Guide to Using SQL: Synonyms and the Rename Statement

Guide to Using SQL: Synonyms and the Rename Statement
Guide to Using SQL:
Synonyms and the Rename Statement
A feature of Oracle Rdb
By Ian Smith
Oracle Rdb Relational Technology Group
Oracle Corporation
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Oracle Rdb Journal – Synonyms and Rename Statement
GUIDE TO USING SQL: SYNONYMS AND THE RENAME STATEMENT.........................................................3
SYNONYMS .....................................................................................................................................................................3
SETTING UP THE DATABASE ...........................................................................................................................................5
OBJECT NAME SPACES ....................................................................................................................................................6
TRACKING DATABASE OBJECT DEPENDENCIES ..............................................................................................................7
DISPLAYING SYNONYMS .................................................................................................................................................7
RMU EXTRACT AND SYNONYMS ....................................................................................................................................8
Example 1: Switching Tables .....................................................................................................................................8
Example 2: Adding an Enhanced Function................................................................................................................9
THE RENAME STATEMENT .........................................................................................................................................11
OVERVIEW OF THE RENAME ACTION .............................................................................................................................11
FREQUENTLY ASKED QUESTIONS ..................................................................................................................................14
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Oracle Rdb Journal – Synonyms and Rename Statement
The Rdb Technical Corner is a regular feature of the Oracle Rdb Web Journal. The examples in
this article use SQL language from Oracle Rdb release 7.1 and later versions.
Guide to Using SQL: Synonyms and the Rename statement
Oracle Rdb release 7.1 introduced a new synonyms feature that allows a database administrator to
define alternate names for existing database objects. This feature is the foundation for the rename
statement that was added in Oracle Rdb release 7.1.2.
Synonyms
The dictionary defines synonym as a word that means exactly the same as another word, or a word
that can be interchanged with another word. This describes the intent of synonyms in Oracle Rdb.
The database administrator has a facility to define equivalent names for an object in the database, in
particular those objects that are commonly referenced in data definition statements (DDL) and data
manipulation statement (DML).
The following table lists those objects that can be given a synonym.
Object Types
Table or
View
DML Usage
Can be referenced in a select,
insert, update, or delete statement.
Subselect clauses referencing the
synonym can appear in various
places as a value for set
assignment, and as a parameter to
call statements.
Domain
Can be referenced in a CAST
function as the source of the data
type. Interactive SQL allows the
domain to be referred to in the edit
using clause, or declare variable
1
DDL Usage
Can be referenced by view, trigger,
constraint, module (declare
transaction clause, default clause
for a module variable), storage map
and index definitions. Target for
alter, comment, drop, grant,
rename table, revoke, and
truncate1.
Can be referenced as the source
data type for a column. Target for
alter, comment on, drop, and
rename.
truncate is only permitted for tables, not views.
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Oracle Rdb Journal – Synonyms and Rename Statement
Sequence
statement.
Can be referenced in a select,
insert, update, or delete statement.
Can appear in various places as a
value for set assignment, and as a
parameter to call statements.
Synonym
Module
Function
Procedure
Can be referenced in a select,
insert, update, or delete statement.
Can appear in various places as a
value for set assignment, and as a
parameter to call statements.
Can be referenced in a call
statement.
Target for alter, comment on,
drop, grant, rename, and revoke.
Can be used as a substitute name in
all DDL statements. Chains of
synonyms can be created. Can be
synonyms target for alter,
comment on, drop, and rename.
Target for alter, comment on,
drop, grant, rename, and revoke.
Can be referenced in a computed
by, automatic as, or default
clause. Target for alter, comment
on, drop, grant, rename, and
revoke.
Can be referenced from functions
procedures, and triggers actions.
Target for alter, comment on,
drop, grant, rename, and revoke.
As database systems grow and applications are enhanced there is a tendency to evolve the naming
conventions, change names of tables, views and routines to better reflect their new and changed
functionality. For example, a stored procedure named ADD_CUSTOMER might grow to verify
credit limits as well as being used to add a new customer. Therefore, it might be natural to change
the name to something like VERIFY_NEW_CUSTOMER.
The problem comes in changing the name in all places before placing the revised application into
production. Synonyms allow both names to be used in the application and database. These
references might exist in SQL module language applications, stored procedures, and interactive
SQL maintenance scripts. Synonyms allow a level of safety when the switch to the new name is
performed.
A curious feature of a synonym is that each may have its own synonym or many synonyms. For
example, a table might have a formal name such as REL001_ORDER_TABLE, a simple name to
allow easier interactive querying ORDER_TABLE, and a lazy name, OT, for people like myself
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Oracle Rdb Journal – Synonyms and Rename Statement
who prefer to type fewer characters. They could also be defined as synonyms for the base table
name, or as chains.
SQL> create synonym ORDER_TABLE for table REL001_ORDER_TABLE;
SQL> create synonym OT for table REL001_ORDER_TABLE;
Or they could be defined as a chain of synonyms, each referencing the previous definition:
SQL> create synonym ORDER_TABLE for table REL001_ORDER_TABLE;
SQL> create synonym OT for table ORDER_TABLE;
The advantage of chaining is that an alter synonym statement can change ORDER_TABLE and
the change will automatically be reflected by all synonyms up the chain. You can just issue a
single alter synonym statement instead of many. The following alter synonym statement will
implicitly change the OT synonym to reference the new table name:
SQL> alter synonym ORDER_TABLE for REL001_ORDER_TABLE_REVISION02;
Oracle Rdb assumes, but does not check, that the new table is identical in structure (column names
and types) to the previously referenced table, or at least contains a superset of columns of the
original table. An incompatible change would be detected at runtime if a routine or trigger, for
instance, referenced a column that no longer existed, or had a different (and incompatible) data
type.
These synonym chains can be connected up to 64 times before Oracle Rdb reports that an internal
limit has been exceeded. This should be ample for most complex environments.
Setting Up the Database
The database administrator must enable support of the synonyms feature using the alter database
… synonyms are enabled2 clause. When this statement is first executed a new system table
named Rdb$OBJECT_SYNONYMS3 is created and used to record the translation of the synonym
to a base object, or to another synonym. This new table can never be removed from the database as
2
This clause is not compatible with multischema is on, and therefore multischema databases do not support the
features described in the paper.
3
Observant Rdb users might notice that a table Rdb$SYNONYMS exists in a multischema database. However, this
table provides a different function and is not used by the synonyms feature.
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Oracle Rdb Journal – Synonyms and Rename Statement
there is no synonyms are disabled clause. However, there is very little overhead having this table
in the database. Use the show database command to determine if synonyms are enabled.
SQL> show database *
Default alias:
Oracle Rdb database in file personnel
Multischema mode is disabled
Synonyms are enabled
…
If the database is already defined as multischema is enabled then you will not be permitted to
enable the synonyms feature. At this time these two settings are mutually exclusive due to the
complexity imposed by the multischema metadata.
Object Name Spaces
The name given to an Oracle Rdb object exists in separate name spaces. For instance, because
Oracle Rdb keeps separate name spaces for objects and you could name a table, an index, a trigger
and a constraint all with the same name.
The name spaces used in Oracle Rdb are:
•
•
•
•
•
•
•
•
•
•
Table, view and sequence
Constraints
Triggers
Indices
Routines (functions and procedures)
Modules
Storage Maps
Storage Areas
Domains
Query Outlines
Sequences and views share the table name space because their name can appear in the similar
locations as a table name. Therefore, using unique names across all three objects allows Oracle
Rdb to choose the correct object.
When you create a synonym the name cannot exist in any other name space. This prevents the user
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Oracle Rdb Journal – Synonyms and Rename Statement
creating an ambiguous synonym. For example:
SQL> create synonym ID_DOM for sequence ID_SEQ;
%RDB-E-NO_META_UPDATE, metadata update failed
-RDMS-E-INVSYNONYM, invalid synonym name "ID_DOM" - name used by another object
Tracking Database Object Dependencies
When various objects are referenced in a data definition statement a dependency row is stored in
the RDB$INTERRELATIONS table. These rows always describe the base objects name and not
the synonym allowing Oracle Rdb to detect a dependency no matter what name is used. In most
cases an error message will report the base object name and not the synonym named in the
command.
Displaying Synonyms
The show synonym command formats the contents of the Rdb$OBJECT_SYNONYMS table. For
example, the following show command displays all synonyms defined in the database:
SQL> show synonyms;
Synonyms in database with filename personnel
CURRENT_POSITION
View
CURRENT_SALARY
View
STAFFING
Table
UNIVERSAL_TIMESTAMP
Domain
CURRENT_JOB
ACTIVE_SALARY_PLAN
EMPLOYEES
STANDARD_DATE
The show commands for each object will also list synonyms below the list of objects. For
example, a show view command will display the names of each view and any synonyms that exist
for views.
SQL> show views;
User tables in database with filename personnel
ACTIVE_SALARY_PLAN
A view.
CURRENT_INFO
A view.
CURRENT_JOB
A view.
CURRENT_POSITION
A synonym for view CURRENT_JOB
CURRENT_SALARY
A synonym for view ACTIVE_SALARY_PLAN
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Oracle Rdb Journal – Synonyms and Rename Statement
If the show table or show view command is given a synonym it will inform you that the given
name was a synonym and then display the details for the base table or view.
SQL> show table (column) e;
E
Information for table EMPLOYEES
A synonym for table EMPLOYEES
Columns for table EMPLOYEES:
Column Name
Data Type
------------------EMPLOYEE_ID
CHAR(5)
… etc …
SQL>
Domain
-----ID_NUMBER
RMU Extract and Synonyms
When synonyms are enabled for a database, the RMU Extract command will include those
synonyms with the table definition when you use the Option=SYNONYMS qualifier. The
synonym chain for that table will follow immediately after the object definition.
An alternate report of synonyms can be extracted using Item=SYNONYMS qualifier on the RMU
Extract command which extracts all the synonyms in one section. While this command is useful to
display the definitions it is not always practical when rebuilding the database as the synonyms
might be referenced by other definitions prior to the synonym being defined.
Example 1: Switching Tables
A financial institution records database transactions in a single table. Their design requires that
every six months the table will be processed to calculate and generate interest payments. They
wish to freeze the table at end of the month processing and resuming on a new table the next day.
This can be accomplished using several tables and a synonym.
Initially the application defines the table, TRANSACTION_DETAILS_001, and the synonym
TRANSACTION_DETAILS. Applications written for this database only refer to the name
TRANSACTION_DETAILS. When it becomes time to process the table, a simple alter synonym
statement is used to switch to another identically defined table, such as
TRANSACTION_DETAILS_002.
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Oracle Rdb Journal – Synonyms and Rename Statement
The alter synonym can be executed online, even if some applications are still running. These may
be reporting applications that have used the previous synonym definition to reference the old table.
Therefore, these applications will have to disconnect or restart at some point to so that the
synonym is reloaded and the new target table located.
How is this different from using a view? While it is true that the view could be dropped and
redefined to reference the new table, the view includes table metadata locking semantics that will
prevent the redirection while applications are active. This complicates the switch to the new table
because the applications would be required to first disconnect and then wait while the view was
changed. On the other hand the synonym change will be allowed online, and the change visible
upon the next attach to the database. Another problem with the view approach is that any
dependencies on the view would be invalidated by the drop view, and there are many options
(create trigger, create index and create storage map) that are not permitted for views.
Synonyms can be used without any such restrictions.
Example 2: Adding an Enhanced Function
When a function changes considerably it may be difficult to test in the database using the same
function name. Therefore, a new function can be created and tested and placed into production for
all new procedures and applications. Rather than editing the existing code it may be easier to use
drop function and add a synonym using the old name of the function.
Consider this simple example. The procedure add_employee is used to insert a new employee into
the employees table, along with accompanying salary_history, job_history rows. It also allocates
and returns a new employee_id.
SQL> create module M
cont>
procedure ADD_EMPLOYEE
cont>
(in :last_name char(40)
cont>
,in :first_name char(30)
cont>
,out :employee_id id_dom);
cont>
begin … end;
cont> end module;
SQL>
At some later time the function is superseded in an upward compatible way by defining the new
parameters to be optional using the default clause, and using varchar instead of the original char
types.
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Oracle Rdb Journal – Synonyms and Rename Statement
SQL> alter module M
cont>
add procedure ADD_EMPLOYEE_DETAILS
cont>
(in :last_name varchar(40)
cont>
,in :first_name varchar(30)
cont>
,out :employee_id id_dom
cont>
,in :address_line_1 varchar(70) default null
cont>
,in :address_line_2 varchar(70) default null);
cont>
begin … end;
cont> end module;
By designing the replacement with the same required parameters the synonym can safely reference
the new routine. In a single transaction the obsolete procedure can be dropped and replaced by a
synonym.
SQL> alter module M
cont>
drop procedure ADD_EMPLOYEE cascade;
SQL>
SQL> create synonym ADD_EMPLOYEE
cont>
for procedure ADD_EMPLOYEE_DETAILS;
Now references to either ADD_EMPLOYEE or ADD_EMPLOYEE_DETAILS will activate the
same routine.
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Oracle Rdb Journal – Synonyms and Rename Statement
The RENAME Statement
The rename statement can be used to change the name of an object in the database. The format of
the command is shown here. The bold keywords are required and the references in brackets [] are
optional.
rename [ object-type ] old-name to new-name;
The effect of this statement is to change the object name within the Oracle Rdb metadata. For
instance, a rename table command will update the Rdb$RELATIONS (table definition),
Rdb$RELATION_FIELDS (column definitions), Rdb$INDICES (table index definitions),
Rdb$INTERRELATIONS (dependency table), and so on. The end result is that each system table
will now reference the new object name.
Object-type can be one of the following keywords: domain, function, module, procedure, profile,
role, sequence, synonym, table, user, or view. If it is omitted Oracle Rdb will search for the named
object among all the system tables and use the first match that it finds. If your database uses the
same name for different object types it is worthwhile using the full statement syntax to avoid any
ambiguity.
The rename statement is a special form of the alter statement. In all cases the rename statement is
equivalent to the corresponding alter … rename to statement4. For the purposes of this article we
will only describe the rename statement but the discussion applies equally to the rename to clause
of the various alter statements.
Overview of the rename action
Consider what occurs when an object is created and used in an Oracle Rdb database. For example,
when a sequence is created a row is written to the Rdb$SEQUENCES system table recording the
name and attributes of the sequence. This name can be now be referenced by data definition
commands such as alter, drop, grant and revoke, and appear within the definitions of constraints,
views, stored procedures, triggers, computed by and automatic as columns. These definitions are
stored in the database for the particular object as well as appearing in dependency rows of the
4
The alter view statement was introduced in Rdb V7.1.4.1, so in prior versions only the rename view statement can be
used.
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Oracle Rdb Journal – Synonyms and Rename Statement
Rdb$INTERRELATIONS system table. This table allows drop and alter statements to detect and
report the usage of these objects.
When SQL processes a rename sequence statement it must change the name in the
Rdb$SEQUENCES table and also the dependency rows in Rdb$INTERRELATIONS. However,
Oracle Rdb makes no attempt to modify the original source definitions provided by the user that
reference the sequence; these SQL sources are effectively descriptive text retained for the database
administrator. Nor can Oracle Rdb modify the external source code stored in module language, or
pre-compiled applications. Therefore, Oracle Rdb implicitly creates a synonym using the old name
of the sequence (or other object) being renamed.
The database must support synonyms; otherwise this error will be reported.
SQL> rename sequence dept_id to next_department_id;
%RDB-E-NO_META_UPDATE, metadata update failed
-RDMS-E-UNSSYNONYM, this database does not have synonyms enabled
When the rename sequence statement is successful you can use the show sequence command to
display all available sequences, including the synonym created with the old name of the sequence.
SQL> show sequence
Sequences in database with filename SQL$DATABASE
NEXT_DEPARTMENT_ID
RESTART_SEQUENCE
DEPT_ID
A synonym for sequence NEXT_DEPARTMENT_ID
Each object listed by the show command has an indication for those that are synonyms. The show
sequence command will accept either the name of the synonym5, or the base sequence name:
SQL> show sequence dept_id
DEPT_ID
NEXT_DEPARTMENT_ID
Sequence Id: 1
Initial Value: 1
Minimum Value: 1
.
.
.
5
A synonym for sequence NEXT_DEPARTMENT_ID
You must be running Oracle Rdb release 7.1.3 or later to have synonym support in the SHOW command.
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Oracle Rdb Journal – Synonyms and Rename Statement
Now either the old-name or the new-name can be used to reference the object. Oracle Rdb relies on
this feature to allow constraints, triggers and routines to continue functioning, as they will continue
to use the old-name.
Over time, as these triggers and constraints are redefined, it might be possible to drop the old
synonym when it no longer referenced. However, keep in mind that Oracle Rdb does not track
external references, such as a SQL Module Language procedure that may be using that old name.
Therefore, it is important that the synonyms created by the rename command not be dropped as
they could leave the database unusable. For instance if the synonym is removed then that part of
the application may no longer function and would report an error similar to the following example:
%RDB-E-OBSOLETE_METADA, request references metadata objects that no longer
exist
-RDMS-F-BAD_SYM, unknown sequence symbol - DEPT_ID
This problem can easily be repaired by creating a synonym with the create synonym statement.
Once the rename completes the old name no longer has dependency rows in the database (for
instance in the system table Rdb$INTERRELATIONS) and so drop and alter cannot protect the
metadata by reporting dependencies. Therefore, later releases6 of Oracle Rdb require DBADMIN
privilege for the drop synonym statement when a rename statement created the synonym.
SQL> show synonym dept_id;
DEPT_ID
for sequence NEXT_DEPARTMENT_ID
Comment:
Generated by RENAME SEQUENCE
SQL>
The comment for the synonym shows that it was created by rename. This comment can be
replaced by the database administrator at any time using the comment on synonym or alter
synonym … comment is statements.
It was stated that the rename statement required that synonyms be enabled on the database. There
is a small set of objects that do not require synonyms at all: roles, users and profiles. These
objects are fully contained in the metadata. Therefore, if you have SQL scripts or applications that
use the names of users or roles, then these applications will need to be modified if one of these
objects is renamed.
6
This privilege requirement is enforced by Oracle Rdb release 7.1.4.4 and 7.2.0.2 and later.
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Oracle Rdb Journal – Synonyms and Rename Statement
Frequently asked Questions
How efficient are synonyms? The first reference to the synonym requires that the Oracle Rdb
server traverse the chain and locate the referenced object. Once found the synonym name and base
object name are stored in the symbol table; future accesses require no extra I/O. Oracle Rdb limits
the longest chain to 64 elements.
What happens if I delete a synonym that makes up a chain? A cascade will be required for
Oracle Rdb to allow this to happen. In this case Oracle Rdb will report an error at runtime as it will
not be able to locate the base object through the chain. The error that is reported will be the same
as seen by the database administrator after a drop … cascade command is executed on a function
or procedure. Such operations succeed but leave partial definitions in the database. Future
references to an object that depends on those missing objects will result in an error reporting the
missing object.
Can I have more than one synonym referencing an object? Yes. You can have as many
synonyms as you like. However, take care not to overwhelm the database programmers with too
many names as these will require extra management.
Can I use synonyms in the SHOW commands? Starting with Oracle Rdb release 7.1.3, SQL
translates the synonyms in the show command. Show will also list any synonyms for the object
type being listed. You can also use show synonym to display all synonyms in the database.
Can I use synonyms in RMU Load and Unload commands? Starting with Oracle Rdb release
7.1.4.4, and 7.2.0.2 RMU will accept synonyms for these commands. They will be translated
internally to the name of the base table or view. Therefore, the record definition file (.RRD) or the
unload interchange file (.UNL) will be created using the base table or view name. RMU Load will
expect to be loading the base table name, so in some cases you may need to use the Match_Name
qualifier with the name of the table as stored in the definition file.
Can I use synonyms in the RMU Extract command? The RMU Extract Option=MATCH
qualifier does not currently support synonyms for objects (other than synonym names for the
Item=SYNONYM qualifier). The wildcard matching supported by RMU Extract uses base object
names only.
What happens if the DBA decides to rename the object back to its old name? You might expect
that the first task would be to drop the synonym created with the old name. However, since this is
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Oracle Rdb Journal – Synonyms and Rename Statement
likely to be a common action SQL handles this automatically, and removes the synonym for you.
Is renaming a SYSTEM table permitted? For obvious reasons this is not allowed as this would
prevent the database from being used. Client tools such as Interactive SQL, ODBC, SQL/Services,
JDBC and so on assume the documented names of the Oracle Rdb system metadata.
What happens when a table has an identity column? In this case both the table and the identity
sequence have the same name. SQL does not allow you to rename the identity sequence.
However, when the table is renamed the identity sequence is also implicitly renamed so that the
sequence and table remain bound together
Oracle Rdb
Guide to Using SQL: Synonyms and the Rename Command
May 2006
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Oracle Rdb Journal – Synonyms and Rename Statement
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