IBM Books

Administration Guide

Chapter 12. System Catalog Statistics

When optimizing SQL queries, the decisions made by the SQL compiler are heavily influenced by the optimizer's model of the database contents. This data model is used by the optimizer to estimate the costs of alternative access paths that could be used to resolve a particular query.

A key element in the data model is the set of statistics gathered about the data contained in the database and stored in the system catalog tables. This includes statistics for tables, indexes, columns, and user-defined functions (UDFs). A change in the data statistics can result in a change in the choice of access plan selected as the most efficient method of accessing the desired data. After running statistics, you may want to rebind applications.

Examples of the statistics available which help define the data model to the optimizer include:

Statistics for objects are updated in the system catalog tables only when explicitly requested. Some or all of the statistics may be updated by:

Additional Information:

The SYSCAT and SYSSTAT catalogs contain information on the statistics gathered. See Appendix J. "Catalog Views":

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