Column statistics are stored permanently in the database in the ISYSCOLSTAT system table. To continually improve the optimizer's performance, the database server automatically updates column statistics during the processing of any SELECT, INSERT, UPDATE, or DELETE statement, including statements on a single row. It does so by monitoring the number of rows that satisfy any predicate that references a table or column, comparing that number to the number of rows estimated, and then, if necessary, updating existing statistics.
With more accurate column statistics available to it, the optimizer can compute better estimates and improve the performance of subsequent queries.
You can set whether to update column statistics using database options. The update_statistics database option controls whether to update column statistics during execution of queries, while the collect_statistics_on_dml_updates database option controls whether to update the statistics during the execution of data-altering DML statements such as LOAD, INSERT, DELETE, and UPDATE.
If you suspect that performance is suffering because your statistics inaccurately reflect the current column values, you may want to execute the statements CREATE STATISTICS or DROP STATISTICS. CREATE STATISTICS deletes old statistics and creates new ones, while DROP STATISTICS only deletes old statistics.
When you execute the CREATE INDEX statement, statistics are automatically created for the index.
When you execute the LOAD TABLE statement, statistics are automatically created for the table.
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