures and formats, to avoid ambiguity between the meanings of database between different DBMS
products, and the non-InnoDB files that may be part of a MySQL database.
See Also database, file-per-table, redo log, system tablespace.
ibbackup_logfile
A supplemental backup file created by the MySQL Enterprise Backup product during a hot backup
operation. It contains information about any data changes that occurred while the backup was running. The
initial backup files, including ibbackup_logfile, are known as a raw backup, because the changes that
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occurred during the backup operation are not yet incorporated. After you perform the apply step to the raw
backup files, the resulting files do include those final data changes, and are known as a prepared backup. At
this stage, the ibbackup_logfile file is no longer necessary.
See Also apply, hot backup, MySQL Enterprise Backup, prepared backup, raw backup.
ibdata file
A set of files with names such as ibdata1, ibdata2, and so on, that make up the InnoDB system
tablespace. These files contain metadata about InnoDB tables, (the data dictionary), and the storage areas
for the undo log, the change buffer, and the doublewrite buffer. They also can contain some or all of
the table data also (depending on whether the file-per-table mode is in effect when each table is created).
When the innodb_file_per_table option is enabled, data and indexes for newly created tables are stored in
separate .ibd files rather than in the system tablespace.
The growth of the ibdata files is influenced by the innodb_autoextend_increment configuration option.
See Also change buffer, data dictionary, doublewrite buffer, file-per-table, .ibd file, innodb_file_per_table,
system tablespace, undo log.
ibtmp file
The InnoDB temporary tablespace data file for non-compressed InnoDB temporary tables and related
objects. The configuration file option, innodb_temp_data_file_path, allows users to define a relative
path for the temporary data file. If innodb_temp_data_file_path is not specified, the default behavior is
to create a single auto- extending 12MB data file named ibtmp1 in the data directory, alongside ibdata1.
See Also temporary tablespace.
ib_logfile
A set of files, typically named ib_logfile0 and ib_logfile1, that form the redo log. Also sometimes
referred to as the log group. These files record statements that attempt to change data in InnoDB tables.
These statements are replayed automatically to correct data written by incomplete transactions, on startup
following a crash.
This data cannot be used for manual recovery; for that type of operation, use the binary log.
See Also binary log, log group, redo log.
ilist
Within an InnoDB FULLTEXT index, the data structure consisting of a document ID and positional
information for a token (that is, a particular word).
See Also FULLTEXT index.
implicit row lock
A row lock that InnoDB acquires to ensure consistency, without you specifically requesting it.
See Also row lock.
in-memory database
A type of database system that maintains data in memory, to avoid overhead due to disk I/O and translation
between disk blocks and memory areas. Some in-memory databases sacrifice durability (the "D" in the ACID
design philosophy) and are vulnerable to hardware, power, and other types of failures, making them more
suitable for read-only operations. Other in-memory databases do use durability mechanisms such as logging
changes to disk or using non-volatile memory.
MySQL features that are address the same kinds of memory-intensive processing include the InnoDB buffer
pool, adaptive hash index, and read-only transaction optimization, the MEMORY storage engine, the
MyISAM key cache, and the MySQL query cache.
See Also ACID, adaptive hash index, buffer pool, disk-based, read-only transaction.
incremental backup
A type of hot backup, performed by the MySQL Enterprise Backup product, that only saves data changed
since some point in time. Having a full backup and a succession of incremental backups lets you reconstruct
backup data over a long period, without the storage overhead of keeping several full backups on hand. You
can restore the full backup and then apply each of the incremental backups in succession, or you can keep
the full backup up-to-date by applying each incremental backup to it, then perform a single restore operation.
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The granularity of changed data is at the page level. A page might actually cover more than one row. Each
changed page is included in the backup.
See Also hot backup, MySQL Enterprise Backup, page.
index
A data structure that provides a fast lookup capability for rows of a table, typically by forming a tree structure
(B-tree) representing all the values of a particular column or set of columns.
InnoDB tables always have a clustered index representing the primary key. They can also have one or
more secondary indexes defined on one or more columns. Depending on their structure, secondary indexes
can be classified as partial, column, or composite indexes.
Indexes are a crucial aspect of query performance. Database architects design tables, queries, and indexes
to allow fast lookups for data needed by applications. The ideal database design uses a covering index
where practical; the query results are computed entirely from the index, without reading the actual table
data. Each foreign key constraint also requires an index, to efficiently check whether values exist in both the
parent and child tables.
Although a B-tree index is the most common, a different kind of data structure is used for hash indexes, as in
the MEMORY storage engine and the InnoDB adaptive hash index.
See Also adaptive hash index, B-tree, child table, clustered index, column index, composite index, covering
index, foreign key, hash index, parent table, partial index, primary key, query, row, secondary index, table.
index cache
A memory area that holds the token data for InnoDB full-text search. It buffers the data to minimize disk I/
O when data is inserted or updated in columns that are part of a FULLTEXT index. The token data is written
to disk when the index cache becomes full. Each InnoDB FULLTEXT index has its own separate index cache,
whose size is controlled by the configuration option innodb_ft_cache_size.
See Also full-text search, FULLTEXT index.
index hint
Extended SQL syntax for overriding the indexes recommended by the optimizer. For example, the FORCE
INDEX, USE INDEX, and IGNORE INDEX clauses. Typically used when indexed columns have unevenly
distributed values, resulting in inaccurate cardinality estimates.
See Also cardinality, index.
index prefix
In an index that applies to multiple columns (known as a composite index), the initial or leading columns of
the index. A query that references the first 1, 2, 3, and so on columns of a composite index can use the index,
even if the query does not reference all the columns in the index.
See Also composite index, index.
index statistics
See statistics.
infimum record
A pseudo-record in an index, representing the gap below the smallest value in that index. If a transaction
has a statement such as SELECT ... FOR UPDATE ... WHERE col < 10;, and the smallest value in
the column is 5, it is a lock on the infimum record that prevents other transactions from inserting even smaller
values such as 0, -10, and so on.
See Also gap, index, pseudo-record, supremum record.
INFORMATION_SCHEMA
The name of the database that provides a query interface to the MySQL data dictionary. (This name is
defined by the ANSI SQL standard.) To examine information (metadata) about the database, you can query
tables such as INFORMATION_SCHEMA.TABLES and INFORMATION_SCHEMA.COLUMNS, rather than using
SHOW commands that produce unstructured output.
The information schema contains some tables that are specific to InnoDB, such as INNODB_LOCKS and
INNODB_TRX. You use these tables not to see how the database is structured, but to get real-time information
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about the workings of InnoDB tables to help with performance monitoring, tuning, and troubleshooting. In
particular, these tables provide information about MySQL features related to compression, and transactions
and their associated locks.
See Also compression, data dictionary, database, InnoDB, lock, transaction.
InnoDB
A MySQL component that combines high performance with transactional capability for reliability, robustness,
and concurrent access. It embodies the ACID design philosophy. Represented as a storage engine; it
handles tables created or altered with the ENGINE=INNODB clause. See The InnoDB Storage Engine for
architectural details and administration procedures, and Optimizing for InnoDB Tables for performance
advice.
In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables and the ENGINE=INNODB
clause is not required. In MySQL 5.1 only, many of the advanced InnoDB features require enabling the
component known as the InnoDB Plugin. See InnoDB as the Default MySQL Storage Engine for the
considerations involved in transitioning to recent releases where InnoDB tables are the default.
InnoDB tables are ideally suited for hot backups. See MySQL Enterprise Backup for information about the
MySQL Enterprise Backup product for backing up MySQL servers without interrupting normal processing.
See Also ACID, hot backup, storage engine, transaction.
innodb_autoinc_lock_mode
The innodb_autoinc_lock_mode option controls the algorithm used for auto-increment locking.
When you have an auto-incrementing primary key, you can use statement-based replication only
with the setting innodb_autoinc_lock_mode=1. This setting is known as consecutive lock mode,
because multi-row inserts within a transaction receive consecutive auto-increment values. If you have
innodb_autoinc_lock_mode=2, which allows higher concurrency for insert operations, use row-based
replication rather than statement-based replication. This setting is known as interleaved lock mode, because
multiple multi-row insert statements running at the same time can receive autoincrement values that are
interleaved. The setting innodb_autoinc_lock_mode=0 is the previous (traditional) default setting and
should not be used except for compatibility purposes.
See Also auto-increment locking, mixed-mode insert, primary key.
innodb_file_format
The innodb_file_format option determines the file format for all InnoDB tablespaces created after you
specify a value for this option. To create tablespaces other than the system tablespace, you must also use
the file-per-table option. Currently, you can specify the Antelope and Barracuda file formats.
See Also Antelope, Barracuda, file format, file-per-table, innodb_file_per_table, system tablespace,
tablespace.
innodb_file_per_table
A very important configuration option that affects many aspects of InnoDB file storage, availability of features,
and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is
disabled by default. The innodb_file_per_table option turns on file-per-table mode, which stores each
newly created InnoDB table and its associated index in its own .ibd file, outside the system tablespace.
This option affects the performance and storage considerations for a number of SQL statements, such as
DROP TABLE and TRUNCATE TABLE.
This option is needed to take full advantage of many other InnoDB features, such as such as table
compression, or backups of named tables in MySQL Enterprise Backup.
This option was once static, but can now be set using the SET GLOBAL command.
For reference information, see innodb_file_per_table. For usage information, see Using Per-Table
Tablespaces.
See Also compression, file-per-table, .ibd file, MySQL Enterprise Backup, system tablespace.
innodb_lock_wait_timeout
The innodb_lock_wait_timeout option sets the balance between waiting for shared resources to
become available, or giving up and handling the error, retrying, or doing alternative processing in your
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application. Rolls back any InnoDB transaction that waits more than a specified time to acquire a lock.
Especially useful if deadlocks are caused by updates to multiple tables controlled by different storage
engines; such deadlocks are not detected automatically.
See Also deadlock, deadlock detection, lock, wait.
innodb_strict_mode
The innodb_strict_mode option controls whether InnoDB operates in strict mode, where conditions that
are normally treated as warnings, cause errors instead (and the underlying statements fail).
This mode is the default setting in MySQL 5.5.5 and higher.
See Also strict mode.
insert
One of the primary DML operations in SQL. The performance of inserts is a key factor in data warehouse
systems that load millions of rows into tables, and OLTP systems where many concurrent connections might
insert rows into the same table, in arbitrary order. If insert performance is important to you, you should learn
about InnoDB features such as the insert buffer used in change buffering, and auto-increment columns.
See Also auto-increment, change buffering, data warehouse, DML, InnoDB, insert buffer, OLTP, SQL.
insert buffer
Former name for the change buffer. Now that change buffering includes delete and update operations as
well as inserts, "change buffer" is the preferred term.
See Also change buffer, change buffering.
insert buffering
The technique of storing secondary index changes due to INSERT operations in the insert buffer rather than
writing them immediately, so that the physical writes can be performed to minimize random I/O. It is one of the
types of change buffering; the others are delete buffering and purge buffering.
Insert buffering is not used if the secondary index is unique, because the uniqueness of new values cannot
be verified before the new entries are written out. Other kinds of change buffering do work for unique indexes.
See Also change buffer, change buffering, delete buffering, insert buffer, purge buffering, unique index.
instance
A single mysqld daemon managing a data directory representing one or more databases with a set of
tables. It is common in development, testing, and some replication scenarios to have multiple instances on
the same server machine, each managing its own data directory and listening on its own port or socket. With
one instance running a disk-bound workload, the server might still have extra CPU and memory capacity to
run additional instances.
See Also data directory, database, disk-bound, mysqld, replication, server.
instrumentation
Modifications at the source code level to collect performance data for tuning and debugging. In MySQL,
data collected by instrumentation is exposed through a SQL interface using the INFORMATION_SCHEMA and
PERFORMANCE_SCHEMA databases.
See Also INFORMATION_SCHEMA, Performance Schema.
intention exclusive lock
See intention lock.
intention lock
A kind of lock that applies to the table level, used to indicate what kind of lock the transaction intends
to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the
same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other
transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an
intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The
two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding
operations that are compatible. For more details on this locking mechanism, see InnoDB Lock Modes.
See Also lock, lock mode, locking.
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intention shared lock
See intention lock.
inverted index
A data structure optimized for document retrieval systems, used in the implementation of InnoDB full-text
search. The InnoDB FULLTEXT index, implemented as an inverted index, records the position of each word
within a document, rather than the location of a table row. A single column value (a document stored as a text
string) is represented by many entries in the inverted index.
See Also full-text search, FULLTEXT index, ilist.
IOPS
Acronym for I/O operations per second. A common measurement for busy systems, particularly OLTP
applications. If this value is near the maximum that the storage devices can handle, the application can
become disk-bound, limiting scalability.
See Also disk-bound, OLTP, scalability.
isolation level
One of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is
the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of
results when multiple transactions are making changes and performing queries at the same time.
From highest amount of consistency and protection to the least, the isolation levels supported by InnoDB are:
SERIALIZABLE, REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED.
With InnoDB tables, many users can keep the default isolation level (REPEATABLE READ) for all operations.
Expert users might choose the read committed level as they push the boundaries of scalability with OLTP
processing, or during data warehousing operations where minor inconsistencies do not affect the aggregate
results of large amounts of data. The levels on the edges (SERIALIZABLE and READ UNCOMMITTED)
change the processing behavior to such an extent that they are rarely used.
See Also ACID, READ COMMITTED, READ UNCOMMITTED, REPEATABLE READ, SERIALIZABLE,
transaction.
J
join
A query that retrieves data from more than one table, by referencing columns in the tables that hold identical
values. Ideally, these columns are part of an InnoDB foreign key relationship, which ensures referential
integrity and that the join columns are indexed. Often used to save space and improve query performance
by replacing repeated strings with numeric IDs, in a normalized data design.
See Also foreign key, index, normalized, query, referential integrity.
K
KEY_BLOCK_SIZE
An option to specify the size of data pages within an InnoDB table that uses compressed row format. The
default is 8 kilobytes. Lower values risk hitting internal limits that depend on the combination of row size and
compression percentage.
See Also compressed row format.
L
latch
A lightweight structure used by InnoDB to implement a lock for its own internal memory structures, typically
held for a brief time measured in milliseconds or microseconds. A general term that includes both mutexes
(for exclusive access) and rw-locks (for shared access). Certain latches are the focus of InnoDB performance
tuning, such as the data dictionary mutex. Statistics about latch use and contention are available through the
Performance Schema interface.
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See Also data dictionary, lock, locking, mutex, Performance Schema, rw-lock.
list
The InnoDB buffer pool is represented as a list of memory pages. The list is reordered as new pages are
accessed and enter the buffer pool, as pages within the buffer pool are accessed again and are considered
newer, and as pages that are not accessed for a long time are evicted from the buffer pool. The buffer pool is
actually divided into sublists, and the replacement policy is a variation of the familiar LRU technique.
See Also buffer pool, eviction, LRU, sublist.
lock
The high-level notion of an object that controls access to a resource, such as a table, row, or internal data
structure, as part of a locking strategy. For intensive performance tuning, you might delve into the actual
structures that implement locks, such as mutexes and latches.
See Also latch, lock mode, locking, mutex.
lock escalation
An operation used in some database systems that converts many row locks into a single table lock, saving
memory space but reducing concurrent access to the table. InnoDB uses a space-efficient representation for
row locks, so that lock escalation is not needed.
See Also locking, row lock, table lock.
lock mode
A shared (S) lock allows a transaction to read a row. Multiple transactions can acquire an S lock on that same
row at the same time.
An exclusive (X) lock allows a transaction to update or delete a row. No other transaction can acquire any kind
of lock on that same row at the same time.
Intention locks apply to the table level, and are used to indicate what kind of lock the transaction intends to
acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same
table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions
from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared
(IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase
process allows the lock requests to be resolved in order, without blocking locks and corresponding operations
that are compatible.
See Also intention lock, lock, locking.
locking
The system of protecting a transaction from seeing or changing data that is being queried or changed by
other transactions. The locking strategy must balance reliability and consistency of database operations (the
principles of the ACID philosophy) against the performance needed for good concurrency. Fine-tuning the
locking strategy often involves choosing an isolation level and ensuring all your database operations are safe
and reliable for that isolation level.
See Also ACID, concurrency, isolation level, latch, lock, mutex, transaction.
locking read
A SELECT statement that also performs a locking operation on an InnoDB table. Either SELECT ... FOR
UPDATE or SELECT ... LOCK IN SHARE MODE. It has the potential to produce a deadlock, depending on
the isolation level of the transaction. The opposite of a non-locking read. Not allowed for global tables in a
read-only transaction.
See Also deadlock, isolation level, locking, non-locking read, read-only transaction.
log
In the InnoDB context, "log" or "log files" typically refers to the redo log represented by the ib_logfile* files.
Another log area, which is physically part of the system tablespace, is the undo log.
Other kinds of logs that are important in MySQL are the error log (for diagnosing startup and runtime
problems), binary log (for working with replication and performing point-in-time restores), the general query
log (for diagnosing application problems), and the slow query log (for diagnosing performance problems).
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See Also binary log, error log, general query log, ib_logfile, redo log, slow query log, system tablespace, undo
log.
log buffer
The memory area that holds data to be written to the log files that make up the redo log. It is controlled by
the innodb_log_buffer_size configuration option.
See Also log file, redo log.
log file
One of the ib_logfileN files that make up the redo log. Data is written to these files from the log buffer
memory area.
See Also ib_logfile, log buffer, redo log.
log group
The set of files that make up the redo log, typically named ib_logfile0 and ib_logfile1. (For that
reason, sometimes referred to collectively as ib_logfile.)
See Also ib_logfile, redo log.
logical
A type of operation that involves high-level, abstract aspects such as tables, queries, indexes, and other
SQL concepts. Typically, logical aspects are important to make database administration and application
development convenient and usable. Contrast with physical.
See Also logical backup, physical.
logical backup
A backup that reproduces table structure and data, without copying the actual data files. For example, the
mysqldump command produces a logical backup, because its output contains statements such as CREATE
TABLE and INSERT that can re-create the data. Contrast with physical backup. A logical backup offers
flexibility (for example, you could edit table definitions or insert statements before restoring), but can take
substantially longer to restore than a physical backup.
See Also backup, mysqldump, physical backup, restore.
loose_
In MySQL 5.1, a prefix added to InnoDB configuration options when installing the InnoDB Plugin after server
startup, so any new configuration options not recognized by the current level of MySQL do not cause a startup
failure. MySQL processes configuration options that start with this prefix, but gives a warning rather than a
failure if the part after the prefix is not a recognized option.
See Also plugin.
low-water mark
A value representing a lower limit, typically a threshold value at which some corrective action begins or
becomes more aggressive. Contrast with high-water mark.
See Also high-water mark.
LRU
An acronym for "least recently used", a common method for managing storage areas. The items that
have not been used recently are evicted when space is needed to cache newer items. InnoDB uses the
LRU mechanism by default to manage the pages within the buffer pool, but makes exceptions in cases
where a page might be read only a single time, such as during a full table scan. This variation of the
LRU algorithm is called the midpoint insertion strategy. The ways in which the buffer pool management
differs from the traditional LRU algorithm is fine-tuned by the options innodb_old_blocks_pct,
innodb_old_blocks_time, and the new MySQL 5.6 options innodb_lru_scan_depth and
innodb_flush_neighbors.
See Also buffer pool, eviction, full table scan, midpoint insertion strategy, page.
LSN
Acronym for "log sequence number". This arbitrary, ever-increasing value represents a point in time
corresponding to operations recorded in the redo log. (This point in time is regardless of transaction
boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB during crash
recovery and for managing the buffer pool.
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Prior to MySQL 5.6.3, the LSN was a 4-byte unsigned integer. The LSN became an 8-byte unsigned integer in
MySQL 5.6.3 when the redo log file size limit increased from 4GB to 512GB, as additional bytes were required
to store extra size information. Applications built on MySQL 5.6.3 or later that use LSN values should use 64bit rather than 32-bit variables to store and compare LSN values.
In the MySQL Enterprise Backup product, you can specify an LSN to represent the point in time from which
to take an incremental backup. The relevant LSN is displayed by the output of the mysqlbackup command.
Once you have the LSN corresponding to the time of a full backup, you can specify that value to take a
subsequent incremental backup, whose output contains another LSN for the next incremental backup.
See Also crash recovery, incremental backup, MySQL Enterprise Backup, redo log, transaction.
M
.MRG file
A file containing references to other tables, used by the MERGE storage engine. Files with this extension are
always included in backups produced by the mysqlbackup command of the MySQL Enterprise Backup
product.
See Also MySQL Enterprise Backup, mysqlbackup command.
.MYD file
A file that MySQL uses to store data for a MyISAM table.
See Also .MYI file, MySQL Enterprise Backup, mysqlbackup command.
.MYI file
A file that MySQL uses to store indexes for a MyISAM table.
See Also .MYD file, MySQL Enterprise Backup, mysqlbackup command.
master server
Frequently shortened to "master". A database server machine in a replication scenario that processes the
initial insert, update, and delete requests for data. These changes are propagated to, and repeated on, other
servers known as slave servers.
See Also replication, slave server.
master thread
An InnoDB thread that performs various tasks in the background. Most of these tasks are I/O related, such
as writing changes from the insert buffer to the appropriate secondary indexes.
To improve concurrency, sometimes actions are moved from the master thread to separate background
threads. For example, in MySQL 5.6 and higher, dirty pages are flushed from the buffer pool by the page
cleaner thread rather than the master thread.
See Also buffer pool, dirty page, flush, insert buffer, page cleaner, thread.
MDL
Acronym for "metadata lock".
See Also metadata lock.
memcached
A popular component of many MySQL and NoSQL software stacks, allowing fast reads and writes for single
values and caching the results entirely in memory. Traditionally, applications required extra logic to write the
same data to a MySQL database for permanent storage, or to read data from a MySQL database when it
was not cached yet in memory. Now, applications can use the simple memcached protocol, supported by
client libraries for many languages, to communicate directly with MySQL servers using InnoDB or MySQL
Cluster tables. These NoSQL interfaces to MySQL tables allow applications to achieve higher read and write
performance than by issuing SQL commands directly, and can simplify application logic and deployment
configurations for systems that already incorporated memcached for in-memory caching.
The memcached interface to InnoDB tables is available in MySQL 5.6 and higher; see InnoDB Integration
with memcached for details. The memcached interface to MySQL Cluster tables is available in MySQL Cluster
7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html for details.
See Also InnoDB, NoSQL.
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merge
To apply changes to data cached in memory, such as when a page is brought into the buffer pool, and any
applicable changes recorded in the change buffer are incorporated into the page in the buffer pool. The
updated data is eventually written to the tablespace by the flush mechanism.
See Also buffer pool, change buffer, flush, tablespace.
metadata lock
A type of lock that prevents DDL operations on a table that is being used at the same time by another
transaction. For details, see Metadata Locking.
Enhancements to online operations, particularly in MySQL 5.6 and higher, are focused on reducing the
amount of metadata locking. The objective is for DDL operations that do not change the table structure
(such as CREATE INDEX and DROP INDEX for InnoDB tables) to proceed while the table is being queried,
updated, and so on by other transactions.
See Also DDL, lock, online, transaction.
metrics counter
A feature implemented by the innodb_metrics table in the information_schema, in MySQL 5.6 and
higher. You can query counts and totals for low-level InnoDB operations, and use the results for performance
tuning in combination with data from the performance_schema.
See Also counter, INFORMATION_SCHEMA, Performance Schema.
midpoint insertion strategy
The technique of initially bringing pages into the InnoDB buffer pool not at the "newest" end of the list,
but instead somewhere in the middle. The exact location of this point can vary, based on the setting of the
innodb_old_blocks_pct option. The intent is that blocks that are only read once, such as during a full
table scan, can be aged out of the buffer pool sooner than with a strict LRU algorithm.
See Also buffer pool, full table scan, LRU, page.
mini-transaction
An internal phase of InnoDB processing, when making changes at the physical level to internal data
structures during DML operations. A mini-transaction has no notion of rollback; multiple mini-transactions
can occur within a single transaction. Mini-transactions write information to the redo log that is used during
crash recovery. A mini-transaction can also happen outside the context of a regular transaction, for example
during purge processing by background threads.
See Also commit, crash recovery, DML, physical, purge, redo log, rollback, transaction.
mixed-mode insert
An INSERT statement where auto-increment values are specified for some but not all of the new rows.
For example, a multi-value INSERT could specify a value for the auto-increment column in some cases and
NULL in other cases. InnoDB generates auto-increment values for the rows where the column value was
specified as NULL. Another example is an INSERT ... ON DUPLICATE KEY UPDATE statement, where
auto-increment values might be generated but not used, for any duplicate rows that are processed as UPDATE
rather than INSERT statements.
Can cause consistency issues between master and slave servers in a replication configuration. Can require
adjusting the value of the innodb_autoinc_lock_mode configuration option.
See Also auto-increment, innodb_autoinc_lock_mode, master server, replication, slave server.
multi-core
A type of processor that can take advantage of multi-threaded programs, such as the MySQL server.
multiversion concurrency control
See MVCC.
mutex
Informal abbreviation for "mutex variable". (Mutex itself is short for "mutual exclusion".) The low-level object
that InnoDB uses to represent and enforce exclusive-access locks to internal in-memory data structures.
Once the lock is acquired, any other process, thread, and so on is prevented from acquiring the same lock.
Contrast with rw-locks, which allow shared access. Mutexes and rw-locks are known collectively as latches.
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See Also latch, lock, Performance Schema, Pthreads, rw-lock.
MVCC
Acronym for "multiversion concurrency control". This technique lets InnoDB transactions with certain
isolation levels to perform consistent read operations; that is, to query rows that are being updated by other
transactions, and see the values from before those updates occurred. This is a powerful technique to increase
concurrency, by allowing queries to proceed without waiting due to locks held by the other transactions.
This technique is not universal in the database world. Some other database products, and some other MySQL
storage engines, do not support it.
See Also ACID, concurrency, consistent read, isolation level, lock, transaction.
my.cnf
The name, on UNIX or Linux systems, of the MySQL option file.
See Also my.ini, option file.
my.ini
The name, on Windows systems, of the MySQL option file.
See Also my.cnf, option file.
mysql
The mysql program is the command-line interpreter for the MySQL database. It processes SQL statements,
and also MySQL-specific commands such as SHOW TABLES, by passing requests to the mysqld daemon.
See Also mysqld, SQL.
MySQL Enterprise Backup
A licensed product that performs hot backups of MySQL databases. It offers the most efficiency and
flexibility when backing up InnoDB tables, but can also back up MyISAM and other kinds of tables.
See Also hot backup, InnoDB.
mysqlbackup command
A command-line tool of the MySQL Enterprise Backup product. It performs a hot backup operation for
InnoDB tables, and a warm backup for MyISAM and other kinds of tables. See MySQL Enterprise Backup for
more information about this command.
See Also hot backup, MySQL Enterprise Backup, warm backup.
mysqld
The mysqld program is the database engine for the MySQL database. It runs as a UNIX daemon or
Windows service, constantly waiting for requests and performing maintenance work in the background.
See Also mysql.
mysqldump
A command that performs a logical backup of some combination of databases, tables, and table data. The
results are SQL statements that reproduce the original schema objects, data, or both. For substantial amounts
of data, a physical backup solution such as MySQL Enterprise Backup is faster, particularly for the restore
operation.
See Also logical backup, MySQL Enterprise Backup, physical backup, restore.
N
natural key
A indexed column, typically a primary key, where the values have some real-world significance. Usually
advised against because:
• If the value should ever change, there is potentially a lot of index maintenance to re-sort the clustered
index and update the copies of the primary key value that are repeated in each secondary index.
• Even seemingly stable values can change in unpredictable ways that are difficult to represent correctly
in the database. For example, one country can change into two or several, making the original country
code obsolete. Or, rules about unique values might have exceptions. For example, even if taxpayer IDs
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are intended to be unique to a single person, a database might have to handle records that violate that
rule, such as in cases of identity theft. Taxpayer IDs and other sensitive ID numbers also make poor
primary keys, because they may need to be secured, encrypted, and otherwise treated differently than other
columns.
Thus, it is typically better to use arbitrary numeric values to form a synthetic key, for example using an autoincrement column.
See Also auto-increment, primary key, secondary index, synthetic key.
neighbor page
Any page in the same extent as a particular page. When a page is selected to be flushed, any neighbor
pages that are dirty are typically flushed as well, as an I/O optimization for traditional hard disks. In MySQL
5.6 and up, this behavior can be controlled by the configuration variable innodb_flush_neighbors; you
might turn that setting off for SSD drives, which do not have the same overhead for writing smaller batches of
data at random locations.
See Also dirty page, extent, flush, page.
next-key lock
A combination of a record lock on the index record and a gap lock on the gap before the index record.
See Also gap lock, locking, record lock.
non-blocking I/O
An industry term that means the same as asynchronous I/O.
See Also asynchronous I/O.
non-locking read
A query that does not use the SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE
clauses. The only kind of query allowed for global tables in a read-only transaction. The opposite of a
locking read.
See Also locking read, query, read-only transaction.
non-repeatable read
The situation when a query retrieves data, and a later query within the same transaction retrieves what
should be the same data, but the queries return different results (changed by another transaction committing
in the meantime).
This kind of operation goes against the ACID principle of database design. Within a transaction, data should
be consistent, with predictable and stable relationships.
Among different isolation levels, non-repeatable reads are prevented by the serializable read and
repeatable read levels, and allowed by the consistent read, and read uncommitted levels.
See Also ACID, consistent read, isolation level, READ UNCOMMITTED, REPEATABLE READ,
SERIALIZABLE, transaction.
normalized
A database design strategy where data is split into multiple tables, and duplicate values condensed into
single rows represented by an ID, to avoid storing, querying, and updating redundant or lengthy values. It is
typically used in OLTP applications.
For example, an address might be given a unique ID, so that a census database could represent the
relationship lives at this address by associating that ID with each member of a family, rather than storing
multiple copies of a complex value such as 123 Main Street, Anytown, USA.
For another example, although a simple address book application might store each phone number in the
same table as a person's name and address, a phone company database might give each phone number a
special ID, and store the numbers and IDs in a separate table. This normalized representation could simplify
large-scale updates when area codes split apart.
Normalization is not always recommended. Data that is primarily queried, and only updated by deleting
entirely and reloading, is often kept in fewer, larger tables with redundant copies of duplicate values. This data
representation is referred to as denormalized, and is frequently found in data warehousing applications.
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See Also denormalized, foreign key, OLTP, relational.
NoSQL
A broad term for a set of data access technologies that do not use the SQL language as their primary
mechanism for reading and writing data. Some NoSQL technologies act as key-value stores, only accepting
single-value reads and writes; some relax the restrictions of the ACID methodology; still others do not require
a pre-planned schema. MySQL users can combine NoSQL-style processing for speed and simplicity with
SQL operations for flexibility and convenience, by using the memcached API to directly access some kinds of
MySQL tables. The memcached interface to InnoDB tables is available in MySQL 5.6 and higher; see InnoDB
Integration with memcached for details. The memcached interface to MySQL Cluster tables is available in
MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html for details.
See Also ACID, InnoDB, memcached, schema, SQL.
NOT NULL constraint
A type of constraint that specifies that a column cannot contain any NULL values. It helps to preserve
referential integrity, as the database server can identify data with erroneous missing values. It also helps
in the arithmetic involved in query optimization, allowing the optimizer to predict the number of entries in an
index on that column.
See Also column, constraint, NULL, primary key, referential integrity.
NULL
A special value in SQL, indicating the absence of data. Any arithmetic operation or equality test involving a
NULL value, in turn produces a NULL result. (Thus it is similar to the IEEE floating-point concept of NaN, "not
a number".) Any aggregate calculation such as AVG() ignores rows with NULL values, when determining how
many rows to divide by. The only test that works with NULL values uses the SQL idioms IS NULL or IS NOT
NULL.
NULL values play a part in index operations, because for performance a database must minimize the
overhead of keeping track of missing data values. Typically, NULL values are not stored in an index, because
a query that tests an indexed column using a standard comparison operator could never match a row with a
NULL value for that column. For the same reason, unique indexes do not prevent NULL values; those values
simply are not represented in the index. Declaring a NOT NULL constraint on a column provides reassurance
that there are no rows left out of the index, allowing for better query optimization (accurate counting of rows
and estimation of whether to use the index).
Because the primary key must be able to uniquely identify every row in the table, a single-column primary
key cannot contain any NULL values, and a multi-column primary key cannot contain any rows with NULL
values in all columns.
Although the Oracle database allows a NULL value to be concatenated with a string, InnoDB treats the result
of such an operation as NULL.
See Also index, primary key, SQL.
O
.OPT file
A file containing database configuration information. Files with this extension are always included in backups
produced by the mysqlbackup command of the MySQL Enterprise Backup product.
See Also MySQL Enterprise Backup, mysqlbackup command.
off-page column
A column containing variable-length data (such as BLOB and VARCHAR) that is too long to fit on a B-tree
page. The data is stored in overflow pages. The DYNAMIC row format in the InnoDB Barracuda file format is
more efficient for such storage than the older COMPACT row format.
See Also B-tree, Barracuda, overflow page.
OLTP
Acronym for "Online Transaction Processing". A database system, or a database application, that runs a
workload with many transactions, with frequent writes as well as reads, typically affecting small amounts of
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data at a time. For example, an airline reservation system or an application that processes bank deposits. The
data might be organized in normalized form for a balance between DML (insert/update/delete) efficiency and
query efficiency. Contrast with data warehouse.
With its row-level locking and transactional capability, InnoDB is the ideal storage engine for MySQL tables
used in OLTP applications.
See Also data warehouse, DML, InnoDB, query, row lock, transaction.
online
A type of operation that involves no downtime, blocking, or restricted operation for the database. Typically
applied to DDL. Operations that shorten the periods of restricted operation, such as fast index creation, have
evolved into a wider set of online DDL operations in MySQL 5.6.
In the context of backups, a hot backup is an online operation and a warm backup is partially an online
operation.
See Also DDL, Fast Index Creation, hot backup, online DDL, warm backup.
online DDL
A feature that improves the performance, concurrency, and availability of InnoDB tables during DDL
(primarily ALTER TABLE) operations. See InnoDB and Online DDL for details.
The details vary according to the type of operation. In some cases, the table can be modified concurrently
while the ALTER TABLE is in progress. The operation might be able to be performed without doing
a table copy, or using a specially optimized type of table copy. Space usage is controlled by the
innodb_online_alter_log_max_size configuration option.
This feature is an enhancement of the Fast Index Creation feature in MySQL 5.5 and the InnoDB Plugin for
MySQL 5.1.
See Also DDL, Fast Index Creation, online.
optimistic
A methodology that guides low-level implementation decisions for a relational database system. The
requirements of performance and concurrency in a relational database mean that operations must be started
or dispatched quickly. The requirements of consistency and referential integrity mean that any operation
could fail: a transaction might be rolled back, a DML operation could violate a constraint, a request for a lock
could cause a deadlock, a network error could cause a timeout. An optimistic strategy is one that assumes
most requests or attempts will succeed, so that relatively little work is done to prepare for the failure case.
When this assumption is true, the database does little unnecessary work; when requests do fail, extra work
must be done to clean up and undo changes.
InnoDB uses optimistic strategies for operations such as locking and commits. For example, data changed
by a transaction can be written to the data files before the commit occurs, making the commit itself very fast,
but requiring more work to undo the changes if the transaction is rolled back.
The opposite of an optimistic strategy is a pessimistic one, where a system is optimized to deal with
operations that are unreliable and frequently unsuccessful. This methodology is rare in a database system,
because so much care goes into choosing reliable hardware, networks, and algorithms.
See Also commit, concurrency, DML, locking, pessimistic.
optimizer
The MySQL component that determines the best indexes and join order to use for a query, based on
characteristics and data distribution of the relevant tables.
See Also index, join, query, table.
option
A configuration parameter for MySQL, either stored in the option file or passed on the command line.
For the options that apply to InnoDB tables, each option name starts with the prefix innodb_.
See Also InnoDB, option file.
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option file
The file that holds the configuration options for the MySQL instance. Traditionally, on Linux and UNIX this
file is named my.cnf, and on Windows it is named my.ini.
See Also configuration file, my.cnf, option.
overflow page
Separately allocated disk pages that hold variable-length columns (such as BLOB and VARCHAR) that are too
long to fit on a B-tree page. The associated columns are known as off-page columns.
See Also B-tree, off-page column, page.
P
.PAR file
A file containing partition definitions. Files with this extension are always included in backups produced by the
mysqlbackup command of the MySQL Enterprise Backup product.
See Also MySQL Enterprise Backup, mysqlbackup command.
page
A unit representing how much data InnoDB transfers at any one time between disk (the data files) and
memory (the buffer pool). A page can contain one or more rows, depending on how much data is in each
row. If a row does not fit entirely into a single page, InnoDB sets up additional pointer-style data structures so
that the information about the row can be stored in one page.
One way to fit more data in each page is to use compressed row format. For tables that use BLOBs or large
text fields, compact row format allows those large columns to be stored separately from the rest of the row,
reducing I/O overhead and memory usage for queries that do not reference those columns.
When InnoDB reads or writes sets of pages as a batch to increase I/O throughput, it reads or writes an extent
at a time.
All the InnoDB disk data structures within a MySQL instance share the same page size.
See Also buffer pool, compact row format, compressed row format, data files, extent, page size, row.
page cleaner
An InnoDB background thread that flushes dirty pages from the buffer pool. Prior to MySQL 5.6, this
activity was performed by the master thread
See Also buffer pool, dirty page, flush, master thread, thread.
page size
For releases up to and including MySQL 5.5, the size of each InnoDB page is fixed at 16 kilobytes. This
value represents a balance: large enough to hold the data for most rows, yet small enough to minimize the
performance overhead of transferring unneeded data to memory. Other values are not tested or supported.
Starting in MySQL 5.6, the page size for an InnoDB instance can be either 4KB, 8KB, or 16KB, controlled
by the innodb_page_size configuration option. You set the size when creating the MySQL instance, and
it remains constant afterwards. The same page size applies to all InnoDB tablespaces, both the system
tablespace and any separate tablespaces created in file-per-table mode.
Smaller page sizes can help performance with storage devices that use small block sizes, particularly for SSD
devices in disk-bound workloads, such as for OLTP applications. As individual rows are updated, less data is
copied into memory, written to disk, reorganized, locked, and so on.
See Also disk-bound, file-per-table, instance, OLTP, page, SSD, system tablespace, tablespace.
parent table
The table in a foreign key relationship that holds the initial column values pointed to from the child table.
The consequences of deleting, or updating rows in the parent table depend on the ON UPDATE and ON
DELETE clauses in the foreign key definition. Rows with corresponding values in the child table could be
automatically deleted or updated in turn, or those columns could be set to NULL, or the operation could be
prevented.
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See Also child table, foreign key.
partial backup
A backup that contains some of the tables in a MySQL database, or some of the databases in a MySQL
instance. Contrast with full backup.
See Also backup, full backup, table.
partial index
An index that represents only part of a column value, typically the first N characters (the prefix) of a long
VARCHAR value.
See Also index, index prefix.
Performance Schema
The performance_schema schema, in MySQL 5.5 and up, presents a set of tables that you can query to
get detailed information about the performance characteristics of many internal parts of the MySQL server.
See Also latch, mutex, rw-lock.
persistent statistics
A feature in MySQL 5.6 that stores index statistics for InnoDB tables on disk, providing better plan stability
for queries.
See Also index, optimizer, plan stability, query, table.
pessimistic
A methodology that sacrifices performance or concurrency in favor of safety. It is appropriate if a high
proportion of requests or attempts might fail, or if the consequences of a failed request are severe. InnoDB
uses what is known as a pessimistic locking strategy, to minimize the chance of deadlocks. At the
application level, you might avoid deadlocks by using a pessimistic strategy of acquiring all locks needed by a
transaction at the very beginning.
Many built-in database mechanisms use the opposite optimistic methodology.
See Also deadlock, locking, optimistic.
phantom
A row that appears in the result set of a query, but not in the result set of an earlier query. For example, if a
query is run twice within a transaction, and in the meantime, another transaction commits after inserting a
new row or updating a row so that it matches the WHERE clause of the query.
This occurrence is known as a phantom read. It is harder to guard against than a non-repeatable read,
because locking all the rows from the first query result set does not prevent the changes that cause the
phantom to appear.
Among different isolation levels, phantom reads are prevented by the serializable read level, and allowed
by the repeatable read, consistent read, and read uncommitted levels.
See Also consistent read, isolation level, non-repeatable read, READ UNCOMMITTED, REPEATABLE
READ, SERIALIZABLE, transaction.
physical
A type of operation that involves hardware-related aspects such as disk blocks, memory pages, files, bits,
disk reads, and so on. Typically, physical aspects are important during expert-level performance tuning and
problem diagnosis. Contrast with logical.
See Also logical, physical backup.
physical backup
A backup that copies the actual data files. For example, the mysqlbackup command of the MySQL
Enterprise Backup product produces a physical backup, because its output contains data files that can be
used directly by the mysqld server, resulting in a faster restore operation. Contrast with logical backup.
See Also backup, logical backup, MySQL Enterprise Backup, restore.
PITR
Acronym for point-in-time recovery.
See Also point-in-time recovery.
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plan stability
A property of a query execution plan, where the optimizer makes the same choices each time for a given
query, so that performance is consistent and predictable.
See Also query, query execution plan.
plugin
In MySQL 5.1 and earlier, a separately installable form of the InnoDB storage engine that includes features
and performance enhancements not included in the built-in InnoDB for those releases.
For MySQL 5.5 and higher, the MySQL distribution includes the very latest InnoDB features and performance
enhancements, known as InnoDB 1.1, and there is no longer a separate InnoDB Plugin.
This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin
but not the built-in InnoDB, or vice versa.
See Also built-in, InnoDB.
point-in-time recovery
The process of restoring a backup to recreate the state of the database at a specific date and time.
Commonly abbreviated PITR. Because it is unlikely that the specified time corresponds exactly to the time
of a backup, this technique usually requires a combination of a physical backup and a logical backup. For
example, with the MySQL Enterprise Backup product, you restore the last backup that you took before the
specified point in time, then replay changes from the binary log between the time of the backup and the PITR
time.
See Also backup, logical backup, MySQL Enterprise Backup, physical backup, PITR.
prefix
See index prefix.
prepared backup
A set of backup files, produced by the MySQL Enterprise Backup product, after all the stages of applying
binary logs and incremental backups are finished. The resulting files are ready to be restored. Prior to the
apply steps, the files are known as a raw backup.
See Also binary log, hot backup, incremental backup, MySQL Enterprise Backup, raw backup, restore.
primary key
A set of columns -- and by implication, the index based on this set of columns -- that can uniquely identify
every row in a table. As such, it must be a unique index that does not contain any NULL values.
InnoDB requires that every table has such an index (also called the clustered index or cluster index), and
organizes the table storage based on the column values of the primary key.
When choosing primary key values, consider using arbitrary values (a synthetic key) rather than relying on
values derived from some other source (a natural key).
See Also clustered index, index, natural key, synthetic key.
process
An instance of an executing program. The operating system switches between multiple running processes,
allowing for a certain degree of concurrency. On most operating systems, processes can contain multiple
threads of execution that share resources. Context-switching between threads is faster than the equivalent
switching between processes.
See Also concurrency, thread.
pseudo-record
An artificial record in an index, used for locking key values or ranges that do not currently exist.
See Also infimum record, locking, supremum record.
Pthreads
The POSIX threads standard, which defines an API for threading and locking operations on UNIX and Linux
systems. On UNIX and Linux systems, InnoDB uses this implementation for mutexes.
See Also mutex.
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purge
A type of garbage collection performed by a separate thread, running on a periodic schedule. The purge
includes these actions: removing obsolete values from indexes; physically removing rows that were marked
for deletion by previous DELETE statements.
See Also crash recovery, delete, doublewrite buffer.
purge buffering
The technique of storing index changes due to DELETE operations in the insert buffer rather than writing
them immediately, so that the physical writes can be performed to minimize random I/O. (Because delete
operations are a two-step process, this operation buffers the write that normally purges an index record
that was previously marked for deletion.) It is one of the types of change buffering; the others are insert
buffering. and delete buffering
See Also change buffer, change buffering, delete buffering, insert buffer, insert buffering.
purge lag
Another name for the InnoDB history list. Related to the innodb_max_purge_lag configuration option.
See Also history list, purge.
purge thread
A thread within the InnoDB process that is dedicated to performing the periodic purge operation. In MySQL
5.6 and higher, multiple purge threads are enabled by the innodb_purge_threads configuration option.
See Also purge, thread.
Q
query
In SQL, an operation that reads information from one or more tables. Depending on the organization of data
and the parameters of the query, the lookup might be optimized by consulting an index. If multiple tables are
involved, the query is known as a join.
For historical reasons, sometimes discussions of internal processing for statements use "query" in a broader
sense, including other types of MySQL statements such as DDL and DML statements.
See Also DDL, DML, index, join, SQL, table.
query execution plan
The set of decisions made by the optimizer about how to perform a query most efficiently, including which
index or indexes to use, and the order in which to join tables. Plan stability involves the same choices being
made consistently for a given query.
See Also index, join, plan stability, query.
query log
See general query log.
quiesce
To reduce the amount of database activity, often in preparation for an operation such as an ALTER TABLE,
a backup, or a shutdown. Might or might not involve doing as much flushing as possible, so that InnoDB
does not continue doing background I/O.
In MySQL 5.6 and higher, the syntax FLUSH TABLES ... FOR EXPORT writes some data to disk for
InnoDB tables that make it simpler to back up those tables by copying the data files.
See Also backup, flush, InnoDB, shutdown.
R
RAID
Acronym for "Redundant Array of Inexpensive Drives". Spreading I/O operations across multiple drives
enables greater concurrency at the hardware level, and improves the efficiency of low-level write operations
that otherwise would be performed in sequence.
See Also concurrency.
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random dive
A technique for quickly estimating the number of different values in a column (the column's cardinality).
InnoDB samples pages at random from the index and uses that data to estimate the number of different
values. This operation occurs when each table is first opened.
Originally, the number of sampled pages was fixed at 8; now, it is determined by the setting of the
innodb_stats_sample_pages parameter.
The way the random pages are picked depends on the setting of the
innodb_use_legacy_cardinality_algorithm parameter. The default setting (OFF) has better randomness than in
older releases.
See Also cardinality.
raw backup
The initial set of backup files produced by the MySQL Enterprise Backup product, before the changes
reflected in the binary log and any incremental backups are applied. At this stage, the files are not ready to
restore. After these changes are applied, the files are known as a prepared backup.
See Also binary log, hot backup, ibbackup_logfile, incremental backup, MySQL Enterprise Backup, prepared
backup, restore.
READ COMMITTED
An isolation level that uses a locking strategy that relaxes some of the protection between transactions,
in the interest of performance. Transactions cannot see uncommitted data from other transactions, but they
can see data that is committed by another transaction after the current transaction started. Thus, a transaction
never sees any bad data, but the data that it does see may depend to some extent on the timing of other
transactions.
When a transaction with this isolation level performs UPDATE ... WHERE or DELETE ... WHERE
operations, other transactions might have to wait. The transaction can perform SELECT ... FOR UPDATE,
and LOCK IN SHARE MODE operations without making other transactions wait.
See Also ACID, isolation level, locking, REPEATABLE READ, SERIALIZABLE, transaction.
READ UNCOMMITTED
The isolation level that provides the least amount of protection between transactions. Queries employ
a locking strategy that allows them to proceed in situations where they would normally wait for another
transaction. However, this extra performance comes at the cost of less reliable results, including data that has
been changed by other transactions and not committed yet (known as dirty read). Use this isolation level only
with great caution, and be aware that the results might not be consistent or reproducible, depending on what
other transactions are doing at the same time. Typically, transactions with this isolation level do only queries,
not insert, update, or delete operations.
See Also ACID, dirty read, isolation level, locking, transaction.
read view
An internal snapshot used by the MVCC mechanism of InnoDB. Certain transactions, depending on their
isolation level, see the data values as they were at the time the transaction (or in some cases, the statement)
started. Isolation levels that use a read view are REPEATABLE READ, READ COMMITTED, and READ
UNCOMMITTED.
See Also isolation level, MVCC, READ COMMITTED, READ UNCOMMITTED, REPEATABLE READ,
transaction.
read-ahead
A type of I/O request that prefetches a group of pages (an entire extent) into the buffer pool
asynchronously, in anticipation that these pages will be needed soon. The linear read-ahead technique
prefetches all the pages of one extent based on access patterns for pages in the preceding extent, and is
part of all MySQL versions starting with the InnoDB Plugin for MySQL 5.1. The random read-ahead technique
prefetches all the pages for an extent once a certain number of pages from the same extent are in the buffer
pool. Random read-ahead is not part of MySQL 5.5, but is re-introduced in MySQL 5.6 under the control of the
innodb_random_read_ahead configuration option.
See Also buffer pool, extent, page.
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read-only transaction
A type of transaction that can be optimized for InnoDB tables by eliminating some of the bookkeeping
involved with creating a read view for each transaction. Can only perform non-locking read queries. It can
be started explicitly with the syntax START TRANSACTION READ ONLY, or automatically under certain
conditions. See Optimizations for Read-Only Transactions for details.
See Also non-locking read, read view, transaction.
record lock
A lock on an index record. For example, SELECT c1 FOR UPDATE FROM t WHERE c1 = 10; prevents
any other transaction from inserting, updating, or deleting rows where the value of t.c1 is 10. Contrast with
gap lock and next-key lock.
See Also gap lock, lock, next-key lock.
redo
The data, in units of records, recorded in the redo log when DML statements make changes to InnoDB
tables. It is used during crash recovery to correct data written by incomplete transactions. The everincreasing LSN value represents the cumulative amount of redo data that has passed through the redo log.
See Also crash recovery, DML, LSN, redo log, transaction.
redo log
A disk-based data structure used during crash recovery, to correct data written by incomplete transactions.
During normal operation, it encodes requests to change InnoDB table data, which result from SQL statements
or low-level API calls through NoSQL interfaces. Modifications that did not finish updating the data files
before an unexpected shutdown are replayed automatically.
The redo log is physically represented as a set of files, typically named ib_logfile0 and ib_logfile1.
The data in the redo log is encoded in terms of records affected; this data is collectively referred to as redo.
The passage of data through the redo logs is represented by the ever-increasing LSN value. The original 4GB
limit on maximum size for the redo log is raised to 512GB in MySQL 5.6.3.
The disk layout of the redo log is influenced by the configuration options innodb_log_file_size,
innodb_log_group_home_dir, and (rarely) innodb_log_files_in_group. The performance of redo
log operations is also affected by the log buffer, which is controlled by the innodb_log_buffer_size
configuration option.
See Also crash recovery, data files, ib_logfile, log buffer, LSN, redo, shutdown, transaction.
redundant row format
The oldest InnoDB row format, available for tables using the Antelope file format. Prior to MySQL 5.0.3, it
was the only row format available in InnoDB. In My SQL 5.0.3 and later, the default is compact row format.
You can still specify redundant row format for compatibility with older InnoDB tables.
For additional information about InnoDB REDUNDANT row format, see Section 5.2, “COMPACT and
REDUNDANT Row Formats”.
See Also Antelope, compact row format, file format, row format.
referential integrity
The technique of maintaining data always in a consistent format, part of the ACID philosophy. In particular,
data in different tables is kept consistent through the use of foreign key constraints, which can prevent
changes from happening or automatically propagate those changes to all related tables. Related mechanisms
include the unique constraint, which prevents duplicate values from being inserted by mistake, and the NOT
NULL constraint, which prevents blank values from being inserted by mistake.
See Also ACID, FOREIGN KEY constraint, NOT NULL constraint, unique constraint.
relational
An important aspect of modern database systems. The database server encodes and enforces relationships
such as one-to-one, one-to-many, many-to-one, and uniqueness. For example, a person might have zero,
one, or many phone numbers in an address database; a single phone number might be associated with
several family members. In a financial database, a person might be required to have exactly one taxpayer ID,
and any taxpayer ID could only be associated with one person.
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The database server can use these relationships to prevent bad data from being inserted, and to find efficient
ways to look up information. For example, if a value is declared to be unique, the server can stop searching as
soon as the first match is found, and it can reject attempts to insert a second copy of the same value.
At the database level, these relationships are expressed through SQL features such as columns within a
table, unique and NOT NULL constraints, foreign keys, and different kinds of join operations. Complex
relationships typically involve data split between more than one table. Often, the data is normalized, so that
duplicate values in one-to-many relationships are stored only once.
In a mathematical context, the relations within a database are derived from set theory. For example, the OR
and AND operators of a WHERE clause represent the notions of union and intersection.
See Also ACID, constraint, foreign key, normalized.
relevance
In the full-text search feature, a number signifying the similarity between the search string and the data in
the FULLTEXT index. For example, when you search for a single word, that word is typically more relevant
for a row where if it occurs several times in the text than a row where it appears only once.
See Also full-text search, FULLTEXT index.
REPEATABLE READ
The default isolation level for InnoDB. It prevents any rows that are queried from being changed by other
transactions, thus blocking non-repeatable reads but not phantom reads. It uses a moderately strict locking
strategy so that all queries within a transaction see data from the same snapshot, that is, the data as it was at
the time the transaction started.
When a transaction with this isolation level performs UPDATE ... WHERE, DELETE ... WHERE,
SELECT ... FOR UPDATE, and LOCK IN SHARE MODE operations, other transactions might have to wait.
See Also ACID, consistent read, isolation level, locking, phantom, SERIALIZABLE, transaction.
replication
The practice of sending changes from a master database, to one or more slave databases, so that all
databases have the same data. This technique has a wide range of uses, such as load-balancing for better
scalability, disaster recovery, and testing software upgrades and configuration changes. The changes can be
sent between the database by methods called row-based replication and statement-based replication.
See Also row-based replication, statement-based replication.
restore
The process of putting a set of backup files from the MySQL Enterprise Backup product in place for use by
MySQL. This operation can be performed to fix a corrupted database, to return to some earlier point in time,
or (in a replication context) to set up a new slave database. In the MySQL Enterprise Backup product, this
operation is performed by the copy-back option of the mysqlbackup command.
See Also hot backup, MySQL Enterprise Backup, mysqlbackup command, prepared backup, replication.
rollback
A SQL statement that ends a transaction, undoing any changes made by the transaction. It is the opposite
of commit, which makes permanent any changes made in the transaction.
By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL
statement. You must change this setting before you can use the rollback technique.
See Also ACID, commit, transaction.
rollback segment
The storage area containing the undo log, part of the system tablespace.
See Also system tablespace, undo log.
row
The logical data structure defined by a set of columns. A set of rows makes up a table. Within InnoDB data
files, each page can contain one or more rows.
Although InnoDB uses the term row format for consistency with MySQL syntax, the row format is a property
of each table and applies to all rows in that table.
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See Also column, data files, page, row format, table.
row format
The disk storage format for a row from an InnoDB table. As InnoDB gains new capabilities such as
compression, new row formats are introduced to support the resulting improvements in storage efficiency and
performance.
Each table has its own row format, specified through the ROW_FORMAT option. To see the row format for
each InnoDB table, issue the command SHOW TABLE STATUS. Because all the tables in the system
tablespace share the same row format, to take advantage of other row formats typically requires setting the
innodb_file_per_table option, so that each table is stored in a separate tablespace.
See Also compact row format, compressed row format, dynamic row format, fixed row format, redundant row
format, row, table.
row lock
A lock that prevents a row from being accessed in an incompatible way by another transaction. Other rows
in the same table can be freely written to by other transactions. This is the type of locking done by DML
operations on InnoDB tables.
Contrast with table locks used by MyISAM, or during DDL operations on InnoDB tables that cannot be done
with online DDL; those locks block concurrent access to the table.
See Also DDL, DML, InnoDB, lock, locking, online DDL, table lock, transaction.
row-based replication
A form of replication where events are propagated from the master server specifying how to change
individual rows on the slave server. It is safe to use for all settings of the innodb_autoinc_lock_mode
option.
See Also auto-increment locking, innodb_autoinc_lock_mode, master server, replication, slave server,
statement-based replication.
row-level locking
The locking mechanism used for InnoDB tables, relying on row locks rather than table locks. Multiple
transactions can modify the same table concurrently. Only if two transactions try to modify the same row
does one of the transactions wait for the other to complete (and release its row locks).
See Also InnoDB, locking, row lock, table lock, transaction.
rw-lock
The low-level object that InnoDB uses to represent and enforce shared-access locks to internal in-memory
data structures. Once the lock is acquired, any other process, thread, and so on can read the data structure,
but no one else can write to it. Contrast with mutexes, which enforce exclusive access. Mutexes and rw-locks
are known collectively as latches.
See Also latch, lock, mutex, Performance Schema.
S
savepoint
Savepoints help to implement nested transactions. They can be used to provide scope to operations on
tables that are part of a larger transaction. For example, scheduling a trip in a reservation system might
involve booking several different flights; if a desired flight is unavailable, you might roll back the changes
involved in booking that one leg, without rolling back the earlier flights that were successfully booked.
See Also rollback, transaction.
scalability
The ability to add more work and issue more simultaneous requests to a system, without a sudden drop in
performance due to exceeding the limits of system capacity. Software architecture, hardware configuration,
application coding, and type of workload all play a part in scalability. When the system reaches its maximum
capacity, popular techniques for increasing scalability are scale up (increasing the capacity of existing
hardware or software) and scale out (adding new servers and more instances of MySQL). Often paired with
availability as critical aspects of a large-scale deployment.
See Also availability, scale out, scale up.
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scale out
A technique for increasing scalability by adding new servers and more instances of MySQL. For example,
setting up replication, MySQL Cluster, connection pooling, or other features that spread work across a group
of servers. Contrast with scale up.
See Also scalability, scale up.
scale up
A technique for increasing scalability by increasing the capacity of existing hardware or software.
For example, increasing the memory on a server and adjusting memory-related parameters such as
innodb_buffer_pool_size and innodb_buffer_pool_instances. Contrast with scale out.
See Also scalability, scale out.
schema
Conceptually, a schema is a set of interrelated database objects, such as tables, table columns, data types
of the columns, indexes, foreign keys, and so on. These objects are connected through SQL syntax, because
the columns make up the tables, the foreign keys refer to tables and columns, and so on. Ideally, they are
also connected logically, working together as part of a unified application or flexible framework. For example,
the information_schema and performance_schema databases use "schema" in their names to emphasize
the close relationships between the tables and columns they contain.
In MySQL, physically, a schema is synonymous with a database. You can substitute the keyword SCHEMA
instead of DATABASE in MySQL SQL syntax, for example using CREATE SCHEMA instead of CREATE
DATABASE.
Some other database products draw a distinction. For example, in the Oracle Database product, a schema
represents only a part of a database: the tables and other objects owned by a single user.
See Also database, ib-file set, INFORMATION_SCHEMA, Performance Schema.
search index
In MySQL, full-text search queries use a special kind of index, the FULLTEXT index. In MySQL 5.6.4 and
up, InnoDB and MyISAM tables both support FULLTEXT indexes; formerly, these indexes were only available
for MyISAM tables.
See Also full-text search, FULLTEXT index.
secondary index
A type of InnoDB index that represents a subset of table columns. An InnoDB table can have zero, one, or
many secondary indexes. (Contrast with the clustered index, which is required for each InnoDB table, and
stores the data for all the table columns.)
A secondary index can be used to satisfy queries that only require values from the indexed columns. For more
complex queries, it can be used to identify the relevant rows in the table, which are then retrieved through
lookups using the clustered index.
Creating and dropping secondary indexes has traditionally involved significant overhead from copying all the
data in the InnoDB table. The fast index creation feature of the InnoDB Plugin makes both CREATE INDEX
and DROP INDEX statements much faster for InnoDB secondary indexes.
See Also clustered index, Fast Index Creation, index.
segment
A division within an InnoDB tablespace. If a tablespace is analogous to a directory, the segments are
analogous to files within that directory. A segment can grow. New segments can be created.
For example, within a file-per-table tablespace, the table data is in one segment and each associated index
is in its own segment. The system tablespace contains many different segments, because it can hold many
tables and their associated indexes. The system tablespace also includes up to 128 rollback segments
making up the undo log.
Segments grow and shrink as data is inserted and deleted. When a segment needs more room, it is extended
by one extent (1 megabyte) at a time. Similarly, a segment releases one extent's worth of space when all the
data in that extent is no longer needed.
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See Also extent, file-per-table, rollback segment, system tablespace, tablespace, undo log.
selectivity
A property of data distribution, the number of distinct values in a column (its cardinality) divided by the
number of records in the table. High selectivity means that the column values are relatively unique, and can
retrieved efficiently through an index. If you (or the query optimizer) can predict that a test in a WHERE clause
only matches a small number (or proportion) of rows in a table, the overall query tends to be efficient if it
evaluates that test first, using an index.
See Also cardinality, query.
semi-consistent read
A type of read operation used for UPDATE statements, that is a combination of read committed and
consistent read. When an UPDATE statement examines a row that is already locked, InnoDB returns the
latest committed version to MySQL so that MySQL can determine whether the row matches the WHERE
condition of the UPDATE. If the row matches (must be updated), MySQL reads the row again, and this
time InnoDB either locks it or waits for a lock on it. This type of read operation can only happen when the
transaction has the read committed isolation level, or when the innodb_locks_unsafe_for_binlog
option is enabled.
See Also consistent read, isolation level, READ COMMITTED.
SERIALIZABLE
The isolation level that uses the most conservative locking strategy, to prevent any other transactions from
inserting or changing data that was read by this transaction, until it is finished. This way, the same query can
be run over and over within a transaction, and be certain to retrieve the same set of results each time. Any
attempt to change data that was committed by another transaction since the start of the current transaction,
cause the current transaction to wait.
This is the default isolation level specified by the SQL standard. In practice, this degree of strictness is rarely
needed, so the default isolation level for InnoDB is the next most strict, repeatable read.
See Also ACID, consistent read, isolation level, locking, REPEATABLE READ, transaction.
server
A type of program that runs continuously, waiting to receive and act upon requests from another program
(the client). Because often an entire computer is dedicated to running one or more server programs (such as
a database server, a web server, an application server, or some combination of these), the term server can
also refer to the computer that runs the server software.
See Also client, mysqld.
shared lock
A kind of lock that allows other transactions to read the locked object, and to also acquire other shared
locks on it, but not to write to it. The opposite of exclusive lock.
See Also exclusive lock, lock, transaction.
shared tablespace
Another way of referring to the system tablespace.
See Also system tablespace.
sharp checkpoint
The process of flushing to disk all dirty buffer pool pages whose redo entries are contained in certain portion
of the redo log. Occurs before InnoDB reuses a portion of a log file; the log files are used in a circular fashion.
Typically occurs with write-intensive workloads.
See Also dirty page, flush, redo log, workload.
shutdown
The process of stopping the MySQL server. By default, this process does cleanup operations for InnoDB
tables, so it can slow to shut down, but fast to start up later. If you skip the cleanup operations, it is fast to
shut down but must do the cleanup during the next restart.
The shutdown mode is controlled by the innodb_fast_shutdown option.
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See Also fast shutdown, InnoDB, slow shutdown, startup.
slave server
Frequently shortened to "slave". A database server machine in a replication scenario that receives changes
from another server (the master) and applies those same changes. Thus it maintains the same contents as
the master, although it might lag somewhat behind.
In MySQL, slave servers are commonly used in disaster recovery, to take the place of a master servers that
fails. They are also commonly used for testing software upgrades and new settings, to ensure that database
configuration changes do not cause problems with performance or reliability.
Slave servers typically have high workloads, because they process all the DML (write) operations relayed
from the master, as well as user queries. To ensure that slave servers can apply changes from the master
fast enough, they frequently have fast I/O devices and sufficient CPU and memory to run multiple database
instances on the same slave server. For example, the master server might use hard drive storage while the
slave servers use SSDs.
See Also DML, replication, server, SSD.
slow query log
A type of log used for performance tuning of SQL statements processed by the MySQL server. The log
information is stored in a file. You must enable this feature to use it. You control which categories of "slow"
SQL statements are logged. For more information, see The Slow Query Log.
See Also general query log, log.
slow shutdown
A type of shutdown that does additional InnoDB flushing operations before completing. Also known as a
clean shutdown. Specified by the configuration parameter innodb_fast_shutdown=0 or the command
SET GLOBAL innodb_fast_shutdown=0;. Although the shutdown itself can take longer, that time will be
saved on the subsequent startup.
See Also clean shutdown, fast shutdown, shutdown.
snapshot
A representation of data at a particular time, which remains the same even as changes are committed by
other transactions. Used by certain isolation levels to allow consistent reads.
See Also commit, consistent read, isolation level, transaction.
space ID
An identifier used to uniquely identify an InnoDB tablespace within a MySQL instance. The space ID for
the system tablespace is always zero; this same ID applies to all tables within the system tablespace. Each
tablespace file created in file-per-table mode also has its own space ID.
Prior to MySQL 5.6, this hardcoded value presented difficulties in moving InnoDB tablespace files between
MySQL instances. Starting in MySQL 5.6, you can copy tablespace files between instances by using the
transportable tablespace feature involving the statements FLUSH TABLES ... FOR EXPORT, ALTER
TABLE ... DISCARD TABLESPACE, and ALTER TABLE ... IMPORT TABLESPACE. The information
needed to adjust the space ID is conveyed in the .cfg file which you copy along with the tablespace. See
Improved Tablespace Management for details.
See Also .cfg file, file-per-table, .ibd file, system tablespace, tablespace, transportable tablespace.
spin
A type of wait operation that continuously tests whether a resource becomes available. This technique is
used for resources that are typically held only for brief periods, where it is more efficient to wait in a "busy
loop" than to put the thread to sleep and perform a context switch. If the resource does not become available
within a short time, the spin loop ceases and another wait technique is used.
See Also latch, lock, mutex, wait.
SQL
The Structured Query Language that is standard for performing database operations. Often divided into
the categories DDL, DML, and queries. MySQL includes some additional statement categories such as
replication. See Language Structure for the building blocks of SQL syntax, Data Types for the data types to
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use for MySQL table columns, SQL Statement Syntax for details about SQL statements and their associated
categories, and Functions and Operators for standard and MySQL-specific functions to use in queries.
See Also DDL, DML, query, replication.
SSD
Acronym for "solid-state drive". A type of storage device with different performance characteristics than a
traditional hard disk drive (HDD): smaller storage capacity, faster for random reads, no moving parts, and with
a number of considerations affecting write performance. Its performance characteristics can influence the
throughput of a disk-bound workload.
See Also disk-bound, SSD.
startup
The process of starting the MySQL server. Typically done by one of the programs listed in MySQL Server
and Server-Startup Programs. The opposite of shutdown.
See Also shutdown.
statement-based replication
A form of replication where SQL statements are sent from the master server and replayed on the slave
server. It requires some care with the setting for the innodb_autoinc_lock_mode option, to avoid potential
timing problems with auto-increment locking.
See Also auto-increment locking, innodb_autoinc_lock_mode, master server, replication, row-based
replication, slave server.
statistics
Estimated values relating to each InnoDB table and index, used to construct an efficient query execution
plan. The main values are the cardinality (number of distinct values) and the total number of table rows
or index entries. The statistics for the table represent the data in its primary key index. The statistics for a
secondary index represent the rows covered by that index.
The values are estimated rather than counted precisely because at any moment, different transactions can
be inserting and deleting rows from the same table. To keep the values from being recalculated frequently,
you can enable persistent statistics, where the values are stored in InnoDB system tables, and refreshed
only when you issue an ANALYZE TABLE statement.
You can control how NULL values are treated when calculating statistics through the
innodb_stats_method configuration option.
Other types of statistics are available for database objects and database activity through the
INFORMATION_SCHEMA and PERFORMANCE_SCHEMA tables.
See Also cardinality, index, INFORMATION_SCHEMA, NULL, Performance Schema, persistent statistics,
primary key, query execution plan, secondary index, table, transaction.
stemming
The ability to search for different variations of a word based on a common root word, such as singular and
plural, or past, present, and future verb tense. This feature is currently supported in MyISAM full-text search
feature but not in FULLTEXT indexes for InnoDB tables.
See Also full-text search, FULLTEXT index.
stopword
In a FULLTEXT index, a word that is considered common or trivial enough that it is omitted from the search
index and ignored in search queries. Different configuration settings control stopword processing for InnoDB
and MyISAM tables. See Full-Text Stopwords for details.
See Also FULLTEXT index, search index.
storage engine
A component of the MySQL database that performs the low-level work of storing, updating, and querying
data. In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables, superceding MyISAM.
Different storage engines are designed with different tradeoffs between factors such as memory usage versus
disk usage, read speed versus write speed, and speed versus robustness. Each storage engine manages
specific tables, so we refer to InnoDB tables, MyISAM tables, and so on.
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The MySQL Enterprise Backup product is optimized for backing up InnoDB tables. It can also back up tables
handled by MyISAM and other storage engines.
See Also InnoDB, MySQL Enterprise Backup, table type.
strict mode
The general name for the setting controlled by the innodb_strict_mode option. Turning on this setting
causes certain conditions that are normally treated as warnings, to be considered errors. For example, certain
invalid combinations of options related to file format and row format, that normally produce a warning and
continue with default values, now cause the CREATE TABLE operation to fail.
MySQL also has something called strict mode.
See Also file format, innodb_strict_mode, row format.
sublist
Within the list structure that represents the buffer pool, pages that are relatively old and relatively new are
represented by different portions of the list. A set of parameters control the size of these portions and the
dividing point between the new and old pages.
See Also buffer pool, eviction, list, LRU.
supremum record
A pseudo-record in an index, representing the gap above the largest value in that index. If a transaction
has a statement such as SELECT ... FOR UPDATE ... WHERE col > 10;, and the largest value in the
column is 20, it is a lock on the supremum record that prevents other transactions from inserting even larger
values such as 50, 100, and so on.
See Also gap, infimum record, pseudo-record.
surrogate key
Synonym name for synthetic key.
See Also synthetic key.
synthetic key
A indexed column, typically a primary key, where the values are assigned arbitrarily. Often done using an
auto-increment column. By treating the value as completely arbitrary, you can avoid overly restrictive rules
and faulty application assumptions. For example, a numeric sequence representing employee numbers might
have a gap if an employee was approved for hiring but never actually joined. Or employee number 100 might
have a later hiring date than employee number 500, if they left the company and later rejoined. Numeric
values also produce shorter values of predictable length. For example, storing numeric codes meaning
"Road", "Boulevard", "Expressway", and so on is more space-efficient than repeating those strings over and
over.
Also known as a surrogate key. Contrast with natural key.
See Also auto-increment, natural key, primary key, surrogate key.
system tablespace
A small set of data files (the ibdata files) containing the metadata for InnoDB-related objects (the data
dictionary), and the storage areas for the undo log, the change buffer, and the doublewrite buffer.
Depending on the setting of the innodb_file_per_table, when tables are created, it might also contain
table and index data for some or all InnoDB tables. The data and metadata in the system tablespace apply to
all the databases in a MySQL instance.
Prior to MySQL 5.6.7, the default was to keep all InnoDB tables and indexes inside the system tablespace,
often causing this file to become very large. Because the system tablespace never shrinks, storage problems
could arise if large amounts of temporary data were loaded and then deleted. In MySQL 5.6.7 and higher, the
default is file-per-table mode, where each table and its associated indexes are stored in a separate .ibd file.
This new default makes it easier to use InnoDB features that rely on the Barracuda file format, such as table
compression and the DYNAMIC row format.
In MySQL 5.6 and higher, setting a value for the innodb_undo_tablespaces option splits the undo log
into one or more separate tablespace files. These files are still considered part of the system tablespace.
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Keeping all table data in the system tablespace or in separate .ibd files has implications for storage
management in general. The MySQL Enterprise Backup product might back up a small set of large files, or
many smaller files. On systems with thousands of tables, the filesystem operations to process thousands of
.ibd files can cause bottlenecks.
See Also Barracuda, change buffer, compression, data dictionary, database, doublewrite buffer, dynamic
row format, file-per-table, .ibd file, ibdata file, innodb_file_per_table, instance, MySQL Enterprise Backup,
tablespace, undo log.
T
.TRG file
A file containing trigger parameters. Files with this extension are always included in backups produced by
the mysqlbackup command of the MySQL Enterprise Backup product.
See Also MySQL Enterprise Backup, mysqlbackup command, .TRN file.
.TRN file
A file containing trigger namespace information. Files with this extension are always included in backups
produced by the mysqlbackup command of the MySQL Enterprise Backup product.
See Also MySQL Enterprise Backup, mysqlbackup command, .TRG file.
table
Each MySQL table is associated with a particular storage engine. InnoDB tables have particular physical
and logical characteristics that affect performance, scalability, backup, administration, and application
development.
In terms of file storage, each InnoDB table is either part of the single big InnoDB system tablespace, or in a
separate .ibd file if the table is created in file-per-table mode. The .ibd file holds data for the table and all
its indexes, and is known as a tablespace.
InnoDB tables created in file-per-table mode can use the Barracuda file format. Barracuda tables can use the
DYNAMIC row format or the COMPRESSED row format. These relatively new settings enable a number of
InnoDB features, such as compression, fast index creation, and off-page columns
For backward compatibility with MySQL 5.1 and earlier, InnoDB tables inside the system tablespace must use
the Antelope file format, which supports the compact row format and the redundant row format.
The rows of an InnoDB table are organized into an index structure known as the clustered index, with
entries sorted based on the primary key columns of the table. Data access is optimized for queries that
filter and sort on the primary key columns, and each index contains a copy of the associated primary key
columns for each entry. Modifying values for any of the primary key columns is an expensive operation. Thus
an important aspect of InnoDB table design is choosing a primary key with columns that are used in the most
important queries, and keeping the primary key short, with rarely changing values.
See Also Antelope, backup, Barracuda, clustered index, compact row format, compressed row format,
compression, dynamic row format, Fast Index Creation, file-per-table, .ibd file, index, off-page column, primary
key, redundant row format, row, system tablespace, tablespace.
table lock
A lock that prevents any other transaction from accessing a table. InnoDB makes considerable effort to
make such locks unnecessary, by using techniques such as online DDL, row locks and consistent reads
for processing DML statements and queries. You can create such a lock through SQL using the LOCK
TABLE statement; one of the steps in migrating from other database systems or MySQL storage engines is to
remove such statements wherever practical.
See Also consistent read, DML, lock, locking, online DDL, query, row lock, table, transaction.
table scan
See full table scan.
table statistics
See statistics.
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table type
Obsolete synonym for storage engine. We refer to InnoDB tables, MyISAM tables, and so on.
See Also InnoDB, storage engine.
tablespace
A data file that can hold data for one or more InnoDB tables and associated indexes. The system
tablespace contains the tables that make up the data dictionary, and prior to MySQL 5.6 holds all the other
InnoDB tables by default. Turning on the innodb_file_per_table option, the default in MySQL 5.6 and
higher, allows newly created tables to each have their own tablespace, with a separate data file for each
table.
Using multiple tablespaces, by turning on the innodb_file_per_table option, is vital to using many
MySQL features such as table compression and transportable tablespaces, and managing disk usage. See
Using Per-Table Tablespaces and Improved Tablespace Management for details.
Tablespaces created by the built-in InnoDB storage engine are upward compatible with the InnoDB Plugin.
Tablespaces created by the InnoDB Plugin are downward compatible with the built-in InnoDB storage engine,
if they use the Antelope file format.
MySQL Cluster also groups its tables into tablespaces. See MySQL Cluster Disk Data Objects for details.
See Also Antelope, Barracuda, compressed row format, data dictionary, data files, file-per-table, index,
innodb_file_per_table, system tablespace, table.
tablespace dictionary
A representation of the data dictionary metadata for a table, within the InnoDB tablespace. This metadata
can be checked against the .frm file for consistency when the table is opened, to diagnose errors resulting
from out-of-date .frm files. This information is present for InnoDB tables that are part of the system
tablespace, as well as for tables that have their own .ibd file because of the file-per-table option.
See Also data dictionary, file-per-table, .frm file, .ibd file, system tablespace, tablespace.
temporary table
A table whose data does not need to be truly permanent. For example, temporary tables might be used as
storage areas for intermediate results in complicated calculations or transformations; this intermediate data
would not need to be recovered after a crash. Database products can take various shortcuts to improve the
performance of operations on temporary tables, by being less scrupulous about writing data to disk and other
measures to protect the data across restarts.
Sometimes, the data itself is removed automatically at a set time, such as when the transaction ends or when
the session ends. With some database products, the table itself is removed automatically too.
See Also table.
temporary tablespace
The tablespace for non-compressed InnoDB temporary tables and related objects. This tablespace was
introduced in MySQL 5.7.1. The configuration file option, innodb_temp_data_file_path, allows users
to define a relative path for the temporary data file. If innodb_temp_data_file_path is not specified,
the default behavior is to create a single auto-extending 12MB data file named ibtmp1 in the data directory,
alongside ibdata1. The temporary tablespace is recreated on each server start and receives a dynamically
generated space-id, which helps avoid conflicts with existing space-ids. The temporary tablespace cannot
reside on a raw device. Inability or error creating the temporary table is treated as fatal and server startup will
be refused.
The tablespace is removed on normal shutdown or on init abort, which may occur when a user specifies the
wrong startup options, for example. The temporary tablespace is not removed when a crash occurs. In this
case, the database administrator can remove the tablespace manually or restart the server with the same
configuration, which will remove and recreate the temporary tablespace.
See Also ibtmp file.
text collection
The set of columns included in a FULLTEXT index.
139
See Also FULLTEXT index.
thread
A unit of processing that is typically more lightweight than a process, allowing for greater concurrency.
See Also concurrency, master thread, process, Pthreads.
torn page
An error condition that can occur due to a combination of I/O device configuration and hardware failure. If
data is written out in chunks smaller than the InnoDB page size (by default, 16KB), a hardware failure while
writing could result in only part of a page being stored to disk. The InnoDB doublewrite buffer guards against
this possibility.
See Also doublewrite buffer.
TPS
Acronym for "transactions per second", a unit of measurement sometimes used in benchmarks. Its value
depends on the workload represented by a particular benchmark test, combined with factors that you control
such as the hardware capacity and database configuration.
See Also transaction, workload.
transaction
Transactions are atomic units of work that can be committed or rolled back. When a transaction makes
multiple changes to the database, either all the changes succeed when the transaction is committed, or all the
changes are undone when the transaction is rolled back.
Database transactions, as implemented by InnoDB, have properties that are collectively known by the
acronym ACID, for atomicity, consistency, isolation, and durability.
See Also ACID, commit, isolation level, lock, rollback.
transaction ID
An internal field associated with each row. This field is physically changed by INSERT, UPDATE, and
DELETE operations to record which transaction has locked the row.
See Also implicit row lock.
transportable tablespace
A feature that allows a tablespace to be moved from one instance to another. Traditionally, this has not
been possible for InnoDB tablespaces because all table data was part of the system tablespace. In MySQL
5.6 and higher, the FLUSH TABLES ... FOR EXPORT syntax prepares an InnoDB table for copying to
another server; running ALTER TABLE ... DISCARD TABLESPACE and ALTER TABLE ... IMPORT
TABLESPACE on the other server brings the copied data file into the other instance. A separate .cfg file,
copied along with the .ibd file, is used to update the table metadata (for example the space ID) as the
tablespace is imported. See Improved Tablespace Management for usage information.
See Also .ibd file, space ID, system tablespace, tablespace.
troubleshooting
Resources for troubleshooting InnoDB reliability and performance issues include: the Information Schema
tables.
truncate
A DDL operation that removes the entire contents of a table, while leaving the table and related indexes
intact. Contrast with drop. Although conceptually it has the same result as a DELETE statement with no
WHERE clause, it operates differently behind the scenes: InnoDB creates a new empty table, drops the old
table, then renames the new table to take the place of the old one. Because this is a DDL operation, it cannot
be rolled back.
If the table being truncated contains foreign keys that reference another table, the truncation operation uses a
slower method of operation, deleting one row at a time so that corresponding rows in the referenced table can
be deleted as needed by any ON DELETE CASCADE clause. (MySQL 5.5 and higher do not allow this slower
form of truncate, and return an error instead if foreign keys are involved. In this case, use a DELETE statement
instead.
140
See Also DDL, drop, foreign key, rollback.
tuple
A technical term designating an ordered set of elements. It is an abstract notion, used in formal discussions
of database theory. In the database field, tuples are usually represented by the columns of a table row. They
could also be represented by the result sets of queries, for example, queries that retrieved only some columns
of a table, or columns from joined tables.
See Also cursor.
two-phase commit
An operation that is part of a distributed transaction, under the XA specification. (Sometimes abbreviated as
2PC.) When multiple databases participate in the transaction, either all databases commit the changes, or all
databases roll back the changes.
See Also commit, rollback, transaction, XA.
U
undo
Data that is maintained throughout the life of a transaction, recording all changes so that they can be
undone in case of a rollback operation. It is stored in the undo log, also known as the rollback segment,
either within the system tablespace or in separate undo tablespaces.
See Also rollback, rollback segment, system tablespace, transaction, undo log, undo tablespace.
undo buffer
See undo log.
undo log
A storage area that holds copies of data modified by active transactions. If another transaction needs to see
the original data (as part of a consistent read operation), the unmodified data is retrieved from this storage
area.
By default, this area is physically part of the system tablespace. In MySQL 5.6 and higher, you can use the
innodb_undo_tablespaces and innodb_undo_directory configuration options to split it into one or
more separate tablespace files, the undo tablespaces, optionally stored on another storage device such as
an SSD.
The undo log is split into separate portions, the insert undo buffer and the update undo buffer. Collectively,
these parts are also known as the rollback segment, a familiar term for Oracle DBAs.
See Also consistent read, rollback segment, SSD, system tablespace, transaction, undo tablespace.
undo tablespace
One of a set of files containing the undo log, when the undo log is separated from the system tablespace
by the innodb_undo_tablespaces and innodb_undo_directory configuration options. Only applies to
MySQL 5.6 and higher.
See Also system tablespace, undo log.
unique constraint
A kind of constraint that asserts that a column cannot contain any duplicate values. In terms of relational
algebra, it is used to specify 1-to-1 relationships. For efficiency in checking whether a value can be inserted
(that is, the value does not already exist in the column), a unique constraint is supported by an underlying
unique index.
See Also constraint, relational, unique index.
unique index
An index on a column or set of columns that have a unique constraint. Because the index is known not
to contain any duplicate values, certain kinds of lookups and count operations are more efficient than in the
normal kind of index. Most of the lookups against this type of index are simply to determine if a certain value
exists or not. The number of values in the index is the same as the number of rows in the table, or at least the
number of rows with non-null values for the associated columns.
141
The insert buffering optimization does not apply to unique indexes. As a workaround, you can temporarily
set unique_checks=0 while doing a bulk data load into an InnoDB table.
See Also cardinality, insert buffering, unique constraint, unique key.
unique key
The set of columns (one or more) comprising a unique index. When you can define a WHERE condition that
matches exactly one row, and the query can use an associated unique index, the lookup and error handling
can be performed very efficiently.
See Also cardinality, unique constraint, unique index.
V
victim
The transaction that is automatically chosen to be rolled back when a deadlock is detected. InnoDB rolls
back the transaction that has updated the fewest rows.
See Also deadlock, deadlock detection, innodb_lock_wait_timeout.
W
wait
When an operation, such as acquiring a lock, mutex, or latch, cannot be completed immediately, InnoDB
pauses and tries again. The mechanism for pausing is elaborate enough that this operation has its own name,
the wait. Individual threads are paused using a combination of internal InnoDB scheduling, operating system
wait() calls, and short-duration spin loops.
On systems with heavy load and many transactions, you might use the output from the SHOW INNODB
STATUS command to determine whether threads are spending too much time waiting, and if so, how you can
improve concurrency.
See Also concurrency, latch, lock, mutex, spin.
warm backup
A backup taken while the database is running, but that restricts some database operations during the backup
process. For example, tables might become read-only. For busy applications and web sites, you might prefer
a hot backup.
See Also backup, cold backup, hot backup.
warm up
To run a system under a typical workload for some time after startup, so that the buffer pool and other
memory regions are filled as they would be under normal conditions.
This process happens naturally over time when a MySQL server is restarted or subjected to a new workload.
Starting in MySQL 5.6, you can speed up the warmup process by setting the configuration variables
innodb_buffer_pool_dump_at_shutdown=ON and innodb_buffer_pool_load_at_startup=ON, to
bring the contents of the buffer pool back into memory after a restart. Typically, you run a workload for some
time to warm up the buffer pool before running performance tests, to ensure consistent results across multiple
runs; otherwise, performance might be artificially low during the first run.
See Also buffer pool, workload.
Windows
The built-in InnoDB storage engine and the InnoDB Plugin are supported on all the same Microsoft Windows
versions as the MySQL server. The MySQL Enterprise Backup product has more comprehensive support for
Windows systems than the InnoDB Hot Backup product that it supersedes.
See Also InnoDB, MySQL Enterprise Backup, plugin.
workload
The combination and volume of SQL and other database operations, performed by a database application
during typical or peak usage. You can subject the database to a particular workload during performance
testing to identify bottlenecks, or during capacity planning.
142
See Also bottleneck, disk-bound, disk-bound, SQL.
write combining
An optimization technique that reduces write operations when dirty pages are flushed from the InnoDB
buffer pool. If a row in a page is updated multiple times, or multiple rows on the same page are updated, all
of those changes are stored to the data files in a single write operation rather than one write for each change.
See Also buffer pool, dirty page, flush.
X
XA
A standard interface for coordinating distributed transactions, allowing multiple databases to participate in a
transaction while maintaining ACID compliance. For full details, see XA Transactions.
XA Distributed Transaction support is turned on by default. If you are not using this feature, you can disable
the innodb_support_xa configuration option, avoiding the performance overhead of an extra fsync for each
transaction.
See Also commit, transaction, two-phase commit.
Y
young
A characteristic of a page in the InnoDB buffer pool meaning it has been accessed recently, and so is
moved within the buffer pool data structure, so that it will not be flushed soon by the LRU algorithm. This term
is used in some information schema column names of tables related to the buffer pool.
See Also buffer pool, flush, INFORMATION_SCHEMA, LRU, page.
143
144
Index
Symbols
.ARM file, 91
.ARZ file, 91
.cfg file, 96
.frm file, 106
.ibd file, 111
.ibz file, 111
.isl file, 111
.MRG file, 119
.MYD file, 119
.MYI file, 119
.OPT file, 123
.PAR file, 125
.TRG file, 138
.TRN file, 138
A
ACID, 91
adaptive flushing, 91
adaptive hash index, 39, 49, 91
AHI, 92
AIO, 92
ALTER TABLE
ROW_FORMAT, 26
Antelope, 92
Antelope file format, 19
application programming interface (API), 92
apply, 92
asynchronous I/O, 92
atomic, 92
atomic instruction, 92
auto-increment, 93
auto-increment locking, 93
autocommit, 93
availability, 93
B
B-tree, 93
background threads
master, 43, 43
read, 42
write, 42
backticks, 94
backup, 94
Barracuda, 94
Barracuda file format, 19
beta, 94
binary log, 94
binlog, 95
blind query expansion, 95
bottleneck, 95
bounce, 95
buddy allocator, 28, 95
buffer, 95
buffer cache, 44
buffer pool, 95
buffer pool instance, 95
built-in, 96
business rules, 96
C
cache, 96
cardinality, 96
change buffer, 97
change buffering, 73, 97
disabling, 39
checkpoint, 97
checksum, 97
child table, 98
clean page, 98
clean shutdown, 98
client, 98
clustered index, 98
cold backup, 98
column, 98
column index, 98
column prefix, 98
commit, 99
compact row format, 99
compiling, 61
composite index, 99
compressed backup, 99
compressed row format, 99
compression, 100
algorithms, 15
application and schema design, 13
BLOBs, VARCHAR and TEXT, 17
buffer pool, 17
compressed page size, 14
configuration characteristics, 14
data and indexes, 16
data characteristics, 13
enabling for a table, 9
implementation, 15
information schema, 27, 27
innodb_strict_mode, 49
KEY_BLOCK_SIZE, 14
log files, 18
modification log, 16
monitoring, 15
overflow pages, 17
overview, 9
tuning, 12
workload characteristics, 14
compression failure, 100
concurrency, 100
configuration file, 100
configuring, 64
consistent read, 101
constraint, 101
counter, 101
covering index, 101
crash, 101
145
crash recovery, 101
CREATE INDEX, 5
CREATE TABLE
KEY_BLOCK_SIZE, 14
options for table compression, 9
ROW_FORMAT, 26
CRUD, 102
cursor, 102
D
data dictionary, 102
data directory, 102
data files, 102
data warehouse, 102
database, 102
DCL, 103
DDL, 103
deadlock, 103
deadlock detection, 103
delete, 104
delete buffering, 104
denormalized, 104
descending index, 104
dirty page, 104
dirty read, 104
disk-based, 104
disk-bound, 104, 104
DML, 105
document id, 105
doublewrite buffer, 105
downgrading, 69
drop, 105
DROP INDEX, 5
dynamic row format, 105
E
early adopter, 105
error log, 106
eviction, 106
exclusive lock, 106
extent, 106
F
Fast Index Creation, 106
concurrency, 7
crash recovery, 7
examples, 5
implementation, 6
limitations, 8
overview, 5
fast shutdown, 107
file format, 19, 107
Antelope, 17
Barracuda, 9
downgrading, 23
identifying, 23
file format management
downgrading, 69
enabling new file formats, 47
file per table, 48
file-per-table, 107
fill factor, 107
fixed row format, 107
flush, 107
flush list, 108
foreign key, 108
FOREIGN KEY constraint, 108
FOREIGN KEY constraints
and fast index creation, 8
and TRUNCATE_TABLE, 49
FTS, 108
full backup, 108
full table scan, 108
full-text search, 108
FULLTEXT index, 109
fuzzy checkpointing, 109
G
GA, 109
gap, 109
gap lock, 109
general query log, 109
global_transaction, 109
group commit, 42, 110
H
hash index, 110
HDD, 110
heartbeat, 110
high-water mark, 110
history list, 110
hot, 110
hot backup, 110
I
ib-file set, 20, 111
ibbackup_logfile, 111
ibdata file, 112
ibtmp file, 112
ib_logfile, 112
ignore_builtin_innodb, 54
and skip_innodb, 85
ilist, 112
implicit row lock, 112
in-memory database, 112
incremental backup, 112
index, 113
index cache, 113
index dives (for statistics estimation), 50
index hint, 113
index prefix, 113
indexes
creating and dropping, 6
primary (clustered) and secondary, 6
146
infimum record, 113
INFORMATION_SCHEMA, 27, 113
INNODB_CMP table, 27
INNODB_CMPMEM table, 28
INNODB_CMPMEM_RESET table, 28
INNODB_CMP_RESET table, 27
INNODB_LOCKS table, 30
INNODB_LOCK_WAITS table, 31
INNODB_TRX table, 29
InnoDB, 114
troubleshooting
fast index creation, 8
InnoDB Plugin
compatibility, 1
downloading, 2
features, 1
installing, 2
restrictions, 3
innodb_adaptive_flushing, 43
innodb_adaptive_hash_index, 39
and innodb_thread_concurrency, 40
dynamically changing, 49
innodb_additional_mem_pool_size
and innodb_use_sys_malloc, 38
innodb_autoinc_lock_mode, 114
innodb_change_buffering, 39
innodb_concurrency_tickets, 40
innodb_file_format, 19, 114
Antelope, 17
Barracuda, 9
downgrading, 69
enabling new file formats, 47
identifying, 23
innodb_file_format_check, 21
innodb_file_io_threads, 42
innodb_file_per_table, 9, 114
dynamically changing, 48
innodb_io_capacity, 43
innodb_lock_wait_timeout, 114
dynamically changing, 49
innodb_max_dirty_pages_pct, 43
innodb_old_blocks_pct, 44
innodb_old_blocks_time, 44
innodb_read_ahead_threshold, 41
innodb_read_io_threads, 42
innodb_spin_wait_delay, 44
innodb_stats_on_metadata
dynamically changing, 48
innodb_stats_sample_pages, 50
innodb_strict_mode, 49, 115
innodb_thread_concurrency, 40
innodb_thread_sleep_delay, 40
innodb_use_sys_malloc, 38
and innodb_thread_concurrency, 40
innodb_write_io_threads, 42
insert, 115
insert buffer, 115
insert buffering, 73, 115
disabling, 39
installing
binary InnoDB Plugin, 54, 85
instance, 115
instrumentation, 115
intention lock, 115
internal memory allocator
disabling, 38
inverted index, 116
IOPS, 116
isolation level, 116
J
join, 116
K
KEY_BLOCK_SIZE, 9, 14, 116
L
latch, 116
list, 117
lock, 117
lock escalation, 117
lock mode, 117
lock wait timeout, 49
locking, 117
information schema, 27, 29, 35
locking read, 117
log, 117
log buffer, 118
log file, 118
log group, 118
logical, 118
logical backup, 118
loose_, 118
low-water mark, 118
LRU, 118
LRU page replacement, 44
LSN, 118
M
master server, 119
master thread, 119
MDL, 119
memcached, 119
memory allocator
innodb_use_sys_malloc, 38
merge, 120
metadata lock, 120
metrics counter, 120
midpoint insertion, 44
midpoint insertion strategy, 120
mini-transaction, 120
mixed-mode insert, 120
multi-core, 120
mutex, 120
MVCC, 121
147
my.cnf, 121
my.ini, 121
mysql, 121
MySQL Enterprise Backup, 121
mysqlbackup command, 121
mysqld, 121
mysqldump, 121
N
natural key, 121
neighbor page, 122
next-key lock, 122
non-blocking I/O, 122
non-locking read, 122
non-repeatable read, 122
normalized, 122
NoSQL, 123
NOT NULL constraint, 123
NULL, 123
O
off-page column, 123
OLTP, 123
online, 124
online DDL, 124
optimistic, 124
optimizer, 124
optimizer statistics estimation, 48, 50
option, 124
option file, 125
overflow page, 125
innodb_thread_concurrency, 89
parent table, 125
partial backup, 126
partial index, 126
Performance Schema, 126
persistent statistics, 126
pessimistic, 126
phantom, 126
physical, 126
physical backup, 126
PITR, 126
plan stability, 127
plugin, 127
point-in-time recovery, 127
prepared backup, 127
primary key, 127
process, 127
PROCESSLIST
possible inconsistency with
INFORMATION_SCHEMA tables, 36
pseudo-record, 127
Pthreads, 127
purge, 128
purge buffering, 128
purge lag, 128
purge thread, 128
Q
query, 128
query execution plan, 128
quiesce, 128
P
R
page, 125
page cleaner, 125
page size, 125
parameters, deprecated, 89
innodb_file_io_threads, 42
parameters, new, 87
innodb_adaptive_flushing, 43
innodb_change_buffering, 39
innodb_file_format, 47
innodb_file_format_check, 21
innodb_io_capacity, 43
innodb_read_ahead_threshold, 41
innodb_read_io_threads, 42
innodb_spin_wait_delay, 44
innodb_stats_sample_pages, 50
innodb_strict_mode, 49
innodb_use_sys_malloc, 38
innodb_write_io_threads, 42
parameters, with new defaults, 89
innodb_additional_mem_pool_size, 89
innodb_buffer_pool_size, 89
innodb_log_buffer_size, 89
innodb_max_dirty_pages_pct, 43, 89
innodb_sync_spin_loops, 89
RAID, 128
random dive, 129
raw backup, 129
read ahead, 52
linear, 41
random, 41
READ COMMITTED, 129
READ UNCOMMITTED, 129
read view, 129
read-ahead, 129
read-only transaction, 130
record lock, 130
redo, 130
redo log, 130
redundant row format, 130
referential integrity, 130
relational, 130
relevance, 131
REPEATABLE READ, 131
replication, 131
restore, 131
rollback, 131
rollback segment, 131
row, 131
148
row format, 132
row lock, 132
row-based replication, 132
row-level locking, 132
ROW_FORMAT
COMPACT, 25
COMPRESSED, 9, 25
DYNAMIC, 25
REDUNDANT, 25
rw-lock, 132
S
savepoint, 132
scalability, 132
scale out, 133
scale up, 133
schema, 133
search index, 133
secondary index, 133
segment, 133
selectivity, 134
semi-consistent read, 134
SERIALIZABLE, 134
server, 134
shared lock, 134
shared tablespace, 134
sharp checkpoint, 134
SHOW ENGINE INNODB MUTEX, 52
SHOW ENGINE INNODB STATUS
and innodb_adaptive_hash_index, 39
and innodb_use_sys_malloc, 38
shutdown, 134
skip_innodb
and ignore_builtin_innodb, 85
slave server, 135
slow query log, 135
slow shutdown, 135
snapshot, 135
source code, 61
space ID, 135
spin, 135
SQL, 135
SSD, 9, 136
startup, 136
statement-based replication, 136
statistics, 136
stemming, 136
stopword, 136
storage engine, 136
strict mode, 49, 137
sublist, 137
supremum record, 137
surrogate key, 137
synthetic key, 137
system tablespace, 137
T
table, 138
table lock, 138
table scan, 44
table type, 139
tablespace, 139
tablespace dictionary, 139
temporary table, 139
temporary tablespace, 139
text collection, 139
thread, 140
torn page, 140
TPS, 140
transaction, 140
transaction ID, 140
transportable tablespace, 140
troubleshooting, 140
truncate, 140
TRUNCATE TABLE, 49
tuple, 141
two-phase commit, 141
U
undo, 141
undo log, 141
undo tablespace, 141
unique constraint, 141
unique index, 141
unique key, 142
upgrading
converting compressed tables, 68
dynamic plugin, 67
static plugin, 67
V
victim, 142
W
wait, 142
warm backup, 142
warm up, 142
Windows, 142
workload, 142
write combining, 143
X
XA, 143
Y
young, 143
149
150
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