| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Bruce Momjian <bruce(at)momjian(dot)us>, Greg Stark <stark(at)mit(dot)edu>, 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: | 2011-10-12 06:50:28 |
| Message-ID: | 1318402228.1724.186.camel@jdavis |
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| Lists: | pgsql-hackers |
On Tue, 2011-10-11 at 13:22 -0400, Robert Haas wrote:
> The real issue is that the costing estimates need to be accurate, and
> that's where the rubber hits the road. Otherwise, even if we pick the
> right way to scan the table, we may do silly things up the line when
> we go to start constructing the join order. I think we need to beef
> up ANALYZE to gather statistics on the fraction of the pages that are
> marked all-visible, or maybe VACUUM should gather that information.
> The trouble is that if we VACUUM and then ANALYZE, we'll often get
> back a value very close to 100%, but then the real value may diminish
> quite a bit before the next auto-analyze fires. I think if we can
> figure out what to do about that problem we'll be well on our way...
Can you send stats messages to keep track when you unset a bit in the
VM? That might allow it to be more up-to-date.
Regards,
Jeff Davis
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