The Oracle system catalog is comprised of tables and views which allow the user to obtain information about the database, such as what other tables and views are available, their attributes, constraints, etc. Oracle has hundreds of available system catalog relations, only some of which are available to the average user (as many are intended for use only by qualified database administrators).

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http://www.cs.rpi.edu/~sibel/dbs/FALL2005/OracleAtRPI/node11.html