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cardserial）都已编入索引：

    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糟糕）

但是，默认情况下，MySQL仅使用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)

（它甚至不考虑cardserial上的索引，但我猜这是另一个问题。）

使用“USE KEY”或“FORCE KEY”可以使用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索引来加速查询？ cardtype和cardstatus索引似乎都以相同的方式定义，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）。如果进行此类合并，则为an example（单击“查看执行计划”）。注意第一个SELECT的“index_merge”。

免责声明：我绝对不确定上述信息。

在你的情况下，尽管有你的提示，优化器仍然认为直接扫描第二个表比合并索引更快（你的表可能有很多行，因此操作非常大，代价高昂）索引）。

我建议：

ALTER TABLE ADD INDEX CardTypeStatus (cardtype, cardstatus);

这会在两列上创建索引。您的查询可能能够使用此索引。您可能之后想要删除CardType索引：即使查询仅在cardtype列上搜索，查询仍然可以使用双列索引（但如果他们搜索的话，则不会仅cardstatus。

有关多列索引的详细信息：http://dev.mysql.com/doc/refman/5.5/en/multiple-column-indexes.html

MySQL没有使用的特定索引

1 个答案: