In case you aren't comfortable running unreleased planner patches from pgsql-hackers, a workaround was discussed on this list recently:

http://archives.postgresql.org/pgsql-performance/2009-09/msg00036.php

On Wed, 09 Sep 2009 06:05:22 -0400, Heikki Linnakangas <heikki.linnakan...@enterprisedb.com> wrote:

Kevin Kempter wrote:
Hi all I have a large table (>2billion rows) that's partitioned by date based on an epoch int value. We're running a select max(id) where id is the PK. I
have a PK index on each of the partitions, no indexes at all on the base
table.

If I hit a partition table directly I get an index scan as expected:

The planner isn't smart enough to create the plan you're expecting.
There was discussion and even a patch posted recently about that:

http://archives.postgresql.org/pgsql-hackers/2009-07/msg01115.php

It seems the thread petered out, but the concept seems sane.




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