To: Alvaro Herrera <alvherre@alvh.no-ip.org>
cc: Ian Westmacott <ianw@intellivid.com>, pgsql-admin@postgresql.org
Subject: Re: 8.1.8 autovacuum missing databases 
In-reply-to: <20080502161709.GH2320@alvh.no-ip.org> 
References: <1209574908.22509.6.camel@vega.intellivid.com>
	<20185.1209575233@sss.pgh.pa.us>
	<1209576488.22509.9.camel@vega.intellivid.com>
	<22093.1209588237@sss.pgh.pa.us>
	<1209646882.16174.15.camel@vega.intellivid.com>
	<7698.1209650248@sss.pgh.pa.us>
	<1209664799.16174.51.camel@vega.intellivid.com>
	<17328.1209667453@sss.pgh.pa.us>
	<1209668403.16174.60.camel@vega.intellivid.com>
	<17606.1209669106@sss.pgh.pa.us>
	<20080502161709.GH2320@alvh.no-ip.org>
Comments: In-reply-to Alvaro Herrera <alvherre@alvh.no-ip.org>
	message dated "Fri, 02 May 2008 12:17:09 -0400"
Date: Fri, 02 May 2008 12:40:58 -0400
Message-ID: <7391.1209746458@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> I'm wondering if there could be a hidden bug in the age arithmetic
> somehow -- say because it's using unsigned comparison of signed
> variables, or the Max() macro is flawed, or something weird like that.

Umm ... actually, there's a bug in the hand calculations I did at the
start of this thread.  The threshold that 8.1 uses is

                         (int32) ((MaxTransactionId >> 3) * 3 - 100000)

which works out to 1610512733, which is considerably more than the 300M
or so age() values that Ian is showing.  I had somehow convinced myself
that it was less and he should be getting whole-db vacuums, but in
reality he shouldn't yet.  It's only in 8.2 and up that we have a much
smaller threshold for trying to advance datfrozenxid.

So actually, what we are looking at is the not-whole-db logic, and
it's working exactly as designed.  The only unexpected behavior was
the choice not to vacuum some of the DBs, which we now know is explained
by their having a last_autovac_time in the future.

Perhaps it is worth adding a defense to autovac to not believe last
autovac times that are in the future, since the system clock is not
under our control and could go backwards.  Other than that I don't
think there's any bug here.

			regards, tom lane