Data Independence(Logical Data Independence and Physical Data independence) in Database

 

Data Independence(Logical Data Independence and Physical Data independence) in Database 

Architecture for database systems called the three schema architecture.The goal of the three schema architecture is to separate the user applications from the physical database.Data independence capacity to change the schema at one level of a database system without having to change the schema at one level of a database system without having to change the schema at the next higher level.

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 schema or application schema or application programs.Change the conceptual schema to expand the database by adding a item,to change constraints or to reduce the database by removing the data item.External schema that refer only to the remaining data should not be affected.Only the view definition and mappings need to be changed in a DBMS that supports logical data independence.Changes to constraints can be applied to the conceptual schema without affecting the external schema or application programs.

Physical Data Independence

Physical Data Independence is the capacity to change the internal schema without having to change the conceptual schema.The external schema need not be changed as well.Changes to the internal schema may be needed because some physical files were reorganized.Example 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.  

Physical data independence exists in most database and file environment where physical details such as exact location of data on disk,and hardware details of storage encoding,placement,compression,splitting

merging of records and so on are hidden from the user.Application unaware of these details.

The three schema architecture can make it easier to achieve true data independence bot physical and logical.Two level of mappings create an overhead during compilation or execution of a query or program leading to in efficiencies in the DBMS.