From: Sean Davis <sdavis2@mail.nih.gov>
To: Sean Davis <sdavis2@mail.nih.gov>
Cc: PostgreSQL SQL <pgsql-sql@postgresql.org>
Cc: Richard Huxton <dev@archonet.com>
Subject: Re: Self-referencing table question
Date: Wed, 23 Mar 2005 06:04:04 -0500
Message-ID: <117faba5d7a78ad4c2ca7bd81f879224@mail.nih.gov> (raw)
In-Reply-To: <000a01c52f3c$3ee7a980$1f6df345@WATSON>
References: <0af8c6d3baaf3d4a6d918df294019507@mail.nih.gov>
<7e154914b76fb1a7c55912b621400860@mail.nih.gov>
<4240874E.2080301@archonet.com>
<000a01c52f3c$3ee7a980$1f6df345@WATSON>
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
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Cc: sdavis2@mail.nih.gov, dev@archonet.com
Subject: Re: Self-referencing table question
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