logical and physical DI

Question

which is difficult to achieve logical data independence or physical DI and why ?

does change in conceptual level affect both physical level (internal level) and extrenal level

and  does change in phyical level affect both conceptual and external level

Comments

$\color{Red}{\text{Complete, Crystal Clear, Detailed Explanation Here:}}$

Data Independence in Three Level Schema of DBMS

Answer 1

The ability to modify schema definition in one level without affecting schema definition in the next higher level is called data independence. There are two levels of data independence, they are Physical data independence and Logical data independence.

1. Physical data independence is the ability to modify the physical schema without causing application programs to be rewritten. Modifications at the physical level are occasionally necessary to improve performance. It means we change the physical storage/level without affecting the conceptual or external view of the data. The new changes are absorbed by mapping techniques.
2. Logical data independence is the ability to modify the logical schema without causing application program to be rewritten. Modifications at the logical level are necessary whenever the logical structure of the database is altered (for example, when money-market accounts are added to banking system). Logical Data independence means if we add some new columns or remove some columns from table then the user view and programs should not change. For example: consider two users A & B. Both are selecting the fields "EmployeeNumber" and "EmployeeName". If user B adds a new column (e.g. salary) to his table, it will not effect the external view for user A, though the internal schema of the database has been changed for both users A & B.

Logical data independence is more difficult to achieve than physical data independence, since application programs are heavily dependent on the logical structure of the data that they access.

Physical data independence means we change the physical storage/level without affecting the conceptual or external view of the data. Mapping techniques absorbs the new changes.

Comments

@Sanjay, could you elaborate what is physical schema and what do you mean by modifications at the physical level?

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Physical schema is a term used in data management to describe how data is to be represented and stored (files, indices, et al.) in secondary storage using a particular database management system (DBMS) (e.g., Oracle RDBMS, Sybase SQL Server, etc.).

Modifications at the physical level are occasionally necessary to improve performance. 

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@Sanjay. Thanks.