Re: Self-referencing table question

----- Original Message ----- 
From: "Richard Huxton" <dev@archonet.com>
To: "Sean Davis" <sdavis2@mail.nih.gov>
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