Re: 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 Huxton
Archonet Ltd