Data Independance

An ability to modify a schema definition in one level without affecting a schema definition in next higher level.

There are two types of data independence: Physical Data Independence (PDI) and Logical Data Independence (LDI).

1) Physical Data Independence (PDI)

• In ANSI SPARC model PDI separates the Conceptual level from Internal level.
• It allows to change in physical storage devices or way of organizing file without making any changes in conceptual level/view. 
• Modification is necessary at internal level over the time, because after some time storage device may be full so you need to replace it with high capacity storage device or add new storage device.
• Easy to achieve compare to LDI.

2) Logical Data Independence (LDI)

• In ANSI SPARC model LDI separates the External level from Conceptual level.
• It allows to change in conceptual level without making any changes in external level.
• Modification is necessary at logical level is required when database structure is changed/altered.
• Difficult to achieve compare to PDI.