Db2 maintains a set of tables that contain information about the data that Db2 controls. These tables are collectively known as the catalog.
The catalog tables contain information about Db2 objects such as tables, views, and indexes. When you create, alter, or drop an object, Db2 inserts, updates, or deletes rows of the catalog that describe the object.
The Db2 catalog consists of tables of data about everything defined to the Db2 system, including table spaces, indexes, tables, copies of table spaces and indexes, and storage groups. The system database DSNDB06 contains the Db2 catalog.
When you create, alter, or drop any structure, Db2 inserts, updates, or deletes rows of the catalog that describe the structure and tell how the structure relates to other structures. For example, SYSIBM.SYSTABLES is one catalog table that records information when a table is created. Db2 inserts a row into SYSIBM.SYSTABLES that includes the table name, its owner, its creator, and the name of its table space and its database.
To understand the role of the catalog, consider what happens when the EMP table is created. Db2 records the following data:
- Table information
- To record the table name and the name of its owner, its creator, its type, the name of its table space, and the name of its database, Db2 inserts a row into the catalog.
- Column information
- To record information about each column of the table, Db2 inserts the name of the table to which the column belongs, its length, its data type, and its sequence number by inserting a row into the catalog for each column of the table.
- Authorization information
- To record that the owner of the table has authorization to create the table, Db2 inserts a row into the catalog.
Tables in the catalog are like any other database tables with respect to retrieval. If you have authorization, you can use SQL statements to look at data in the catalog tables in the same way that you retrieve data from any other table in the Db2 database. Db2 ensures that the catalog contains accurate object descriptions. If you are authorized to access the specific tables or views on the catalog, you can use SELECT on the catalog. You can use INSERT, UPDATE, and DELETE statements on updatable catalog tables, but you cannot use TRUNCATE or MERGE on the catalog.
The communications database (CDB) is part of the Db2 catalog. The CDB consists of a set of tables that establish conversations with remote database management systems (DBMSs). The distributed data facility (DDF) uses the CDB to send and receive distributed data requests.