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