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

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, Jan 26, 2011 at 3:08 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> It's not obvious to me that it has a use case outside of casts, but
>>> it's certainly possible I'm missing something.

>> A possible example is simplifying X + 0 to X, or X * 0 to 0.

> Oh, I see. The times I've seen an issue with those kinds of
> expressions have always been related to selectivity estimation.

Yeah, helping the planner recognize equivalent cases is at least as
large a reason for wanting this as any direct savings of execution time.

I don't mind confining the feature to casts to start with, but it might
be a good idea to specify the check-function API in a way that would let
it be plugged into a more generally available call-simplification hook
later.

regards, tom lane