From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Jesper Krogh <jesper(at)krogh(dot)cc> |
Cc: | Mladen Gogala <mladen(dot)gogala(at)vmsinfo(dot)com>, "david(at)lang(dot)hm" <david(at)lang(dot)hm>, Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>, Vitalii Tymchyshyn <tivv00(at)gmail(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Slow count(*) again... |
Date: | 2010-10-12 18:58:13 |
Message-ID: | 4292.1286909893@sss.pgh.pa.us |
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Lists: | pgsql-hackers pgsql-performance |
Jesper Krogh <jesper(at)krogh(dot)cc> writes:
> On 2010-10-12 19:07, Tom Lane wrote:
>> Anyway, if anyone is hot to make COUNT(*) faster, that's where to look.
> Just having 32 bytes bytes of "payload" would more or less double
> you time to count if I read you test results correctly?. .. and in the
> situation where diskaccess would be needed .. way more.
> Dividing by pg_relation_size by the amout of tuples in our production
> system I end up having no avg tuple size less than 100bytes.
Well, yeah. I deliberately tested with a very narrow table so as to
stress the per-row CPU costs as much as possible. With any wider table
you're just going to be I/O bound.
> .. without having complete insigt.. a visibillity map that could be used in
> conjunction with indices would solve that. What the cost would be
> of maintaining it is also a factor.
I'm less than convinced that that approach will result in a significant
win. It's certainly not going to do anything to convert COUNT(*) into
an O(1) operation, which frankly is what the complainants are expecting.
There's basically no hope of solving the "PR problem" without somehow
turning COUNT(*) into a materialized-view reference. We've discussed
that in the past, and know how to do it in principle, but the complexity
and distributed overhead are daunting.
regards, tom lane
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