| From: | Peter Geoghegan <peter(at)2ndquadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Bruce Momjian <bruce(at)momjian(dot)us>, Andrew Dunstan <andrew(at)dunslane(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: [PERFORM] encouraging index-only scans |
| Date: | 2012-12-13 15:31:06 |
| Message-ID: | CAEYLb_U1dTg=bXkDL=yvFTrMw5oC13ivgZA=jdQXnbgaDJ+e-Q@mail.gmail.com |
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| Lists: | pgsql-hackers pgsql-performance |
On 13 December 2012 03:51, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> ANALYZE does not set that value, and is not going to start doing so,
> because it doesn't scan enough of the table to derive a trustworthy
> value.
I'm slightly surprised by your remarks here, because the commit
message where the relallvisible column was added (commit
a2822fb9337a21f98ac4ce850bb4145acf47ca27) says:
"Add a column pg_class.relallvisible to remember the number of pages
that were all-visible according to the visibility map as of the last
VACUUM
(or ANALYZE, or some other operations that update pg_class.relpages).
Use relallvisible/relpages, instead of an arbitrary constant, to
estimate how many heap page fetches can be avoided during an
index-only scan."
Have I missed some nuance?
--
Peter Geoghegan http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services
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