Pavel Stehule <pavel.stehule@gmail.com> writes:
> some years ago there was some plans about parser's extensibility. I am
> able write bison extensions, but I thing, so lot of work should be
> done via hooking of transform stage.
This strikes me as next door to useless, because it can only handle
things that look like valid expressions to the existing grammar.
So pretty much all you can do is weird sorts of functions, which are
already accommodated at less effort with existing features such as
function overloading.
A hook check in that particular place is not going to have negligible
performance impact, since it's going to be hit tens or hundreds or
thousands of times per query rather than just once. So it's going to
require more than a marginal use case to persuade me we ought to have
it.
regards, tom lane