Hi Tomas, 2009/9/10 <t...@fuzzy.cz>
> > default_statistics_target = 100 (tried with 500, no change). Vacuum > > analyzed > > before initial query, and after each change to default_statistics_target. > > Modifying the statistics target is useful only if the estimates are > seriously off, which is not your case - so it won't help, at least not > reliably. > > > The same query, with a different "ofid", will occasionally get the more > > optimal plan -- I assume that the distribution of data is the > > differentiator > > there. > > Yes, the difference between costs of the two plans is quite small (11796 > vs. 13153) so it's very sensible to data distribution. > > > Is there any other data I can provide to shed some light on this? > > You may try to play with the 'cost' constants - see this: > > > http://www.postgresql.org/docs/8.4/interactive/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS > > You just need to modify them so that the bitmap index scan / bitmap heap > scan is prefered to plain index scan. > > Just be careful - if set in the postgresql.conf, it affects all the > queries and may cause serious problems with other queries. So it deserves > proper testing ... > > regards > Tomas > Playing around with seq_page_cost (1) and random_page_cost (1), I can get the correct index selected. Applying those same settings to our production server does not produce the optimal plan, though.
