It may be simply refer to, as data about the data, i.e. metadata. Data dictionary, also known by the name of 'metadata' is used to provide the description of the data objects that are used in the data model. It is an indispensable part of a database management system, which acts as a central data repository, defining various aspects pertaining to the data such as their meaning, relationship between them, source of origin, their usage, format, etc.
Data dictionary may prove to be useful for developers in developing programs, controls, procedures and queries so as to manage and manipulate the data model. Further, it may also prove to be beneficial for database administrators, designers and other professional for managing and viewing the information resources.
Typically, a data dictionary usually contains following elements with respect to information about the data.
Based on the usage of a data dictionary by a particular entity, it may be seen under two categories as described below.
If a data dictionary is being used by the users, designers and the database administrators then the data dictionary is called as passive dictionary.
Basically, a passive dictionary is used for the purpose of software documentation only. It enables a developer to maintain a separate database, which is independent and is in the state of non-synchronization to a particular relational database management system.
Further, it needs to update manually by a user and does not produces or implements similar changes in the main relational database management system file due to its non-synchronization with the RDBMS.
Data dictionary being directly used and managed by the Database Management System Software is known as active dictionary. This dictionary is consistent and in synchronization to the relational database management system.
In this form, firstly, active dictionary is updated and consequently changes are implemented in the relational database management system. Hence, most of the relational database management system contains active dictionaries.
Below given are some of the advantages of using a data dictionary.