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. 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.
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.
Logical Data Independence:
Logical data independence is the ability to modify the conceptual schema without having alteration in external schemas or application programs. Alterations in the conceptual schema may include addition or deletion of fresh entities, attributes or relationships and should be possible without having alteration to existing external schemas or having to rewrite application programs.
Physical Data Independence:
Physical data independence is the ability to modify the inner schema without having alteration to the conceptual schemas or application programs. Alteration in the internal schema might include.
* Using new storage devices.
* Using different data structures.
* Switching from one access method to another.
* Using different file organizations or storage structures.
* Modifying indexes.