Tom Lane wrote:
Instead, I'm thinking it might be time to re-introduce some notion of
function execution cost into the system, and make use of that info to
sort WHERE clauses into a reasonable execution order.


That sounds like a straightforward idea.

This example
would be fixed with even a very stupid rule-of-thumb about SQL functions
being more expensive than C functions, but if we're going to go to the
trouble it seems like it'd be a good idea to provide a way to label
user-defined functions with execution costs.

Agreed.

Would a simple constant value be workable, or do we need some more
complex model (and if so what)?

A simple constant would probably be enough. If we want anything fancier than that, it should be up to the author of the function to define the cost model as well. I'm envisioning that you could attach a custom cost function to a user-defined function which would return the estimated CPU cost. And # of rows returned for a set-returning function.

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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