The following query:

SELECT *
FROM t1, t2
WHERE (t2.a , t1.b) IN (
SELECT a , b
FROM t3
);

returns the matching row twice:

-------+

-------+

-------+

whereas the correct result is:

-------+

-------+

-------+

explain:
----------------------------------------------------------------------------------------+

----------------------------------------------------------------------------------------+

----------------------------------------------------------------------------------------+

minimal optimizer_switch: semijoin=on,loosescan=on

full 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=on,materialization=off,in_to_exists=on,semijoin=on,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

bzr version-info:

revision-id: <email address hidden>
date: 2011-08-23 15:51:47 +0300
build-date: 2011-08-26 17:20:27 +0300
revno: 3166
branch-nick: maria-5.3

test case:

CREATE TABLE t1 (b int) ;
INSERT INTO t1 VALUES (1),(5);

CREATE TABLE t2 (a int, PRIMARY KEY (a)) ;
INSERT INTO t2 VALUES (6),(10);

CREATE TABLE t3 (a int, b int, KEY (b)) ;
INSERT INTO t3 VALUES (6,5),(6,2),(8,0),(9,1),(6,5);

SET SESSION optimizer_switch='loosescan=on';

SELECT *
FROM t1, t2
WHERE (t2.a , t1.b) IN (
SELECT a , b
FROM t3
);