Message-ID: <4240874E.2080301@archonet.com>
Date: Tue, 22 Mar 2005 20:59:58 +0000
From: Richard Huxton <dev@archonet.com>
User-Agent: Mozilla Thunderbird 1.0 (X11/20041206)
MIME-Version: 1.0
To: Sean Davis <sdavis2@mail.nih.gov>
Cc: PostgreSQL SQL <pgsql-sql@postgresql.org>
Subject: Re: Self-referencing table question
References: <0af8c6d3baaf3d4a6d918df294019507@mail.nih.gov>
	<7e154914b76fb1a7c55912b621400860@mail.nih.gov>
In-Reply-To: <7e154914b76fb1a7c55912b621400860@mail.nih.gov>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Sean Davis wrote:
> I answer my own question, if only for my own records.  The following  
> query is about 5-6 times faster than the original.  Of course, if  
> anyone else has other ideas, I'd be happy to hear them.
> 
> Sean
> 
> explain analyze select from_id,to_id,val from exprsdb.correlation where  
> from_id in (select to_id from exprsdb.correlation where from_id=2424  
> order by val desc limit 100) and to_id in (select to_id from  
> exprsdb.correlation where from_id=2424 order by val desc limit 100) and  
> val>0.6 and to_id<from_id;

Might not be any faster, but you can do this as a self-join with subquery:

SELECT c1.from_id, c1.to_id, c1.val
FROM
   correlation c1,
   (
     SELECT to_id FROM correlation WHERE from_id=2424
     ORDER BY val DESC LIMIT 100
   ) AS c2
   (
     SELECT to_id FROM correlation WHERE from_id=2424
     ORDER BY val DESC LIMIT 100
   ) AS c3
WHERE
   c1.from_id = c2.to_id
   AND c1.to_id = c3.to_id
   AND c1.val > 0.5
   AND c1.to_id < from_id
;

I think PG should be smart enough nowadays to figure out these two 
queries are basically the same.
-- 
   Richard Huxton
   Archonet Ltd