To: James Cranch <jdc41@cam.ac.uk>
cc: pgsql-performance@postgresql.org
Subject: Re: Rapidly finding maximal rows 
In-reply-to: <Prayer.1.3.4.1110102259170.8390@hermes-2.csi.cam.ac.uk> 
References: <Prayer.1.3.4.1110102259170.8390@hermes-2.csi.cam.ac.uk>
Comments: In-reply-to James Cranch <jdc41@cam.ac.uk>
	message dated "10 Oct 2011 22:59:17 +0100"
Date: Fri, 14 Oct 2011 00:25:53 -0400
Message-ID: <13279.1318566353@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

James Cranch <jdc41@cam.ac.uk> writes:
> I have a slow query. I expect that there is either a more intelligent way 
> to write it, or that one could make some indexes that would speed it up. 
> I've tried various indexes, and am not getting anywhere.
> I'd be grateful for any suggestions. Reasonably full details are below.

Two bits of advice:

1. Avoid unnecessary use of WITH.  It acts as an optimization fence,
which you don't want here.  In particular, the only way to avoid
sorting/aggregating over the whole table is for the outer query's WHERE
conditions on competition_name and academic_year_beginning to get pushed
down into the scans on challenge_entries ... and that can't happen if
there's a WITH in between.

2. Try increasing work_mem.  I think that your first view would work all
right if it had enough work_mem to go for a HashAgg plan instead of
sort-and-group, even without pushdown of the outer WHERE.  It'd
definitely be faster than what you've got, anyway.

The other approach with a window function is probably a lost cause.
Postgres hasn't got a lot of intelligence about optimizing
window-function queries ...

			regards, tom lane