Information pertaining to the structure and usage of data contained in the database, the metadata, is maintained in a data dictionary. The data dictionary is a database itself, documents the data. Each database user can consult the data dictionary to learn what each piece of data and the various synonyms of the data fields mean. In a system where the data dictionary is part of the DBMS(Integrated system) the data dictionary stores information concerning the source of each data-field value, the frequency of its use, and an audit trail concerning updates, including the who and when of each update.
Currently data dictionary systems are available as add-ons to the DBMS.
The data dictionary stores: • Names of relations
• Names of the attributes of each relation
• Domains and lengths of attributes
• Names of views defined on the database, and definitions of those views
