Friday, 9 December 2016

What is data dictionary in a dbms

What is data dictionary in a dbms? / How do we use data dictionary? / Data dictionary, system catalog, metadata / Define data dictionary


Data Dictionary

Data dictionary is a centralized repository that stores definitions about the data elements and their relationships. The DBMS uses the data dictionary to look up the required data component structures and relationships. Data dictionary is referred with different names like ‘system catalogue’, and ‘meta data’ (data about data).

Any changes made in a database structure are automatically recorded in the data dictionary, thereby freeing you from having to modify all of the programs that access the changed structure. For example, Oracle modifies the data dictionary every time a DDL (Data Definition Language) statement is executed.

What does it store?

  • It stores information about,
  • various data elements (database objects) like
    • tables [table names, attributes, attribute size, data types etc],
    • views [view definition],
    • indexes [all indexes and the tables in which they are defined],
    • procedures,
    • functions,
    • triggers etc.
  • integrity constraints information
  • users and privileges
  • space management data etc.

Example:

If you create a table then you are using a DDL statement. This statement will create the table as per the definition and updates the data dictionary by storing the following information;

  • table name, column names, data types, sizes, constraints etc.
  • user privileges as per the user who created that table



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