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The system catalog is a collection of tables and views that contain important information about a database. A system catalog is available for each database. Information in the system catalog defines the structure of the database. For example, the DDL (data dictionary language) for all tables in the database is stored in the system catalog. See Figure 21.1 for an illustration of the system catalog within the database. |
Figure 21.1 The system catalog.
As you can see in Figure 21.1, the system catalog for a database is actually part of the database. Within the database are objects, such as tables, indexes, and views. The system catalog is basically a group of objects that contain information that defines other objects in the database, the structure of the database itself, and various other significant information.
The system catalog for your implementation may be divided into logical groups of objects to provide tables that are accessible by not only the database administrator, but any other database user as well. For example, a user may need to view the particular database privileges that he or she has been granted, but has no need to know about the internal structure or processes of the database. A user typically queries the system catalog to acquire information on the user's own objects and privileges, whereas the DBA needs to be able to inquire about any structure or event within the database. In some implementations, there are system catalog objects that are accessible only to the database administrator.
The system catalog is crucial to the database administrator or any other database user who needs to know about the database's structure and nature. The system catalog allows order to be kept, not only by the database administrator and users, but by the database server itself.