Repeatable in maria-5.3, maria-5.2, maria-5.1, mysql-5.5 . Not repeatable in mysql-5.1. Not influenced by optimizer switches.

The following query:

SELECT t2.b , t1.c
FROM t2 LEFT JOIN t1 ON t2.a != 0
WHERE ( t2.b , t1.c ) NOT IN ( SELECT 2 , 9 );

does not return rows for which the WHERE predicate is true. It returns an empty result in maria and mysql-5.5 and in mysql-5.1 it returns:

b c
9 NULL
9 NULL

test case:

CREATE TABLE t1 (c int) ;

CREATE TABLE t2 (a int, b int) ;
INSERT INTO t2 VALUES (20,9),(20,9);

SELECT t2.b , t1.c
FROM t2 LEFT JOIN t1 ON t2.a != 0
WHERE ( t2.b , t1.c ) NOT IN ( SELECT 2 , 9 );

explain:

optimizer switch:

index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,index_condition_pushdown=off,derived_merge=off,derived_with_keys=off,firstmatch=off,loosescan=off,materialization=off,in_to_exists=on,semijoin=off,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=off,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=off,semijoin_with_cache=off,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on

revision-id: <email address hidden>
date: 2011-07-20 11:56:28 +0400
build-date: 2011-07-20 15:37:47 +0300
revno: 3123
branch-nick: maria-5.3

test case:

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (c int) ;

DROP TABLE IF EXISTS t2;
CREATE TABLE t2 (a int, b int) ;
INSERT INTO t2 VALUES (20,9),(20,9);

SELECT t2.b , t1.c
FROM t2 LEFT JOIN t1 ON t2.a != 0
WHERE ( t2.b , t1.c ) NOT IN ( SELECT 2 , 9 );