The following query:

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

returns

5
5

when executed with loosescan=on, even though t1 contains only one row where a = 5 .

EXPLAIN:

id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY t2 index b b 9 NULL 2 Using where; Using index; LooseScan
1 PRIMARY t3 ref b b 4 test.t2.b 1 Using index; FirstMatch(t2)
1 PRIMARY t1 ALL NULL NULL NULL NULL 4 Using where; Using join buffer (flat, BNL join)

minimal switch: semijoin=on,loosescan=on

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=on,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 12:26:06 +0200
revno: 3273
branch-nick: maria-5.3

test case:

CREATE TABLE t1 (a int);
INSERT INTO t1 VALUES (9),(0),(8),(5);

CREATE TABLE t2 (a int, b varchar(1), KEY (b,a));
INSERT INTO t2 VALUES (5,'r'),(5,'z');

CREATE TABLE t3 (a int, b varchar(1), KEY (b,a));
INSERT INTO t3 VALUES (5,'r'),(5,'z');

SET SESSION optimizer_switch='loosescan=on';

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