Re: Self-referencing table question

On Mar 22, 2005, at 7:07 PM, Sean Davis wrote:

>
> ----- 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.

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