From: Tim Jones <TJones@optio.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: slow query using sub select
Date: Tue, 23 May 2006 09:26:37 -0400
Message-ID: <47668A1334CDBF46927C1A0DFEB223D340EF94@mail.optiosoftware.com> (raw)
that worked like a champ nice call as always!
thanks
Tim Jones
Healthcare Project Manager
Optio Software, Inc.
(770) 576-3555
-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: Monday, May 22, 2006 7:07 PM
To: Tim Jones
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] slow query using sub select
"Tim Jones" <TJones@optio.com> writes:
> I am having a problem with a sub select query being kinda slow. The
> query is as follows:
> select batterycode, batterydescription, observationdate from Battery
> t1 where patientidentifier=611802158 and observationdate = (select
> max(observationdate) from Battery t2 where
> t2.batterycode=t1.batterycode and patientidentifier=611802158) order
by batterydescription.
Yeah, this is essentially impossible for the planner to optimize,
because it doesn't see any way to de-correlate the subselect, so it does
it over again for every row. You might find it works better if you cast
the thing as a SELECT DISTINCT ON problem (look at the "weather report"
example in the SELECT reference page).
regards, tom lane
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Cc: TJones@optio.com, tgl@sss.pgh.pa.us
Subject: Re: slow query using sub select
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