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Re: Where does data in pg_stat_user_tables come from?

From: "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>
To: Josh Berkus <josh(at)agliodbs(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Where does data in pg_stat_user_tables come from?
Date: 2010-09-14 00:12:08
Message-ID: 1284423128.28610.592.camel@jd-desktop.unknown.charter.com (view raw or flat)
Thread:
Lists: pgsql-performance
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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