Andres Freund <andres(at)anarazel(dot)de> writes:
> Ah, brainfade. Probably triggered by the fact that I forgot that we call
> input functions even on NULL (which never quite made sense to me).
That's so that domain_in can reject NULLs if the domain constraints
say to do so.
Mind you, the SQL committee should never have allowed NOT NULL
domain constraints in the first place, because the concept is
fundamentally incompatible with outer joins. But it's there
and we try to honor it in this case.
regards, tom lane