SELECT NULL IS DISTINCT FROM 'не нулл';

SELECT
CASE WHEN ROW('Null','Null') IS DISTINCT FROM ROW('Null','Null')
THEN
	'изменен'
ELSE
	'неизменен'
END;

...
RAISE NOTICE \'_lastold.okei %\', _lastold.okei;
RAISE NOTICE \'NEW.okei %\',  NEW.okei;
_isupdated := (select _lastold.okei  IS DISTINCT FROM  NEW.okei);
--_isupdated := tRUE;
RAISE NOTICE \'_isupdated %\',_isupdated;
...

NOTICE:  _lastold.okei  123 
NOTICE:  NEW.okei <NULL>
NOTICE:  _isupdated f

SELECT   123  IS DISTINCT FROM null
t

--_isupdated :=  (_lastold.okei) IS DISTINCT FROM (NEW.okei);
_isupdated :=  1  IS DISTINCT FROM NULL;

CREATE OR REPLACE FUNCTION f_test_distinct()
  RETURNS SETOF bool AS
'
declare
	_res bool;
	_tmp int4;
	_tmp1 int4;
begin 
	_res := 1 IS DISTINCT FROM 2;
	RAISE NOTICE \'1 IS DISTINCT FROM  2  %\' , _res;
	RETURN NEXT _res;
	_res := 1 IS DISTINCT FROM 1;
	RAISE NOTICE \'1 IS DISTINCT FROM  1  %\' , _res;

	_tmp:=1; _tmp1:=2;
	RETURN NEXT _res;
	_res := _tmp  IS DISTINCT FROM _tmp1;
	RAISE NOTICE \'_tmp:= 1   IS DISTINCT FROM _tmp1:= 2  %\' , _res;

	_tmp:=1; _tmp1:=1;
	RETURN NEXT _res;
	_res := _tmp  IS DISTINCT FROM _tmp1;
	RAISE NOTICE \'_tmp:= 1   IS DISTINCT FROM _tmp1:= 1  %\' , _res;
	RETURN NEXT _res;

	_tmp:=1; _tmp1:=2;
	_res := ROW(_tmp,1)  IS DISTINCT FROM (_tmp1,1);
	RAISE NOTICE \'ROW(_tmp:= 1 , 1 )  IS DISTINCT FROM ROW(_tmp1:= 2 , 1 ) %\' , _res;
	RETURN NEXT _res;

	_res := ROW(1,1)  IS DISTINCT FROM (2,2);
	RAISE NOTICE \'ROW( 1 , 1 )  IS DISTINCT FROM ( 2 , 2 ) %\' , _res;
	RETURN NEXT _res;
return;
end;
'
  LANGUAGE 'plpgsql' VOLATILE;