Question

我们有两个表 - 第一个是相对较大的（联系表）250k行，第二个是小的（用户表，＆lt; 10行）。在mysql 5.6版本上，我接下来解释结果：

EXPLAIN SELECT  
  o0_.id AS id_0,  
  o8_.first_name, 
  o8_.last_name 
FROM  
  contact o0_  
  LEFT JOIN user o8_ ON o0_.user_owner_id = o8_.id  
LIMIT  
  25 OFFSET 100

+----+-------------+-------+-------+---------------+----------------------+---------+------+--------+----------------------------------------------------+
| id | select_type | table | type  | possible_keys | key                  | key_len | ref  | rows   | Extra                                              |
+----+-------------+-------+-------+---------------+----------------------+---------+------+--------+----------------------------------------------------+
|  1 | SIMPLE      | o0_   | index | NULL          | IDX_403263ED9EB185F9 | 5       | NULL | 253030 | Using index                                        |
|  1 | SIMPLE      | o8_   | ALL   | PRIMARY       | NULL                 | NULL    | NULL |      5 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+---------------+----------------------+---------+------+--------+----------------------------------------------------+

套装中的2行（0,00秒）

当我使用force index进行连接时：

EXPLAIN SELECT  
  o0_.id AS id_0,  
  o8_.first_name, 
  o8_.last_name 
FROM  
  contact o0_  
  LEFT JOIN user o8_ force index for join(`PRIMARY`) ON o0_.user_owner_id = o8_.id  
LIMIT  
  25 OFFSET 100

或在用户表上的select子句（first_name，last_name）中显示的字段上添加索引：

alter table user add index(first_name, last_name);

解释结果更改：

    +----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
| id | select_type | table | type   | possible_keys | key                  | key_len | ref                     | rows   | Extra       |
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
|  1 | SIMPLE      | o0_   | index  | NULL          | IDX_403263ED9EB185F9 | 5       | NULL                    | 253030 | Using index |
|  1 | SIMPLE      | o8_   | eq_ref | PRIMARY       | PRIMARY              | 4       | o0_.user_owner_id |      1 | NULL        |
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
    2 rows in set (0,00 sec)

在mysql 5.5版本上，我有相同的解释结果，没有附加索引：

    +----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
| id | select_type | table | type   | possible_keys | key                  | key_len | ref                     | rows   | Extra       |
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
|  1 | SIMPLE      | o0_   | index  | NULL          | IDX_403263ED9EB185F9 | 5       | NULL                    | 255706 | Using index |
|  1 | SIMPLE      | o8_   | eq_ref | PRIMARY       | PRIMARY              | 4       | o0_.user_owner_id |      1 |             |
+----+-------------+-------+--------+---------------+----------------------+---------+-------------------------+--------+-------------+
2 rows in set (0.00 sec)

为什么我需要强制使用PRIMARY索引或在mysql 5.6版本上添加额外的索引？当连接小表时，其他选择会出现相同的行为。

Answer 1

如果你的表有这么少的行，那么进行全表扫描实际上可能比转到索引更快，找到记录然后再回到表中。如果除了查询中的3之外，用户表中还有其他字段，那么您可以考虑添加覆盖索引，但坦白地说，我认为这不会对查询的速度产生重大影响。

Mysql 5.6优化器不使用小表连接中的索引

1 个答案: