5.6 The System Catalog
The system catalog or system data dictionary can be thought of as a database of information about a database. It contains, in table form, a summary of the structure of each database as it appears at a given time. Whenever a base table, view, index, constraint, stored module, or other item of a database schema is created, altered, or dropped, the DBMS automatically updates its entries in the catalog. The system also uses the catalog to check authorizations and to store information for access plans for applications. Users can query the data dictionary using ordinary SQL SELECT commands. However, because the data dictionary is maintained by the DBMS itself, the SQL UPDATE, INSERT, and DELETE commands cannot be executed in ...
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