David Wheeler <david@xxxxxxxxxxxxxx> writes: > But seriously, the documentation says (as if I need to tell you, but > I was reading it again to make sure that I'm not insane): >> IMMUTABLE indicates that the function always returns the same >> result when given the same argument values; that is, it does not do >> database lookups or otherwise use information not directly present >> in its argument list. If this option is given, any call of the >> function with all-constant arguments can be immediately replaced >> with the function value. Sure. As I read it, that's talking about a static transformation: planner sees 2 + 2 (or if you prefer, int4pl(2,2)), planner runs the function and replaces the expression with 4. Nothing there about memoization. It's true that the system *could* memoize (or in our more usual parlance, cache function values) given the assumptions embodied in IMMUTABLE. But we don't, and I don't see any statement in the docs that promises that we do. For 99% of the functions that the planner deals with, memoization would be seriously counterproductive because the function evaluation cost is comparable to if not less than the lookup cost in a memo table. (int4pl is a good case in point.) regards, tom lane
