Data Dictionary - Documentation

Documentation

The term Data Dictionary and Data Repository are used to indicate a more general software utility than a catalogue. A Catalogue is closely coupled with the DBMS Software; it provides the information stored in it to user and the DBA, but it is mainly accessed by the various software modules of the DBMS itself, such as DDL and DML compilers, the query optimiser, the transaction processor, report generators, and the constraint enforcer. On the other hand, a Data Dictionary is a data structure that stores meta-data, i.e., data about data. The Software package for a stand-alone Data Dictionary or Data Repository may interact with the software modules of the DBMS, but it is mainly used by the Designers, Users and Administrators of a computer system for information resource management. These systems are used to maintain information on system hardware and software configuration, documentation, application and users as well as other information relevant to system administration.

If a data dictionary system is used only by the designers, users, and administrators and not by the DBMS Software, it is called a Passive Data Dictionary; otherwise, it is called an Active Data Dictionary or Data Dictionary. An Active Data Dictionary is automatically updated as changes occur in the database. A Passive Data Dictionary must be manually updated.

The data Dictionary consists of record types (tables) created in the database by systems generated command files, tailored for each supported back-end DBMS. Command files contain SQL Statements for CREATE TABLE, CREATE UNIQUE INDEX, ALTER TABLE (for referential integrity), etc., using the specific statement required by that type of database.

Database users and application developers can benefit from an authoritative data dictionary document that catalogs the organization, contents, and conventions of one or more databases. This typically includes the names and descriptions of various tables and fields in each database, plus additional details, like the type and length of each data element. There is no universal standard as to the level of detail in such a document, but it is primarily a weak kind of data.