Mysql 5.1.49 InnoDB/query optimizer acting weird? - mysql

Question

I have the following InnoDB table, which acts kinda weird under MySQL 5.1.49
(mysql Ver 14.14 Distrib 5.1.49, for debian-linux-gnu (x86_64) using readline 6.1)
mysql> desc forum_favorite;
+-----------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+----------------------+------+-----+---------+-------+
| id_member | smallint(5) unsigned | YES | MUL | NULL | |
| id_topic | int(10) unsigned | YES | MUL | NULL | |
+-----------+----------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> show index from forum_favorite;
+----------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| forum_favorite | 1 | id_member | 1 | id_member | A | 2134 | NULL | NULL | YES | BTREE | |
| forum_favorite | 1 | id_topic | 1 | id_topic | A | 3201 | NULL | NULL | YES | BTREE | |
+----------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.00 sec)
Now, check the query:
mysql> SELECT id_topic FROM forum_favorite WHERE (id_member = 2);
+----------+
| id_topic |
+----------+
| 1249 |
| 20209 |
| 91878 |
| 99026 |
| 90257 |
| 1179 |
| 1179 |
+----------+
7 rows in set (0.00 sec)
When I search for a specific topic with a given member, it gives an empty result set. WHY?
mysql> select * from forum_favorite where id_member = 2 and id_topic = 1249;
Empty set (0.00 sec)
But when I search for another topic, it comes back ok...
It can be found without id_member in the where clause:
mysql> select * from forum_favorite where id_topic = 1249;
+-----------+----------+
| id_member | id_topic |
+-----------+----------+
| 2 | 1249 |
+-----------+----------+
1 rows in set (0.00 sec)
Indexes:
mysql> show index from forum_favorite;
+----------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| forum_favorite | 1 | id_member | 1 | id_member | A | 2134 | NULL | NULL | YES | BTREE | |
| forum_favorite | 1 | id_topic | 1 | id_topic | A | 3201 | NULL | NULL | YES | BTREE | |
+----------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.00 sec)
Explain on the incriminating query:
mysql> explain select * from forum_favorite where id_member = 2 and id_topic = 1249;
+----+-------------+----------------+-------------+--------------------+--------------------+---------+------+------+---------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------+-------------+--------------------+--------------------+---------+------+------+---------------------------------------------------------------+
| 1 | SIMPLE | forum_favorite | index_merge | id_member,id_topic | id_member,id_topic | 3,5 | NULL | 1 | Using intersect(id_member,id_topic); Using where; Using index |
+----+-------------+----------------+-------------+--------------------+--------------------+---------+------+------+---------------------------------------------------------------+
1 row in set (0.00 sec)
Explain on the query, with another topicid. WHAT THE HELL???
mysql> explain select * from forum_favorite where (id_member = 2) and id_topic = 20209;
+----+-------------+----------------+------+--------------------+----------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------+------+--------------------+----------+---------+-------+------+-------------+
| 1 | SIMPLE | forum_favorite | ref | id_member,id_topic | id_topic | 5 | const | 1 | Using where |
+----+-------------+----------------+------+--------------------+----------+---------+-------+------+-------------+
1 row in set (0.00 sec
I have migrated from an old mysql 4.x database to mysql 5.1 recently, where the above queries gave consistent results.
What could be the problem?!?! What makes the optimizer go bonkers?

Answer 1

Have you tried to run OPTIMIZE on the table?
UPD:
For InnoDB tables, OPTIMIZE TABLE is mapped to ALTER TABLE, which rebuilds the table to update index statistics and free unused space in the clustered index.
I think the problem is actually in Using intersect(id_member,id_topic);. I would extend the id_topic index to (id_topic, id_member) so that it used a single key for the lookup instead of index merge (which is actually rather rare in MySQL).