Re: [SQL] Self-referencing table question

[Sean Davis]

----- Original Message ----- 
From: "Richard Huxton" <dev@archonet.com>
To: "Sean Davis" <[EMAIL PROTECTED]>
Cc: "PostgreSQL SQL" <pgsql-sql@postgresql.org>
Sent: Tuesday, March 22, 2005 3:59 PM
Subject: Re: [SQL] Self-referencing table question


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,
In another email, I posted what I did (which was what you suggest), along 
with explain analyze output.  It looks like the subquery is 4-6 times 
faster, which is getting into the acceptible for my little web application.

Thanks for the help.
Sean

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