Particular index not used by MySQL - mysql

Question

I'm trying to run the following query in my database:
SELECT * FROM ts_cards WHERE ( cardstatus= 2 OR cardstatus= 3 ) AND ( cardtype= 1 OR cardtype= 2 ) ORDER BY cardserial DESC LIMIT 10;
All three fields (cardstatus, cardtype and cardserial) are indexed:
mysql> SHOW INDEX FROM ts_cards;
+----------+------------+----------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------+------------+----------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+
| ts_cards | 0 | PRIMARY | 1 | card_id | A | 15000134 | NULL | NULL | | BTREE | |
| ts_cards | 1 | CardID | 1 | cardserial | A | 15000134 | NULL | NULL | | BTREE | |
| ts_cards | 1 | CardType | 1 | cardtype | A | 17 | NULL | NULL | | BTREE | |
| ts_cards | 1 | CardHolder | 1 | cardstatusholder | A | 17 | NULL | NULL | | BTREE | |
| ts_cards | 1 | CardExpiration | 1 | cardexpiredstatus | A | 17 | NULL | NULL | | BTREE | |
| ts_cards | 1 | CardStatus | 1 | cardstatus | A | 17 | NULL | NULL | | BTREE | |
+----------+------------+----------------+--------------+-------------------+-----------+-------------+----------+--------+------+------------+---------+
6 rows in set (0.22 sec)
(Yes, I know the index's names suck)
However, by default, MySQL uses only cardstatus' index:
mysql> EXPLAIN SELECT * FROM `ts_cards` WHERE ( cardstatus= 2 OR cardstatus= 3 ) AND ( cardtype= 1 OR cardtype= 2 ) ORDER BY cardserial DESC LIMIT 10;
+----+-------------+----------+-------+---------------------+------------+---------+------+---------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------------+------------+---------+------+---------+-----------------------------+
| 1 | SIMPLE | ts_cards | range | CardType,CardStatus | CardStatus | 1 | NULL | 3215967 | Using where; Using filesort |
+----+-------------+----------+-------+---------------------+------------+---------+------+---------+-----------------------------+
1 row in set (0.00 sec)
(It doesn't even consider the index on cardserial but I guess that's another problem.)
Using "USE KEY" or "FORCE KEY" can make it use cardtype's index, but not both cardtype and cardstatus:
mysql> EXPLAIN SELECT * FROM `ts_cards` FORCE KEY (CardType) WHERE ( cardstatus= 2 OR cardstatus= 3 ) AND ( cardtype= 1 OR cardtype= 2 ) ORDER BY cardserial DESC LIMIT 10;
+----+-------------+----------+-------+---------------+----------+---------+------+---------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------+----------+---------+------+---------+-----------------------------+
| 1 | SIMPLE | ts_cards | range | CardType | CardType | 1 | NULL | 6084861 | Using where; Using filesort |
+----+-------------+----------+-------+---------------+----------+---------+------+---------+-----------------------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM `ts_cards` FORCE KEY (CardType,CardStatus) WHERE ( cardstatus= 2 OR cardstatus= 3 ) AND ( cardtype= 1 OR cardtype= 2 ) ORDER BY cardserial DESC LIMIT 10;
+----+-------------+----------+-------+---------------------+------------+---------+------+---------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------------+------------+---------+------+---------+-----------------------------+
| 1 | SIMPLE | ts_cards | range | CardType,CardStatus | CardStatus | 1 | NULL | 3215967 | Using where; Using filesort |
+----+-------------+----------+-------+---------------------+------------+---------+------+---------+-----------------------------+
1 row in set (0.00 sec)
How can I force MySQL to use BOTH indexes to speed up the query? Both cardtype and cardstatus indexes seem to be defined in the same way yet cardstatus seems to take precedence over cardtype.

Answer 1

IIRC, MySQL cannot use two distinct indexes in the same query. To make use of both indexes, MySQL would need to merge them into one (link to manual). Here is an example if such merge (click on "View Execution Plan"). Notice the "index_merge" of the first SELECT.
Disclaimer: I'm not absolutely sure about the above information.
In your case, despite your hints, the optimizer still considers that the direct scanning of the second table is faster than merging indexes (your tables probably have a very large number of rows, hence a very large, costly-to-manipulate index).
I advise:
ALTER TABLE ADD INDEX CardTypeStatus (cardtype, cardstatus);
This creates an index on both columns. Your query will probably be able to use this index. You may want to drop your CardType index afterwards: queries can still use the two-column index even if they search on the cardtype column only (but not if they search on cardstatus only).
More information about multiple-column indexes: http://dev.mysql.com/doc/refman/5.5/en/multiple-column-indexes.html