Thread: 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
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
On Fri, 30 Sep 2005, Joe Maldonado wrote: > 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. RVC NOT IN IPV is described as NOT(RVC IN IPV) which turns into NOT(RVC = ANY IPV) = ANY does the following: c) If the implied <comparison predicate> is true for at least one row RT in T, then "R <comp op> <some> T" is true. d) If T is empty or if the implied <comparison predicate> is false for every row RT in T, then "R <comp op> <some> T" is false. e) If "R <comp op> <quantifier> T" is neither true nor false, then it is unknown. So, for key NOT IN ('something', NULL) there are two cases, key = 'something', in which case c applies and IN would be true and NOT IN false so the row doesn't get returned key <> 'something', in which case key = 'something' is false and key=NULL is unknown, so e applies and IN is unknown and NOT IN is unknown so the row doesn't get returned.