Re: slow query using sub select

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