Triggers: Detecting if a column value is explicitly set in an UPD ATE statement or not... ("IS NULL" not working?)

I'm having a problem, and can't seem to find a good answer in the mailing
list archives... sorry if I'm missing something obvious!

Postgres version: 8.2.4
O/S: Solaris 10

I want to set a BOOLEAN column value to FALSE by default for all INSERT and
UPDATE statements performed against a particular table _UNLESS_ it's
explicitly set to TRUE in the SQL statement. Here is the trigger I created:

CREATE TABLE table1 ( id INTEGER PRIMARY KEY, data VARCHAR(64), b_flag
BOOLEAN);

CREATE OR REPLACE FUNCTION func1() RETURNS trigger AS $func1$
BEGIN
RAISE NOTICE 'NEW.b_flag=%', NEW.b_flag;
IF ( NEW.b_flag IS NULL ) THEN
NEW.b_flag := FALSE;
END IF;

RETURN NEW;
END;
$func1$ LANGUAGE plpgsql;

CREATE TRIGGER func1 BEFORE INSERT OR UPDATE ON table1 FOR EACH ROW EXECUTE
PROCEDURE func1();

What I get, however, is that for an UPDATE, the "NEW.b_flag" value evaluates
to TRUE if it has been set to TRUE by a previous UPDATE that explicitly sets
it. So...

tqa=> INSERT INTO table1 VALUES ( '1', 'some data');
NOTICE: NEW.b_flag=<NULL>
INSERT 0 1
tqa=> SELECT b_flag FROM table1 WHERE id='1';
b_flag
--------
f
(1 row)

tqa=> UPDATE table1 SET b_flag=TRUE where id='1';
NOTICE: NEW.b_flag=t
UPDATE 1
tqa=> SELECT b_flag FROM table1 WHERE id='1';
b_flag
--------
t
(1 row)

tqa=> UPDATE table1 SET data='new data' where id='1';
NOTICE: NEW.b_flag=t
UPDATE 1
tqa=> SELECT b_flag FROM table1 WHERE id='1';
b_flag
--------
t
(1 row)

As you can see, I put a RAISE NOTICE and verified that for some reason, the
NEW row contains a 'TRUE' value for b_flag, even though I didn't explicitly
set it in the last UPDATE statement. Why does it seem to be reading the
value from the OLD row for that column unless I override it inside the SQL
statement? Is there any way to achieve the desired result without having to
explicitly set 'b_flag' each time I touch a row in the table?

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