Data independence in DBMS
- The three-schema architecture can be used to further explain the concept of data independence, which can be defined as the capacity to change the schema at one level of a database system without having to change the schema at the next higher level. We can define two types of data independence:
- Logical data independence
- Physical data independence
Logical data independence
- Logical data independence is the capacity to change the conceptual schema without having to change external schemas or application programs.
- We may change the conceptual schema to expand the database (by adding a record type or data item), or to reduce the database (by removing a record type or data item).
- In the latter case, external schemas that refer only to the remaining data should not be affected. Only the view definition and the mappings need be changed in a DBMS that supports logical data independence.
- Application programs that reference the external schema constructs must work as before, after the conceptual schema undergoes a logical reorganization.
- Changes to constraints can be applied also to the conceptual schema without affecting the external schemas or application programs.
Physical data independence
- Physical data independence is the capacity to change the internal schema without having to change the conceptual (or external) schemas.
- Changes to the internal schema may be needed because some physical files had to be reorganized—for example, by creating additional access structures—to improve the performance of retrieval or update.
- If the same data as before remains in the database, we should not have to change the conceptual schema.