Jeff Davis <pgsql@j-davis.com> writes:
> Would any form of cost estimate have meaning if the function has side
> effects? If it's a volatile function, doesn't that mean that the planner
> can't avoid or favor executing it?
No, not really. If the function is down inside a sub-select or
something like that, the number of executions could depend on any number
of factors (like whether we put it on the inside or outside of a join)
--- and if it's expensive then we will and should try to arrange the
query to minimize the number of executions. We're certainly not going
to drop back to all-plain-nestloops just because the query contains one
volatile function.
(Now mind you, a smart user will probably avoid using a volatile
function like that, but I don't think it's an adequate argument for
saying that we don't need cost information.)
regards, tom lane