Second Level of Data Independence Data Independence Type

Second Level of Data Independence Data Independence Type
Data independence
Data independence is the type of data transparency that matters for a centralised DBMS. It refers to the
immunity of user applications to changes made in the definition and organization of data.
Physical data independence deals with hiding the details of the storage structure from user applications.
The application should not be involved with these issues, since there is no difference in the operation
carried out against the data.
The data independence and operation independence together gives the feature of data abstraction. There
are two levels of data independence.
The logical structure of the data is known as the 'schema definition'. In general, if a user application
operates on a subset of the attributes of a relation, it should not be affected later when new attributes are
added to the same relation. Logical data independence indicates that the conceptual schema can be
changed without affecting the existing schemas.
Second Level of Data Independence
The physical structure of the data is referred to as "physical data description". Physical data independence
deals with hiding the details of the storage structure from user applications. The application should not be
involved with these issues since, conceptually, there is no difference in the operations carried out against
the data. There are three types of data independence:
1. Logical data independence: The ability to change the logical (conceptual) schema without
changing the External schema (User View) is called logical data independence. For example, the
addition or removal of new entities, attributes, or relationships to the conceptual schema should
be possible without having to change existing external schemas or having to rewrite existing
application programs.
2. Physical data independence: The ability to change the physical schema without changing the
logical schema is called physical data independence. For example, a change to the internal
schema, such as using different file organization or storage structures, storage devices, or
indexing strategy, should be possible without having to change the conceptual or external
schemas.
3. View level data independence: always independent no effect, because there doesn't exist any
other level above view level.
Data Independence
Data independence can be explained as follows: Each higher level of the data architecture is immune to
changes of the next lower level of the architecture.
The logical scheme stays unchanged even though the storage space or type of some data is changed for
reasons of optimization or reorganization. In this external schema does not change. In this internal schema
changes may be required due to some physical schema were reorganized here. Physical data independence
is present in most databases and file environment in which hardware storage of encoding, exact location
of data on disk,merging of records, so on this are hidden from user.
One of the biggest advantage of databases is data independence. It means we can change the conceptual
schema at one level without affecting the data at another level. It also means we can change the structure
of a database without affecting the data required by users and programs. This feature was not available in
the file oriented approach.
Data Independence Type
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The ability to modify schema definition in one level without affecting schema definition in the next higher
level is called data independence. There are two levels of data independence, they are Physical data
independence and Logical data independence.
1. Physical data independence is the ability to modify the physical schema without causing
application programs to be rewritten. Modifications at the physical level are occasionally
necessary to improve performance. It means we change the physical storage/level without
affecting the conceptual or external view of the data. The new changes are absorbed by mapping
techniques.
2. Logical data independence is the ability to modify the logical schema without causing application
program to be rewritten. Modifications at the logical level are necessary whenever the logical
structure of the database is altered (for example, when money-market accounts are added to
banking system). Logical Data independence means if we add some new columns or remove
some columns from table then the user view and programs should not change. For example:
consider two users A & B. Both are selecting the fields "EmployeeNumber" and
"EmployeeName". If user B adds a new column (e.g. salary) to his table, it will not effect the
external view for user A, though the internal schema of the database has been changed for both
users A & B.
Logical data independence is more difficult to achieve than physical data independence, since application
programs are heavily dependent on the logical structure of the data that they access.
Physical data independence means we change the physical storage/level without affecting the conceptual
or external view of the data. Mapping techniques absorbs the new changes.
If a database system is not multi-layered, then it becomes difficult to make any changes in
the database system. Database systems are designed in multi-layers as we learnt earlier.
Data Independence
A database system normally contains a lot of data in addition to users’ data. For example, it
stores data about data, known as metadata, to locate and retrieve data easily. It is rather
difficult to modify or update a set of metadata once it is stored in the database. But as a
DBMS expands, it needs to change over time to satisfy the requirements of the users. If the
entire data is dependent, it would become a tedious and highly complex job.
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Metadata itself follows a layered architecture, so that when we change data at one layer, it
does not affect the data at another level. This data is independent but mapped to each other.
Logical Data Independence
Logical data is data about database, that is, it stores information about how data is managed
inside. For example, a table (relation) stored in the database and all its constraints, applied
on that relation.
Logical data independence is a kind of mechanism, which liberalizes itself from actual data
stored on the disk. If we do some changes on table format, it should not change the data
residing on the disk.
Physical Data Independence
All the schemas are logical, and the actual data is stored in bit format on the disk. Physical
data independence is the power to change the physical data without impacting the schema
or logical data.
For example, in case we want to change or upgrade the storage system itself − suppose we
want to replace hard-disks with SSD − it should not have any impact on the logical data or
schemas.
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