MySQL表中具有相同字段的两个索引

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

Ther是两个索引：id_type和id_type,date.

据我所知,如果我们有两个字段的索引,我们可以将它用作第一个字段的单个索引.

我可以删除索引id_type而不会丢失性能吗？

更新：问这个问题引起注意,有时不同索引中的同一个字段具有不同的基数.

最佳答案

MysqL 5.7.9 – 删除id_type索引并没有什么区别.多列索引(type_date)适用于两个查询.

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      |         |               |
+--------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+-------------