| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Greg Stark <stark(at)mit(dot)edu> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Bruce Momjian <bruce(at)momjian(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: | 2011-10-12 15:18:03 |
| Message-ID: | 13771.1318432683@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Greg Stark <stark(at)mit(dot)edu> writes:
> On Wed, Oct 12, 2011 at 3:29 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The problem is precisely that the pages a query is going to read are
>> likely to *not* be a random sample, but to be correlated with
>> recently-dirtied pages.
> Sure, but I was suggesting aiming for the nth percentile rather than a
> linear factor which I don't know has any concrete meaning.
Well, I have no problem with using a more complicated estimation
equation, but it might be nice to get some field experience with the
thing before we start complicating matters.
regards, tom lane
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