Database Management System Basic Concepts By Sumit Singh

Data Independence - Achievement of Layered Architecture of DBMS

A major objective for three- level architecture is to provide data independence, which means that upper levels are unaffected by changes in the lower levels.

1. Logical data independence

2. Physical data independence

1. Logical data independence: - Logical data independence indicates that the conceptual schema can be changed without affecting the existing schemas. The change would be absorbed by the mapping between the external and conceptual levels. Logical data independence also insulates application programs from operations such as combining two records in one or splitting an existing record in two or more records. This would require a change in the external/conceptual mapping so to leave the external view unchanged.

2. Physical data independence: - Physical data independence indicates that physical storage structures or devices could be changed without affecting conceptual schema. The change would be absorbed by the mapping between conceptual and internal levels. Physical data independence is achieved by the presence of the internal level of the database and the mapping or transformation from the conceptual level of the database to the internal level. Conceptual level to internal level mapping, therefore provides a means to go from the conceptual view to the internal view and hence to the stored data in the database (physical records).

If there is a need to change the file organization or the type of physical device used as a result of the growth in the database or new technology, a change is required in the conceptual/internal mapping between he conceptual and internal; levels. This change is necessary to maintain the conceptual level invariant. The physical data independence criterion to maintain the conceptual level does note specify storage structures or the access methods used to retrieve the data from the physical storage medium. Making the conceptual schema physical physically data independent means that the external schema, which is d3fined on the conceptual schema, is in turn physically data independent.

The logical data independence is difficult to achieve than physical data independence as it requires the flexibility in the design of the database and programmer has to foresee the future requirements or modifications in then design.