The following query:

SELECT *
FROM t1
WHERE (t1.b) IN (
SELECT c
FROM t2
)
ORDER BY t1.a;

returns
a b
3888 20

when executed with firstmatch=off

and

a b
3888 20
3914 17

for all other execution plans.

EXPLAIN:

id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t1 ALL NULL NULL NULL NULL 2 Using where; Using filesort; Start temporary
1 PRIMARY t2 ref c c 5 test.t1.b 1 Using index; End temporary

minimal switch: semijoin=on,firstmatch=off ; join_cache_level=3..8
full 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=on,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: 2011-11-07 16:39:02 +0400
build-date: 2011-11-08 11:48:08 +0200
revno: 3273
branch-nick: maria-5.3

test case:

--source include/have_innodb.inc

CREATE TABLE t1 (a int NOT NULL , b int) ENGINE=InnoDB;
INSERT INTO t1 VALUES (3914,17),(3888,20);

CREATE TABLE t2 (c int, KEY (c)) ENGINE=InnoDB;
INSERT INTO t2 VALUES (17),(20);

SET SESSION join_cache_level=3;
SET SESSION optimizer_switch='firstmatch=OFF';

SELECT *
FROM t1
WHERE (t1.b) IN (
SELECT c
FROM t2
)
ORDER BY t1.a;