From: Tom Lane <tgl@redhat.com>
To: Andrew Dunstan <andrew@dunslane.net>
cc: pgsql-perform <pgsql-performance@postgresql.org>
Subject: Re: encouraging index-only scans
In-reply-to: <50C8F1EC.9010801@dunslane.net>
References: <50C8F1EC.9010801@dunslane.net>
Comments: In-reply-to Andrew Dunstan <andrew@dunslane.net>
	message dated "Wed, 12 Dec 2012 16:06:52 -0500"
Date: Wed, 12 Dec 2012 16:32:33 -0500
Message-ID: <15321.1355347953@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

Andrew Dunstan <andrew@dunslane.net> writes:
> A client is testing a migration from 9.1 to 9.2, and has found that a 
> large number of queries run much faster if they use index-only scans. 
> However, the only way he has found to get such a plan is by increasing 
> the seq_page_cost to insanely high levels (3.5). Is there any approved 
> way to encourage such scans that's a but less violent than this?

Is the pg_class.relallvisible estimate for the table realistic?  They
might need a few more VACUUM and ANALYZE cycles to get it into the
neighborhood of reality, if not.

Keep in mind also that small values of random_page_cost necessarily
decrease the apparent advantage of index-only scans.  If you think 3.5
is an "insanely high" setting, I wonder whether you haven't driven those
numbers too far in the other direction to compensate for something else.

			regards, tom lane


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