On Fri, Sep 30, 2005 at 11:59:26PM -0400, Bruce Momjian wrote: > This is expected. The main TODO items is: > > * Allow accurate statistics to be collected on indexes with more than > one column or expression indexes, perhaps using per-index statistics > > Basically, we don't have multi-column or expression statistics. ANALYZE > just analyzes columns, even if an expression index exists.
But the row count estimates imply that expression index queries do use column statistics, presumably as a proxy in the absence of expression statistics. This looks like a relevant commit: http://archives.postgresql.org/pgsql-committers/2004-02/msg00124.php The behavior I observed is that the planner does appear to use column statistics when planning an expression index query, but it doesn't appear to honor a column's non-default statistics target. In other words: * Row count estimates for expression index queries (at least simple ones) are reasonably accurate for the N most common column values, where N is the value of default_statistics_target when ANALYZE was run. * Specifically setting the column's statistics target with ALTER TABLE SET STATISTICS doesn't result in better statistics for expression index queries. That difference in behavior seems odd: if default_statistics_target has an effect, why doesn't ALTER TABLE SET STATISTICS? -- Michael Fuhr ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
