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row-level stats and last analyze time

From: Neil Conway <neilc(at)samurai(dot)com>
To: Guillaume Lelarge <guillaume(at)lelarge(dot)info>
Cc: pgsql-docs(at)postgresql(dot)org, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: row-level stats and last analyze time
Date: 2007-04-24 21:38:59
Message-ID: 1177450739.16415.249.camel@localhost.localdomain (view raw or flat)
Thread:
Lists: pgsql-docspgsql-hackers
[ CC'ing -hackers ]

On Sun, 2007-04-22 at 16:10 +0200, Guillaume Lelarge wrote:
> This patch adds a sentence on monitoring.sgml explaining that 
> stats_row_level needs to be enabled if user wants to get last 
> vacuum/analyze execution time.

This behavior was introduced in r1.120 of postmaster/pgstat.c:

    Modify pgstats code to reduce performance penalties from
    oversized stats data files: avoid creating stats
    hashtable entries for tables that aren't being touched
    except by vacuum/analyze [...]

which included other modifications to reduce the pgstat I/O volume in
8.1. I don't think this particular change was wise: the reduction in
pgstat volume is pretty marginal, and it is counter-intuitive for
stats_row_level to effect whether the last ANALYZE / VACUUM is recorded.
(Plus, the optimization is not even enabled with the default
postgresql.conf settings.)

-Neil



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