I wouldn't mind being able to provide hints to the planner. For example, I have some set-returning functions that typically return 10-100 rows and I usually have a good idea of how many rows a particular set of inputs will generate, and sometimes I know other characteristics about those rows as well. But the planner, not knowing any better, always assumes the functions will return 1000 rows, which presumably affects the plan, sometimes for the worse. Here's an example of a problem I've had:
http://archives.postgresql.org/pgsql-performance/2004-08/msg00236.php
I've wondered if the ability to provide hints about the expected output from set-returning functions would be useful.
Hmmm, you could jank that by making a new GUC:
SET expected_srf_rows TO 152; SELECT * FROM myfunction();
Chris
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