Re: BUG #17575: row(NULL) IS NULL inconsistent with IS NOT DISTINCT FROM NULL

PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> I noticed strange behavior of ROW(NULL):
> I expect, that IS NULL operator should give the same result as IS NOT
> DISTINCT FROM NULL
> similarly, IS NOT NULL operator should give the same result as IS DISTINCT
> FROM NULL:

I don't see any particular reason to expect that. row(NULL)
is a row object containing one null field, which is in fact not
identical to a null composite value. The SQL spec dictates that
IS NULL should return true for both cases, but that doesn't mean
that no other operator is allowed to distinguish them. I'd say
that this is a wart of IS NULL rather than desirable behavior
we should copy elsewhere.

> Next point:
> I expected that IS NULL and IS NOT NULL operators must always return
> opposite results, but:

You have not read the SQL standard, then. It's quite clear
about that.

> Can we fix or document this PG issue?

... nor our documentation. See

https://www.postgresql.org/docs/current/functions-comparison.html

para beginning "If the expression is row-valued,", near the
bottom of the page.

regards, tom lane