not quite expected behaviour when using IN clause

Hello all,

I apologize for the wide distribution but we recently ran into an
interesting behaviour using PostgreSQL 8.0.3 and did not know whether this
was a bug or intended behaviour.

When an IN clause contains a NULL value the entire in clause is considered
as being false, thus no records are returned.

Why doesn't IN evaluate NULL as a value?

so for example:

SELECT count(*) FROM test WHERE key NOT IN ('something');
returns the count of rows...

where
SELECT count(*) FROM test WHERE key NOT IN ('something', NULL);
does not. table test does not have any NULL values in the key column.

the query plans follow...

mazu=# EXPLAIN ANALYZE SELECT count(*) FROM test WHERE key NOT IN
('something');
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=100000022.44..100000022.44 rows=1 width=0) (actual time=
0.664..0.665 rows=1 loops=1)
-> Seq Scan on test (cost=100000000.00..100000020.38 rows=826 width=0)
(actual time=0.030..0.349 rows=168 loops=1)
Filter: (("key")::text <> 'something'::text)
Total runtime: 0.826 ms
(4 rows)

mazu=# EXPLAIN ANALYZE SELECT count(*) FROM test WHERE key NOT IN
('something', NULL);
QUERY PLAN
------------------------------------------------------------------------------------------------------------
Aggregate (cost=100000022.44..100000022.44 rows=1 width=0) (actual time=
0.027..0.029 rows=1 loops=1)
-> Result (cost=100000000.00..100000020.38 rows=826 width=0) (actual time=
0.002..0.002 rows=0 loops=1)
One-Time Filter: NULL::boolean
-> Seq Scan on test (cost=100000000.00..100000020.38 rows=826 width=0)
(never executed)
Filter: (("key")::text <> 'something'::text)
Total runtime: 0.110 ms
(6 rows)

--
Joe Maldonado