Re: pg_autovacuum: short, wide tables

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Mark Reid <mail(at)markreid(dot)org>
Cc: "Matthew T(dot) O'Connor" <matthew(at)zeut(dot)net>, pgsql-bugs(at)postgresql(dot)org, Jan Wieck <JanWieck(at)Yahoo(dot)com>
Subject: Re: pg_autovacuum: short, wide tables
Date: 2005-07-08 16:57:28
Message-ID: 24101.1120841848@sss.pgh.pa.us
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Mark Reid <mail(at)markreid(dot)org> writes:
> I think the issue is that a single update to the main table causes a
> whole bunch of updates to the toast table. So in my case (with the
> vacuum output attached previously), a thousand updates to the main table
> entails tens of thousands of updates to the toast table.

Exactly. If autovac were looking at the properties of the toast table
it would think a vacuum pass was warranted sooner than it thinks from
just looking at the main table.

Admittedly this doesn't come into play unless you have a fairly large
number of toast chunks per main-table row, so the rows in question have
to be really wide (dozens of KB even after compression) before it gets
to be a big deal.

regards, tom lane

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