Subject: Re: Where does data in pg_stat_user_tables come from?
From: "Joshua D. Drake" <jd@commandprompt.com>
Reply-To: jd@commandprompt.com
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <4C8EB813.6070204@agliodbs.com>
References: <4C8EAE5E.70201@agliodbs.com>
	<1284421288.28610.501.camel@jd-desktop.unknown.charter.com>
	<4C8EB813.6070204@agliodbs.com>
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Organization: Command Prompt, Inc.
Date: Mon, 13 Sep 2010 17:12:08 -0700
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On Mon, 2010-09-13 at 16:47 -0700, Josh Berkus wrote:
> On 9/13/10 4:41 PM, Joshua D. Drake wrote:
> > On Mon, 2010-09-13 at 16:06 -0700, Josh Berkus wrote:
> >> All,
> >>
> >> I've been looking at pg_stat_user_tables (in 8.3, because of a project I
> >> have), and it appears that autovacuum, and only autovaccum, updates the
> >> data for this view.  This means that one can never have data in
> >> pg_stat_user_tables which is completely up-to-date, and if autovacuum is
> >> off, the view is useless.
> > 
> > As I recall its kept in shared_buffers (in some kind of counter) and
> > updated only when it is requested or when autovacuum fires. This was
> > done because we used to write stats every 500ms and it was a bottleneck.
> > (IIRC)
> 
> Yes, looks like it only gets updated on SELECT or on autovacuum.
> 
> Thing is, a full VACUUM ANALYZE on the database, or even just ANALYZE,
> should update some of the counters.  And currently it doesnt, resulting
> in pg_class.reltuples often being far more up to date than
> pg_stat_user_tables.n_live_tup.  And frankly, no way to reconcile those
> two stats.

If you select from pg_stat_user_tables, the counters should be
reasonably close unless your default_statistics_target is way off and
then pg_class.reltuples would be wrong.

Joshua D. Drake


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