Sam Mason <sam@samason.me.uk> writes:
> I have a large problem understanding the real purpose casts so maybe I'm
> missing something. My problem is that I don't understand the purpose
> of trying to provide a "standard" way of converting between arbitrary
> datatypes,
I think the reason CREATE CAST exists is exactly that the cast mechanism
*isn't* intended to provide conversions between any arbitrary pair of
datatypes. It's only intended to provide conversions in those cases
where the conversion semantics are obvious to some degree or other.
Since that's somewhat in the eye of the beholder, we allow the user
to adjust edge cases by creating/removing casts --- but there's no
expectation that when you define a new datatype, you'll provide casts
to or from unrelated types.
regards, tom lane