为什么选择查询行为与单引号不同，在哪里条件且没有单引号

Question

我在使用单引号执行查询时没有观察到单行引号的不同行为。

用单引号（'121'）我很快得到了结果，查询使用的索引很好。但没有单引号，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

由于场id的定义是varchar，而不单引号的数据库将不得不作出一个隐式转换将铸造领域id的（或其他功能将其转换为varchar），其将阻止使用索引idx_1055

的阅读这篇文章吧：Type Conversion in Expression Evaluation