开发同学，这么写不能走索引

数据库干货铺 2020-04-01

303

一个字符类型的、一个int类型的，查询的时候到底会不会走索引，其实很多工作了几年的开发人员有时也会晕，下面就用具体事例来测试一下。

准备工作

先准备2张表，以备后续测试使用。

表1：创建表test1,总共3列，其中id 是主键（int）,c_no 为int型，且有索引，c_2为普通字段

/*创建表test1 */
create table  test1(id int primary key,c_no  int ,c_2 varchar(1),key c_no(c_no));


/* 插入一些测试数据 */
insert  into test1 values(1,1,'0'),(2,2,'1'),(3,4,'1'),(4,6,'0'),(5,7,''1),(6,11,'2'),(7,5,'3'),(8,100,'0'),(9,30,'1'),(10,50,'0');

表2：创建表test1,总共3列，其中id 是主键（int）,c_no 为字符型，且有索引，c_2为普通字段

/* 创建test2 */
create table  test2(id int primary key  auto_increment,c_no  varchar(11) ,c2 varchar(2),key c_no(c_no));


/* 插入一些测试数据 */
 insert  into test2 values(1,'5','1'),(4,'100','0'),(3,'30','1'),(10,'500','0'),(11,'20','0'),(12,'20a','0'),(15,'020b','1');

两张表的差异是c_no的字段类型不同。

等值查询测试

2.1 测试test1

test1.c_no字段为int类型，下面分别用整型和字符串进行比较，查看是否走索引。对应的执行计划如下：

mysql> explain  select *  from test1 where c_no='100';
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | test1 | NULL       | ref  | c_no          | c_no | 5       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
/* 均走了索引 */
mysql> explain  select *  from test1 where c_no=100;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | test1 | NULL       | ref  | c_no          | c_no | 5       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

可见，两种方式均走索引了，且走的是c_no的索引，类型为ref为const（常量的等值查询），扫行数为1

也就是说当表中的字段类型为整型时，无论查询用字符串类型的数字还是int类型的数字均能走索引。其中用int类型的值查询能走索引可以容易理解，那么，字符型的为什么能走？其实这里的字符类型做了隐式转化，上例中就相当于

mysql> explain  select *  from test1 where c_no=CAST('100' as UNSIGNED);
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | test1 | NULL       | ref  | c_no          | c_no | 5       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

2.2 测试 test2 表

以同样的方式测试一下test2的查询情况

先测试正常情况下字符型与字符型比较，结果可想而知，可以走索引，如下：

mysql> explain  select *  from test2 where c_no='100';
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | test2 | NULL       | ref  | c_no          | c_no | 47      | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

而，如果是整型再查呢？结果如下（很遗憾，不能走索引了）

mysql> explain  select *  from test2 where c_no=100;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | test2 | NULL       | ALL  | c_no          | NULL | NULL    | NULL |    7 |    14.29 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)

也就是说，表中字段为字符类型的时候，查询的值为整型时，无法走索引了。

那这句相当于如下情况：

mysql> explain  select *  from test2 where cast(c_no  as  unsigned)=100;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | test2 | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    7 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

也就是c_no做了隐式转化。因为如果是100做了影视转化，那么结果应该是可以走索引，例如：

mysql> explain  select *  from test2 where c_no=cast(100 as char);
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | test2 | NULL       | ref  | c_no          | c_no | 47      | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

由此，我们也应证了如果字段做了函数计算后，该列上即使有索引也无法使用（MySQL8.0之前的版本）。

2.3 进一步测试

其实针对test2表还可以测试一点，进一步证明是c_no字段做了隐式转化，例如：

mysql> select  * from test2 where c_no=20;
+----+------+------+
| id | c_no | c2   |
+----+------+------+
| 11 | 20   | 0    |
| 12 | 20a  | 0    |
| 15 | 020b | 1    |
+----+------+------+
3 rows in set, 2 warnings (0.00 sec)

另外，看到了2个警告，内容如下：

mysql> show warnings;
+---------+------+------------------------------------------+
| Level   | Code | Message                                  |
+---------+------+------------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: '20a'  |
| Warning | 1292 | Truncated incorrect DOUBLE value: '020b' |
+---------+------+------------------------------------------+
2 rows in set (0.00 sec)