Re: ALTER TYPE 3: add facility to identify further no-work cases

Robert Haas <robertmhaas@gmail.com> writes:
> Well, if you're positive we're eventually going to want this in
> pg_proc, we may as well add it now.  But I'm not too convinced it's
> the right general API.  The number of people writing exactly x + 0 or
> x * 0 in a query has got to be vanishingly small; I'm not eager to add
> additional parse analysis time to every SQL statement that has a
> function in it just to detect those cases.

Actually, you've got that backwards: the facility I've got in mind would
cost next to nothing when not used.  The place where we'd want to insert
this in eval_const_expressions has already got its hands on the relevant
pg_proc row, so checking for a nonzero hook-function reference would be
a matter of a couple of instructions.  If we go with a pg_cast entry
then we're going to have to add a pg_cast lookup for every cast, whether
it turns out to be optimizable or not; which is going to cost quite a
lot more.  The intermediate hook function I was sketching might be
worthwhile from a performance standpoint even if we don't expose the
more general feature to users, just because it would be possible to
avoid useless pg_cast lookups (by not installing the hook except on
pg_proc entries for which there's a relevant CAST WHEN function to call).

> Even slightly more
> complicated problems seem intractable - e.g. (x + 1) = x can be
> simplified to constant false, and NOT ((x + 1) = x) can be simplified
> to x IS NOT NULL, but under the proposed API those would have to hang
> off of =(int4,int4), which seems pretty darn ugly.

True, but where else are you going to hang them off of?  Not that I was
particularly thinking of doing either one of those.
        regards, tom lane