Null comparisons and the transform_null_equals run-time parameter

When the run-time parameter transform_null_equals is on, shouldn't two
variables with NULL values evaluate as equal? They don't seem to.

At the bottom of this message is a little test function. It tries all
comparisons of NULL-valued variables and NULL constants, both before and
after turning transform_null_equals on. Here's what it returns:

transform_null_equals OFF: NULL = NULL -> Unknown

transform_null_equals OFF: v1 = NULL -> Unknown

transform_null_equals OFF: NULL = v2 -> Unknown

transform_null_equals OFF: v1 = v2 -> Unknown

transform_null_equals ON: NULL = NULL -> True

transform_null_equals ON: v1 = NULL -> True

transform_null_equals ON: NULL = v2 -> True

transform_null_equals ON: v1 = v2 -> Unknown

My problem is in the last line: Comparing two NULL variables produces an
unknown result. I need it to evaluate as True, like the preceding three
comparisons.

Any suggestions?

~ TIA

~ Ken

CREATE OR REPLACE FUNCTION test() RETURNS varchar AS

$BODY$

DECLARE

v1 VARCHAR;

v2 VARCHAR;

s VARCHAR := '';

BEGIN

v1 := Null;

v2 := Null;

IF NULL = NULL THEN

s := s || 'transform_null_equals OFF: NULL = NULL -> True ';

ELSIF NOT (NULL = NULL) THEN

s := s || 'transform_null_equals OFF: NULL = NULL -> False ';

ELSE

s := s || 'transform_null_equals OFF: NULL = NULL -> Unknown ';

END IF;

s := s || chr(10);

IF v1 = NULL THEN

s := s || 'transform_null_equals OFF: v1 = NULL -> True ';

ELSIF NOT (v1 = NULL) THEN

s := s || 'transform_null_equals OFF: v1 = NULL -> False ';

ELSE

s := s || 'transform_null_equals OFF: v1 = NULL -> Unknown ';

END IF;

s := s || chr(10);

IF NULL = v2 THEN

s := s || 'transform_null_equals OFF: NULL = v2 -> True ';

ELSIF NOT (NULL = v2) THEN

s := s || 'transform_null_equals OFF: NULL = v2 -> False ';

ELSE

s := s || 'transform_null_equals OFF: NULL = v2 -> Unknown ';

END IF;

s := s || chr(10);

IF v1 = v2 THEN

s := s || 'transform_null_equals OFF: v1 = v2 - > True ';

ELSIF NOT v1 = v2 THEN

s := s || 'transform_null_equals OFF: v1 = v2 -> False ';

ELSE

s := s || 'transform_null_equals OFF: v1 = v2 -> Unknown ';

END IF;

s := s || chr(10);

SET LOCAL transform_null_equals TO ON;

IF NULL = NULL THEN

s := s || 'transform_null_equals ON: NULL = NULL -> True ';

ELSIF NOT (NULL = NULL) THEN

s := s || 'transform_null_equals ON: NULL = NULL -> False ';

ELSE

s := s || 'transform_null_equals ON: NULL = NULL -> Unknown ';

END IF;

s := s || chr(10);

IF v1 = NULL THEN

s := s || 'transform_null_equals ON: v1 = NULL -> True ';

ELSIF NOT (v1 = NULL) THEN

s := s || 'transform_null_equals ON: v1 = NULL -> False ';

ELSE

s := s || 'transform_null_equals ON: v1 = NULL -> Unknown ';

END IF;

s := s || chr(10);

IF NULL = v2 THEN

s := s || 'transform_null_equals ON: NULL = v2 -> True ';

ELSIF NOT (NULL = v2) THEN

s := s || 'transform_null_equals ON: NULL = v2 -> False ';

ELSE

s := s || 'transform_null_equals ON: NULL = v2 -> Unknown ';

END IF;

s := s || chr(10);

IF v1 = v2 THEN

s := s || 'transform_null_equals ON: v1 = v2 -> True ';

ELSIF NOT v1 = v2 THEN

s := s || 'transform_null_equals ON: v1 = v2 -> False ';

ELSE

s := s || 'transform_null_equals ON: v1 = v2 -> Unknown ';

END IF;

RETURN s;

END;

$BODY$

LANGUAGE plpgsql VOLATILE;

SELECT test();