From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tim Kane <tim.kane@gmail.com>
cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: 9.2.4 vs 9.3.0 query planning (sort merge join vs hash join)
In-reply-to: <CF9664D0.7CB56%tim.kane@gmail.com>
References: <CF9664D0.7CB56%tim.kane@gmail.com>
Comments: In-reply-to Tim Kane <tim.kane@gmail.com>
 message dated "Mon, 12 May 2014 11:45:20 +0100"
Date: Mon, 12 May 2014 07:23:00 -0400
Message-ID: <17739.1399893780@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

Tim Kane <tim.kane@gmail.com> writes:
> Is there a propensity for 9.2.4 to prefer a sort-merge-join, in place of a
> hash join?

Not particularly; I don't think there's any actual difference in the cost
estimation equations between 9.2 and 9.3.  The two plans you show are
close enough in estimated cost that the ordering of their costs might be
coming out differently just as a matter of random variation in statistics.

It'd be worth double-checking the work_mem setting on both systems,
though, as (IIRC) an undersized work_mem hurts the estimate for hashes
more than for sorts.

			regards, tom lane


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