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mysql varchar int 123 走索引吗？

2020年03月20日  | 移动技术网IT编程  | 我要评论

结论：

当mysql中字段为int类型时，搜索条件where num='111' 与where num=111都可以使用该字段的索引。
当mysql中字段为varchar类型时，搜索条件where num='111' 可以使用索引，where num=111 不可以使用索引

验证过程：

    建表语句：

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create table `gyl` (
  `id` int(11) not null auto_increment,
  `str` varchar(255) not null,
  `num` int(11) not null default '0',
  `obj` varchar(255) default null,
  primary key (`id`),
  key `str_x` (`str`),
  key `num_x` (`num`)
) engine=innodb  default charset=utf8;

　　向表中使用自复制语句插入数据

            insert into gyl (`str`,`num`)values(123123,'12313');

            insert into gyl (`str`,`num`) select `str`,`num` from gyl;

更改数据 update gyl set num=id,str=id

结果：

 

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mysql> explain
select * from gyl where str=123123 limit 1;
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | extra       |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
|  1 | simple      | gyl   | all  | str_x         | null | null    | null | 262756 | using where |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
1 row in set
mysql> explain select * from gyl where str='123123' limit 1;
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------------+
| id | select_type | table | type | possible_keys | key   | key_len | ref   | rows   | extra       |
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------------+
|  1 | simple      | gyl   | ref  | str_x         | str_x | 257     | const | 131378 | using where |
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------------+
1 row in set
 
mysql> explain select * from gyl where num='12313' limit 1;;
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+
| id | select_type | table | type | possible_keys | key   | key_len | ref   | rows   | extra |
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+
|  1 | simple      | gyl   | ref  | num_x         | num_x | 4       | const | 131378 |       |
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+
1 row in set
 
1065 - query was empty
mysql> explain select * from gyl where num=12313 limit 1;
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+
| id | select_type | table | type | possible_keys | key   | key_len | ref   | rows   | extra |
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+
|  1 | simple      | gyl   | ref  | num_x         | num_x | 4       | const | 131378 |       |
+----+-------------+-------+------+---------------+-------+---------+-------+--------+-------+
1 row in set

字段类型不同造成的隐式转换，导致索引失效

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