Re: Excessive planner time for some queries with high statistics

On Sat, Nov 5, 2011 at 1:26 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Stuart Bishop <stuart(at)stuartbishop(dot)net> 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(at)stuartbishop(dot)net>
http://www.stuartbishop.net/