mysql主键返回的结果少于复合复合索引

Question

我继承了一个存在一些设计问题的数据库架构

请注意，桌子上还有另外 9 个键，我没有在下面列出，有问题的键看起来像

+-------+------------+----------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name                   | Seq_in_index | Column_name  | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| users |          0 | PRIMARY                    |            1 | userid       | A         |         604 |     NULL | NULL   |      | BTREE      |         |               |
| users |          1 | userid_2                   |            1 | userid       | A         |         604 |     NULL | NULL   |      | BTREE      |         |               |
| users |          1 | userid_2                   |            2 | age          | A         |         604 |     NULL | NULL   | YES  | BTREE      |         |               |
| users |          1 | userid_2                   |            3 | image        | A         |         604 |      255 | NULL   | YES  | BTREE      |         |               |
| users |          1 | userid_2                   |            4 | gender       | A         |         604 |     NULL | NULL   | YES  | BTREE      |         |               |
| users |          1 | userid_2                   |            5 | last_login   | A         |         604 |     NULL | NULL   | YES  | BTREE      |         |               |
| users |          1 | userid_2                   |            6 | latitude     | A         |         604 |     NULL | NULL   | YES  | BTREE      |         |               |
| users |          1 | userid_2                   |            7 | longitude    | A         |         604 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+----------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

在具有以下字段的表中。

+--------------------------------+---------------------+------+-----+-------------------+----------------+
| Field                          | Type                | Null | Key | Default           | Extra          |
+--------------------------------+---------------------+------+-----+-------------------+----------------+
| userid                         | int(11)             | NO   | PRI | NULL              | auto_increment |
| age                            | int(11)             | YES  |     | NULL              |                |
| image                          | varchar(500)        | YES  |     |                   |                |
| gender                         | varchar(10)         | YES  |     | NULL              |                |
| last_login                     | timestamp           | YES  | MUL | NULL              |                |
| latitude                       | varchar(20)         | YES  | MUL | NULL              |                |
| longitude                      | varchar(20)         | YES  |     | NULL              |                |
+--------------------------------+---------------------+------+-----+-------------------+----------------+

运行解释语句并强制它使用 userid_2，它使用 522 行

describe SELECT userid, age FROM users USE INDEX(userid_2) WHERE `userid` >=100 and age >27   limit 10 ;
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
|  1 | SIMPLE      | users | index | userid_2      | userid_2 | 941     | NULL |  522 | Using where; Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
1 row in set (0.02 sec)

如果我不强制它使用索引，它只是使用 主键，它只包含用户 ID 并且只使用 261 行

mysql> describe SELECT userid, age FROM users  WHERE userid >=100 and age >27   limit 10 ;
+----+-------------+-------+-------+--------------------------------------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys                              | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+--------------------------------------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | users | range | PRIMARY,users_user_ids_key,userid,userid_2 | PRIMARY | 4       | NULL |  261 | Using where |
+----+-------------+-------+-------+--------------------------------------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

问题

1. 为什么在使用复合复合索引时检查更多行？
2. 如果查询中未指定 userid_2 索引，为什么该查询不使用？

Answer 1

该行数只是基于索引值分布的估计值。

你有两个选择：

1. 执行ANALYZE TABLE mytable 重新计算分布，然后重试describe
2. 不要担心无关紧要的事情...rows 无论如何只是一个估计