MySQL 未使用的特定索引

Question

我正在尝试在我的数据库中运行以下查询：

SELECT * FROM ts_cards WHERE ( cardstatus= 2 OR cardstatus= 3 ) AND ( cardtype= 1 OR cardtype= 2 ) ORDER BY cardserial DESC LIMIT 10;

所有三个字段（cardstatus、cardtype 和carderial）都被索引：

    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)

（是的，我知道索引的名字很烂）

然而，默认情况下，MySQL 只使用 cardstatus 的索引：

    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)

（它甚至不考虑carderial上的索引，但我想这是另一个问题。）

使用“USE KEY”或“FORCE KEY”可以使其使用cardtype的索引，但不能同时使用cardtype和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)

如何强制 MySQL 使用 BOTH 索引来加快查询速度？ cardtype 和 cardstatus 索引的定义方式似乎相同，但 cardstatus 似乎优先于 cardtype。

Answer 1

IIRC，MySQL 不能在同一个查询中使用两个不同的索引。要使用这两个索引，MySQL 需要将它们合并为一个 (link to manual)。如果这样的合并，这里是an example（点击“查看执行计划”）。注意第一个SELECT 的“index_merge”。

免责声明：我不绝对确定上述信息。

在您的情况下，尽管有您的提示，优化器仍然认为直接扫描第二个表比合并索引更快（您的表可能有非常多的行，因此非常大，操作成本高索引）。

我建议：

ALTER TABLE ADD INDEX CardTypeStatus (cardtype, cardstatus);

这会在两列上创建一个索引。您的查询可能能够使用此索引。您可能想在之后删除您的 CardType 索引：查询仍然可以使用两列索引，即使他们仅搜索 cardtype 列（但如果他们仅搜索 cardstatus )。

更多关于多列索引的信息：http://dev.mysql.com/doc/refman/5.5/en/multiple-column-indexes.html