Re: 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