| From: | Greg Stark <stark(at)mit(dot)edu> |
|---|---|
| To: | Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov> |
| Cc: | Aidan Van Dyk <aidan(at)highrise(dot)ca>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jeff Davis <pgsql(at)j-davis(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: COUNT(*) and index-only scans |
| Date: | 2011-10-12 15:30:00 |
| Message-ID: | CAM-w4HO=zSNmyZcrtx9PYxyqxjObEgO+YcP_mRA4AUucZgh6hA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Oct 12, 2011 at 4:26 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
>> But it matters a *lot* when they heap pages are not in shared
>> buffers
>
> Yeah, obviously it matters more if you actually need to add a random
> disk read.
To be fair the indexes are also random I/O. So the case that really
matters is when the index fits in RAM but the heap does not.
--
greg
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