Re: Slow SQL query (14-15 seconds)

Bruno Baguette <bruno.baguette@gmail.com> writes:
> Le 13/11/08 14:31, Tom Lane a �crit :
>> 16.511 * 818 = 13505.998, so this is all but about 100 msec of the
>> runtime.  Can't tell if there's any easy way to improve it.  In
>> pre-8.4 releases trying to convert the EXISTS into an IN might help.

> Can you explain why a IN is fastest than an EXISTS subquery ?

The planner is smarter about IN than EXISTS --- it can usually convert
the former into a join plan instead of a subplan.  (This situation will
improve in 8.4.)

> Do you think I can improve again the performance of that query ?

You've still got a subplan in there, not quite sure why.  Anyway,
increasing work_mem might get it to change to a hashed subplan,
which'd likely be faster.

            regards, tom lane