The word catalog signifies a list of items. When DB2 is installed, DB2 creates a set of tables which stores the information about DB2 objects as and when it gets created/updated, like definition of the objects, security information, statistical information etc. The information of every object known and available to DB2 is stored in these system catalog tables.
The schema for all catalog tables is SYSIBM. Thus, when a new table is created, DB2 updates the information of the table in SYSIBM.SYSTABLES and SYSIBM.SYSCOLUMNS table. If an index is created, a row gets inserted in SYSIBM.SYSINDEXES table. When a plan is created, the table SYSPLAN gets updated with the plan name. Application develops can not use insert/update statements to modify these catalog tables.
Information in catalog table gets updated via RUNSTATS utility.
Refer to the link for set of catalog tables maintained in DB2.
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