Re: Re: Increasing work_mem and shared_buffers on Postgres 9.2 significantly slows down queries

"Gunnar \"Nick\" Bluth" <gunnar.bluth@xxxxxxxxxxx> · Tue, 06 Nov 2012 21:50:21 +0100

    Am 06.11.2012 21:24, schrieb Petr
      Praus:

    On 6 November 2012 14:17, Gunnar "Nick" Bluth <gunnar.bluth@xxxxxxxxxxx>
      wrote:

          Am 06.11.2012 21:08, schrieb Petr Praus:

              2MB: http://explain.depesz.com/s/ul1

              4MB: http://explain.depesz.com/s/IlVu

              10MB: http://explain.depesz.com/s/afx3

              12MB: http://explain.depesz.com/s/i0vQ

          See the change in the plan between 10MB and 12MB, directly at
          top level? That narrows the thing down quite a bit.

          Though I wonder why this didn't show in the original plans...

        Yes, the 2,4 and 10 are the same, the only difference is
          number of buckets. But with 12, it makes completely different
          choices, it decides to make sequential scans and hash right
          joins instead of merge joins. And those sequential scans take
          a loong time. Could this be caused by some missing indices
          perhaps?

    Well, you do have indices, as we can clearly see.

        The original plans I posted at the start are the same as
          the 12MB plan, I'm not sure why is that, I really hope I
          didn't make some sort of mistake there.

    I had been wondering why you didn't have any indices, tbth. However,
    the execution times still grow with work_mem, which is interesting
    independent of the actual plan change...

        Thanks for your help by the way! :-)

    Oh, no worries there... this is by far the most interesting
    challenge I've encountered in months ;-)

    But I do admit that I've reached the end of the ladder now. No idea
    how you can improve your runtime yet. Probably 

    - using full text search on "personinfo"

    - try different join_collapse_limit / from_collapse_limit /
    enable_hashjoin values

    The most pragmatic approach is probably to just stick with work_mem
    = 1MB (or less) ;-), but that may potentially bite you later.

    -- 
Gunnar "Nick" Bluth
RHCE/SCLA

Mobil   +49 172 8853339
Email: gunnar.bluth@xxxxxxxxxxx
__________________________________________________________________________
In 1984 mainstream users were choosing VMS over UNIX.  Ten years later
they are choosing Windows over UNIX.  What part of that message aren't you
getting? - Tom Payne