Re: IMMUTABLE?

On May 15, 2006, at 21:31, Tom Lane wrote:

> 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.

Oh, I see. So it's more like a constant or C macro.

> 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.)

Yes, but there are definitely programming cases where memoization/
caching definitely helps. And it's easy to tell for a given function
whether or not it really helps by simply trying it with CACHED and
without.

Would this be a simple thing to implement?

Best,

David