Comment 1 for bug 934342

The bug is reproducible with LEFT JOIN as well.

With Elena's settings and
set join_cache_level = 6
I get the following execution plan:

MariaDB [test]> EXPLAIN EXTENDED SELECT * FROM t2 LEFT JOIN t1 ON ( a = c ) WHERE ( b, c ) IN ( SELECT b, c FROM t2 t );
+----+--------------+-------------+--------+---------------+--------------+---------+-----------+------+----------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+-------------+--------+---------------+--------------+---------+-----------+------+----------+--------------------------+
| 1 | PRIMARY | t2 | ALL | b | NULL | NULL | NULL | 3 | 100.00 | |
| 1 | PRIMARY | <subquery2> | eq_ref | distinct_key | distinct_key | 10 | func,func | 1 | 100.00 | |
| 1 | PRIMARY | t1 | ref | a | a | 4 | test.t.c | 2 | 100.00 | Using where; Using index |
| 2 | MATERIALIZED | t | ALL | b | NULL | NULL | NULL | 3 | 100.00 | |
+----+--------------+-------------+--------+---------------+--------------+---------+-----------+------+----------+--------------------------+

I get a wrong result set when executing by this plan.

If I use
set join_cache_level = 0;
I get the execution plan:

MariaDB [test]> EXPLAIN EXTENDED SELECT * FROM t2 LEFT JOIN t1 ON ( a = c ) WHERE ( b, c ) IN ( SELECT b, c FROM t2 t );
+----+--------------+-------------+--------+---------------+--------------+---------+-----------+------+----------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+-------------+--------+---------------+--------------+---------+-----------+------+----------+--------------------------+
| 1 | PRIMARY | t2 | ALL | b | NULL | NULL | NULL | 3 | 100.00 | |
| 1 | PRIMARY | <subquery2> | eq_ref | distinct_key | distinct_key | 10 | func,func | 1 | 100.00 | |
| 1 | PRIMARY | t1 | ref | a | a | 4 | test.t2.c | 2 | 100.00 | Using where; Using index |
| 2 | MATERIALIZED | t | ALL | b | NULL | NULL | NULL | 3 | 100.00 | |
+----+--------------+-------------+--------+---------------+--------------+---------+-----------+------+----------+--------------------------+
4 rows in set, 1 warning (0.00 sec)

I get the right result set when executing by this plan.

The plans differ only in line 3:
the first plan uses test.t.c to build the key to access table t1,
while the second plan uses test.t2.c for this purpose.