Data independence
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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, and vice-versa.
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.
[edit] 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.
[edit] 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.
We may also look upon the two data independence :
1.Logical Data Independence : The ability to change the logical schema without changing the physical schema is called Logical Data Independence.
Example -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.
Example : A change to the internal schema, such as using different file organisation or storage structures, storage devices, or indexing strategy, should be possible without having to change the conceptual or external schemas
Related Articles: Network transparency, Replication transparency, Fragmentation transparency.
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