Data independence

Data independence

Data independence is the type of data transparency that matters for a centralized DBMS. It refers to the immunity of user applications to make changes 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.

Contents

First level

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

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 two 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 affect, because there doesn't exist any other level above view level.

Data Independence Types

Data independence has two types: 1. Physical Independence and 2. Logical Independence. With knowledge about the three-schemes architecture the term 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.

Physical Independence: The logical scheme stays unchanged even though the storage space or type of some data is changed for reasons of optimisation or reorganisation.
[The ability to change the physical schema without changing the logical schema is called as Physical Data Independence.]


Logical Independence: The external scheme may stay unchanged for most changes of the logical scheme. This is especially desirable as the application software does not need to be modified or newly translated.
[The ability to change the logical schema without changing the external schema or application programs is called as Logical Data Independence.]

See also



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