Efficient deletions?

Hello, 
This is my first message to this list. I'm curently trying to
improve my postgres skillsm I've assigned myself a project involving a
database whose dataset is quite larger than those I had to use until
now.
I'm curently facing a trouble with the DELETE query: I need to
delete from a table a set of rows choosen by a sub-select. The query
curently looks like this:
DELETE FROM foo WHERE (col1, col2) IN (SELECT...)

It seems from the output of ANALYSE that the sub-query is executed for
each foo rows. That's really not efficient as the set returned by the
sub-select is constant.
I had a similar trouble with a SELECT query but it was eavily
solved by placing one of the sub-select in the FROM part of the query.
Unfortunately it look that can't be done this way with DELETE.
Is there a way to instruct the planner that this sub-query is
constant over the time of the DELETE execution? Or is there another
more efficent way to express this?
I conducted tests with a sub-select returning about 20,000
rows and a foo table of 370,000 rows, out of patience I had to
interrupt the query after five hours. That worries me as this query is
part of an update process that could be fired several times a day.


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