Re: [SQL] Self-referencing table question

[Sean Davis]

On Mar 22, 2005, at 7:07 PM, Sean Davis wrote:
----- 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.
Oops, I DID do a different query in my previous email than what you 
suggest in the your email.  Testing both against each other, the two 
queries--using subselects in 'in' and doing a self-join via 
subquery--have basically the same performance.

Thanks again for the help.
Sean
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