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

Am 06.11.2012 21:24, schrieb Petr Praus:
> On 6 November 2012 14:17, Gunnar "Nick" Bluth
> <gunnar(dot)bluth(at)pro-open(dot)de <mailto:gunnar(dot)bluth(at)pro-open(dot)de>> 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(dot)bluth(at)pro-open(dot)de
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