On Sat, Nov 5, 2011 at 1:26 AM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: > Stuart Bishop <stuart@xxxxxxxxxxxxxxxx> writes: >> We also found this problem did not occur on one of our staging >> systems, which had a default statistics target of 100. Lowering the >> statistics on the relavant columns from 1000 to 100 and reanalyzing >> made the overhead unnoticeable. > > eqjoinsel() is O(N^2) in the number of entries in the MCV lists. > I wouldn't expect this to be an issue unless comparison is pretty > expensive, but maybe those are string not integer columns? No, per http://paste.ubuntu.com/726193/ there are only integer columns being used. >> Thoughts on IRC was this might be a regression in 8.4.9, but I haven't >> got earlier versions to test with at the moment. > > eqjoinsel has worked like that for many years. Are you claiming you > didn't see this behavior in a prior release? If so, which one? I'm repeating speculation from discussions on IRC with pg devs in my timezone. I haven't tested with earlier versions. I can't confirm if this problem appeared with 8.4.9 or not - it is only recently that our query times got to the stage where we can start worrying about milliseconds instead of seconds :-) Previously, we have never noticed the planner overhead so had set default_statistics_target to 1000 to minimize the chance of bad plans due to a skewed sample. 150ms+ seems excessive though in this simple case. -- Stuart Bishop <stuart@xxxxxxxxxxxxxxxx> http://www.stuartbishop.net/ -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
