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