The DB2 database maintains internal information and statistics about the physical layout of data in the database. These internal statistics are used by the prefetch and I/O scheduling threads to plan operations in advance and can therefore have a very large impact on performance. With regular database activity, the statistics can become incorrect and therefore begin to have an adverse affect on I/O planning. This is especially true after major loads of new data, creating indexes on tables and heavy table activity involving large numbers of delete or update queries.

DB2 provides several tools to assist in updating these statistics and therefore enable continued and accurate I/O planning. These tools can be run from the db2 command prompt and are called RUNSTATS, REORG and REORGCHK tools. They should be run regularly to ensure optimal database performance.

See the System Catalog Statistics section in the DB2 Administration Guide and the section on CLP commands in the DB2 Command Reference.