Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, Mar 5, 2010 at 5:32 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> It's arguably a bug, but since we lack consensus on whether NULL and
>> ROW(NULL,NULL,...) are the same thing, it's difficult to make a
>> bulletproof case either way.
> Have we or can we somehow document why this happens?
The reason it happens is that the assignment target is a "row" variable,
meaning that it doesn't have concrete existence as a tuple but is just
an alias for a list of scalar variables. So there is no way for it to
represent an atomic NULL; setting each of the individual scalars to NULL
is possible but the result acts more like ROW(NULL,NULL,...).
I'm not sure about documenting that. It seems like an implementation
detail. If we had consensus that the two cases either should or should
not be distinguishable, we could work towards making that happen; but
lacking such consensus I'm hesitant to touch it at all.
regards, tom lane