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

On Friday, August 5, 2022, PG Bug reporting form <noreply@postgresql.org>
wrote:

The following bug has been logged on the website:

Bug reference: 17575
Logged by: Alexey Borschev
Email address: aborschev@gmail.com
PostgreSQL version: 14.4
Operating system: Ubuntu
Description:

Hi, PG hackers!
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:

SELECT row(NULL::int) = row(NULL::int) AS "test= "
, row(NULL::int) IS NULL AS IS_NULL
, row(NULL::int) IS NOT NULL AS NOT_NULL
, row(NULL::int) IS DISTINCT FROM NULL AS
IS_DISTINCT_FROM_NULL

, row(NULL::int) IS NOT DISTINCT FROM NULL AS
NOT_DISTINCT_FROM_NULL

test= | is_null | not_null | is_distinct_from_null |
not_distinct_from_null
--------+---------+----------+-----------------------+------
------------------
| t | f | t | f

But here row(NULL::int) IS NULL -> true, and row(NULL::int) IS NOT DISTINCT
FROM NULL -> false !

Next point:

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

SELECT row(NULL::int, 'Bob'::TEXT) IS NULL AS
Row_IsNULL
, row(NULL::int, 'Bob'::TEXT) IS NOT NULL AS
Row_NotNULL ;

row_isnull | row_notnull
------------+-------------
f | f
- They both return False on same input!

Can we fix or document this PG issue?

It is documented. The paragraph just below the tip.

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

David J.

PG Bug reporting form <noreply@postgresql.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

Thank You, David and Tom!
Now these tricks around row(null) are much clearer to me!
I think this issue should be closed.

пт, 5 авг. 2022 г. в 17:23, Tom Lane <tgl@sss.pgh.pa.us>: