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Stats for multi-column indexes

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Stats for multi-column indexes
Date: 2007-03-20 00:56:00
Message-ID: 1174352160.23455.453.camel@dogma.v10.wvs (view raw or flat)
Thread:
Lists: pgsql-hackers
I know the idea has come up a few times to do cross-column statistics to
improve plans when the data distributions are dependent. I found a
couple references in the archives:

http://archives.postgresql.org/pgsql-hackers/2006-09/msg02118.php
http://archives.postgresql.org/pgsql-hackers/2006-08/msg00924.php

We can already keep stats for a functional index. Is there a reason we
can't keep stats for a multi-column index?

Out of the two situations outlined by Jim Nasby here:

http://archives.postgresql.org/pgsql-hackers/2006-08/msg00948.php

It would not help with joins, but it would help with columns that are
referred to as a group (e.g. a=1 AND b<20).

Regards,
	Jeff Davis


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