Re: Function execution costs 'n all that

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