To: Royce Ausburn <royce.ml@inomial.com>
cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Ineffective autovacuum 
In-reply-to: <059E65EF-8C66-4DA6-835C-936D6592C94B@inomial.com> 
References: <059E65EF-8C66-4DA6-835C-936D6592C94B@inomial.com>
Comments: In-reply-to Royce Ausburn <royce.ml@inomial.com>
	message dated "Tue, 27 Sep 2011 13:45:33 +1000"
Date: Tue, 27 Sep 2011 00:21:22 -0400
Message-ID: <14103.1317097282@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Royce Ausburn <royce.ml@inomial.com> writes:
> I have a problem with autovacuum apparently not doing the job I need it to do.

Hm, I wonder whether you're getting bit by bug #5759, which was fixed
after 8.3.12.

> I have a table named datasession that is frequently inserted, updated and deleted from.  Typically the table will have a few thousand rows in it.  Each row typically survives a few days and is updated every 5 - 10 mins.  The application receives unreliable, potentially duplicate data from its source, so this table is heavily used for synchronising application threads as well.  A typical access pattern is:

> - tx begin
> - SELECT FOR UPDATE on a single row
> - Do some application processing (1 - 100 ms)
> - Possibly UPDATE the row
> - tx commit

Transactions of that form would not interfere with autovacuum.  You'd
need something that wants exclusive lock, like a schema change.

> I've read some recent threads and found a discussion (below) on auto vacuum that mentions auto vacuum will be cancelled when a client requests a lock that auto vacuum is using… My questions:
> 1) Does it look like I'm affected by the same problem as in the below discussion?

Not unless you're seeing a lot of "canceling autovacuum task" messages
in the postmaster log.

			regards, tom lane