Content-Type: text/plain; charset=UTF-8
Cc: Peter Hussey <peter@labkey.com>,
	pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Questions on query planner, join types, and work_mem
From: Alvaro Herrera <alvherre@commandprompt.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
In-reply-to: <18551.1280275502@sss.pgh.pa.us>
References: <AANLkTikMwVZKDYLitnsK31R=VNOuTH8NtmDHDGkBR7k8@mail.gmail.com>
	<18551.1280275502@sss.pgh.pa.us>
Date: Wed, 28 Jul 2010 00:01:51 -0400
Message-Id: <1280289550-sup-6155@alvh.no-ip.org>
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Excerpts from Tom Lane's message of mar jul 27 20:05:02 -0400 2010:
> Peter Hussey <peter@labkey.com> writes:

> > 2) How is work_mem used by a query execution?
> 
> Well, the issue you're hitting is that the executor is dividing the
> query into batches to keep the size of the in-memory hash table below
> work_mem.  The planner should expect that and estimate the cost of
> the hash technique appropriately, but seemingly it's failing to do so.
> Since you didn't provide EXPLAIN ANALYZE output, though, it's hard
> to be sure.

Hmm, I wasn't aware that hash joins worked this way wrt work_mem.  Is
this visible in the explain output?  If it's something subtle (like an
increased total cost), may I suggest that it'd be a good idea to make it
explicit somehow in the machine-readable outputs?