例如我们有表:

CREATE TABLE `my_tbl` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `id_type` int(11) NOT NULL,
 `date` date NOT NULL,
 `other_fields` varchar(200) CHARACTER SET latin1 NOT NULL,
 PRIMARY KEY (`id`),
 KEY `id_type` (`id_type`),
 KEY `type_date` (`id_type`,`date`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8

有两个索引:id_type 和id_type, date。

据我所知，如果我们有包含两个字段的索引，我们可以将其用作第一个字段的单个索引。

我可以在不损失性能的情况下删除索引 id_type 吗？

更新:问这个问题是因为注意到有时不同索引中的相同字段具有不同的基数。

最佳答案

MySQL 5.7.9 - 删除 id_type 索引没有任何区别。多列索引(type_date)应用于两个查询。

解释查询输出:

mysql> explain SELECT id_type,date FROM my_tbl WHERE id_type='some';
+----+-------------+--------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | my_tbl | NULL       | ref  | type_date     | type_date | 4       | const |    1 |   100.00 | Using index |
+----+-------------+--------+------------+------+---------------+-----------+-----



mysql> explain  SELECT id_type FROM my_tbl WHERE id_type='some';
+----+-------------+--------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra       |
        +----+-------------+--------+------------+------+---------------+-----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | my_tbl | NULL       | ref  | type_date     | type_date | 4       | const |    1 |   100.00 | Using index |
        +----+-------------+--------+------------+------+---------------+---------


mysql> show indexes from my_tbl;
+--------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table  | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| my_tbl |          0 | PRIMARY   |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| my_tbl |          1 | type_date |            1 | id_type     | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| my_tbl |          1 | type_date |            2 | date        | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+--------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+-------------

关于mysql - MySQL 表中具有相同字段的两个索引，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/36350596/