| From: | Bruce Momjian <bruce(at)momjian(dot)us> |
|---|---|
| To: | Greg Stark <stark(at)mit(dot)edu> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: COUNT(*) and index-only scans |
| Date: | 2011-10-11 16:43:59 |
| Message-ID: | 201110111643.p9BGhxf09653@momjian.us |
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| Thread: | |
| Lists: | pgsql-hackers |
Greg Stark wrote:
> On Mon, Oct 10, 2011 at 9:17 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > My intention was to allow it to consider any covering index. ?You're
> > thinking about the cost estimate, which is really entirely different.
> >
>
> Is there any reason to consider more than one? I would have expected
> the narrowest one to be the best choice. There's something to be said
> for using the same index consistently but we already have that problem
> and make no attempt to do that. And partial indexes might be better
> but then we would already be considering them if their constraints are
> satisfied.
Actually, I think the smallest non-partial one on disk might be the best
--- that is very easy to find out.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
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