Question

我在使用单引号和没有单引号执行查询时观察到了不同的行为。

使用单引号（'121'）我快速得到了结果，查询正在使用索引。但是没有单引号，查询不使用索引，响应时间很长。

mysql> explain  select * from tbl_n1 where id =121;
+----+-------------+----------------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table                | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+----------------------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | tbl_n1               | ALL  | n1_id      | NULL | NULL    | NULL | 286929 | Using where |
+----+-------------+----------------------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)

mysql> explain  select * from tbl_n1 where id ='121';
+----+-------------+----------------------+------+---------------+----------+---------+-------+------+-------------+
| id | select_type | table                | type | possible_keys | key      | key_len | ref   | rows | Extra       |
+----+-------------+----------------------+------+---------------+----------+---------+-------+------+-------------+
|  1 | SIMPLE      | tbl_n1               | ref  | n1_id       | n1_id  | 53      | const |   29 | Using where |
+----+-------------+----------------------+------+---------------+----------+---------+-------+------+-------------+
1 row in set (0.00 sec)


show create table tbl_n1\G
....
....
 `id` varchar(10)  DEFAULT NULL,
....
....
....

Answer 1

由于字段id的定义为varchar，如果没有单引号，数据库必须对字段id进行隐式转换强制转换（或将其转换为varchar的任何函数），这将阻止使用索引idx_1055

阅读这篇文章：Type Conversion in Expression Evaluation

为什么select where查询行为与where where where条件不同而没有单引号

1 个答案: