by Dinesh Thakur

A metadata (also called the data dictionary) is the data about the data. It is the self describing nature of the database that provides program-data independence. It is also called as the System Catalog. It holds the following information about each data element in the databases, it normally includes:

+ Name

+ Type

+ Range of values

+ Source

+ Access authorization

+ Indicates which application programs use the data so that, when a change in a data structure is contemplated, a list of the affected programs can be generated.

Data dictionary is used to actually control the database operation, data integrity and accuracy. Metadata is used by developers to develop the programs, queries, controls and procedures to manage and manipulate the data. Metadata is available to database administrators (DBAs), designers and authorized user as on-line system documentation. This improves the control of database administrators (DBAs) over the information system and the user's understanding and use of the system.

Active and Passive Data Dictionaries

Data dictionary may be either active or passive. An active data dictionary (also called integrated data dictionary) is managed automatically by the database management software. Consistent with the current structure and definition of the database. Most of the relational database management systems contain active data dictionaries that can be derived from their system catalog.

The passive data dictionary (also called non-integrated data dictionary) is the one used only for documentation purposes. Data about fields, files, people and so on, in the data processing environment are. Entered into the dictionary and cross-referenced. Passive dictionary is simply a self-contained application. It is managed by the users of the system and is modified whenever the structure of the database is changed. Since this modification must be performed manually by the user, it is possible that the data dictionary will not be current with the current structure of the database. However, the passive data dictionaries may be maintained as a separate database. Thus, it allows developers to remain independent from using a particular relational database management system. It may be extended to contain information about organizational data that is not computerized.

Importance of Data Dictionary

Data dictionary is essential in DBMS because of the following reasons:

• Data dictionary provides the name of a data element, its description and data structure in which it may be found.

• Data dictionary provides great assistance in producing a report of where a data element is used in all programs that mention it.

• It is also possible to search for a data name, given keywords that describe the name. For example, one might want to determine the name of a variable that stands for net pay. Entering keywords would produce a list of possible identifiers and their definitions. Using keywords one can search the dictionary to locate the proper identifier to use in a program.

These days, commercial data dictionary packages are available to facilitate entry, editing and to use the data elements.