mysql主鍵返回的結果少於復合復合索引

Question

我繼承了一個數據庫架構，其中存在一些設計問題

請注意，桌上還有另外9個鍵，我下面沒有列出，這些鍵看起來像

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

運行一條說明語句並強制其使用userid_2 ，它使用522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)

如果我不強迫它使用索引，那只是使用主鍵 ，它僅由userid組成，並且僅使用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) 

問題

1. 為什么使用復合復合索引時要檢查更多行？
2. 如果查詢中未指定userid_2索引，為什么不使用該查詢？

Answer 1

該行計數僅是基於索引值分布的估計。

您有兩種選擇：

1. 執行ANALYZE TABLE mytable以重新計算分布，然后重試describe
2. 不必擔心無關緊要的東西... rows僅僅是一個估計