From: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
---|---|
To: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
Cc: | PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: is a special cost for external sort? |
Date: | 2013-07-18 22:30:20 |
Message-ID: | CAMkU=1y4+ei780uVBWAnVNad_xAmi5viZBudVax6kzBk4-RVmQ@mail.gmail.com |
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On Thu, Jul 18, 2013 at 9:14 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
> Hello
>
> I found a slow query with large external sort. I expect, so external
> sort should be penalized. Is it?
It tries to, but it doesn't seem to be much good at it. In
particular, I think it does a poor job of estimating the CPU cost of
an external sort relative to an in-memory sort of a slightly smaller
data set. In my experience adding the single tuple that actually
pushes you over the work_mem limit costs about 3x in CPU.
It is one of those things I started looking into a few times, but
never got anywhere before getting distracted.
Cheers,
Jeff
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