Re: Extremely irregular query performance

=?iso-8859-1?Q?Jean-Philippe_C=F4t=E9?= <jean-philippe.cote@crt.umontreal.ca> writes:
> I'm running version 8.1 on a dedicated Sun v20 server (2 AMD x64's)
> with 4Gb of RAM. I have recently noticed that the performance of
> some more complex queries is extremely variable and irregular.
> For example, I currently have a query that returns a small number
> of rows (5) by joining a dozen of tables.

A dozen tables?  You're exceeding the geqo_threshold and getting a plan
that has some randomness in it.  You could either increase
geqo_threshold if you can stand the extra planning time, or try
increasing geqo_effort to get it to search a little harder and hopefully
find a passable plan more often.  See

http://www.postgresql.org/docs/8.1/static/geqo.html
http://www.postgresql.org/docs/8.1/static/runtime-config-query.html#RUNTIME-CONFIG-QUERY-GEQO

I'm kinda surprised that you don't get better results with the default
settings.  We could tinker some more with the defaults, if you can
provide evidence about better values ...

            regards, tom lane