MySQL索引失效之隐式转换

常见索引失效：

1. 条件索引字段"不干净"：函数操作、运算操作

2. 隐式类型转换：字符串转数值；其他类型转换

3. 隐式字符编码转换：按字符编码数据长度大的方向转换，避免数据截取

一、常见索引失效场景

root@test 10:50 > show create table t_numG
*************************** 1. row ***************************
       Table: t_num
Create Table: CREATE TABLE `t_num` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `c1` int(11) NOT NULL,
  `c2` varchar(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `ix_c1` (`c1`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4
​
root@test 10:51 > select * from t_num;
+----+----+----+
| id | c1 | c2 |
+----+----+----+
|  1 | -2 | -2 |
|  2 | -1 | -1 |
|  3 |  0 |  0 |
|  4 |  1 |  1 |
|  5 |  2 |  2 |
+----+----+----+
​
# 在c1字段上加上索引
root@test 10:52 > alter table t_num add index ix_c1(c1);
​
# 标准使用情况下，索引有效
root@test 10:55 > explain select * from t_num where c1 = -1;
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | t_num | NULL       | ref  | ix_c1         | ix_c1 | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------+

1、条件字段函数操作

# 在where中c1上加上abs()绝对值函数，可以看到type=ALL，全表扫描，在Server层进行绝对值处理后进行比较
root@test 10:58 > explain select * from t_num where abs(c1) = 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t_num | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    5 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

如上，对索引字段做函数操作，即where条件列上不干净时，可能会破坏索引值的有序性（按照c1的值有序组织索引树），因此优化器就决定放弃走索引树搜索功能。

但是，条件字段函数操作下，也并非完全的走全表扫描，优化器并非完全的放弃该字段索引。

# 选择查询的数据，只有id和c1字段，可以看到type=index，使用到了ix_c1索引
root@test 10:59 > explain select id,c1 from t_num where abs(c1) = 1;
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key   | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | t_num | NULL       | index | NULL          | ix_c1 | 4       | NULL |    5 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-------+---------+------+------+----------+--------------------------+

如上，由于ix_c1索引树是根节点c1和叶子节点id构造的，虽然因为c1上的函数操作导致放弃索引定位，但优化器可以选择遍历该索引树，使用覆盖索引（Using index），无需回表，将所需的id和c1数据返回Server层后进行后续的abs()和where过滤。

2、条件字段运算操作

# where条件里，对c1进行运算操作
root@test 11:03 > explain select * from t_num where c1 + 1 = 2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t_num | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    5 |   100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

如上，虽然“+1”的操作并没有破坏c1索引的有序性，但优化器仍然没有使用该索引快速定位。因此，等号左边，注意优化掉索引字段上的运算操作。

3、隐式类型转换

# 在c2字段上加上索引
root@test 12:30 > alter table t_num add index ix_c2(c2);
​
# 标准使用情况下（注：c2是varchar类型的），索引有效
root@test 12:30 > explain select * from t_num where c2 = "2";
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | t_num | NULL       | ref  | ix_c2         | ix_c2 | 42      | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------+
​
# 去掉等号右边值的引号，即字符串和数值进行比较，索引失效
root@test 12:30 > explain select * from t_num where c2 = 2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t_num | NULL       | ALL  | ix_c2         | NULL | NULL    | NULL |    5 |    20.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

如上，c2字段是varchar类型，是字符串和数值的比较，此时，MySQL是将字符串转换成数字，即此处的c2被CAST(c2 AS signed int)，这就相当于对条件字段做了函数操作，优化器放弃走树索引定位。

4、隐式字符编码转换

# 创建一个t_cou表，表结构基本和前面的t_num相同，唯一不同的设置是表字符集CHARSET=utf8
root@test 14:02 > show create table t_couG
*************************** 1. row ***************************
       Table: t_cou
Create Table: CREATE TABLE `t_cou` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `c1` int(11) NOT NULL,
  `c2` varchar(10) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `ix_c1` (`c1`),
  KEY `ix_c2` (`c2`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8
​
root@test 14:02 > insert into t_cou select * from t_num;
​
# join表，t_num和t_cou通过c2字段进行关联查询
root@test 14:03 > select n.* from t_num n
    -> join t_cou c
    -> on n.c2 = c.c2
    -> where n.c1 = 1;
+----+----+----+
| id | c1 | c2 |
+----+----+----+
|  4 |  1 | 1  |
+----+----+----+
​
root@test 14:23 > explain select n.* from t_num n join t_cou c  on n.c2 = c.c2 where c.c1 = 1;
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | c     | NULL       | ref  | ix_c1         | ix_c1 | 4       | const |    1 |   100.00 | NULL                  |
|  1 | SIMPLE      | n     | NULL       | ref  | ix_c2         | ix_c2 | 42      | func  |    1 |   100.00 | Using index condition |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
# 执行计划分析:
# 1.操作的c表，使用了ix_c1定位到一行数据
# 2.从c表定位到的行数据，拿到c2字段去操作n表，t_cou称为驱动表，t_num称为被驱动表
# 3.ref=func说明使用了函数操作，指的是n.c2=CONVERT(c.c2 USING utf8mb4)
# 4.同时Using index condition，ix_c2读取查询时，使用被下推的条件过滤，满足条件的才回表
​
root@test 14:23 > explain select n.* from t_num n join t_cou c  on n.c2 = c.c2 where n.c1 = 1;
+----+-------------+-------+------------+-------+---------------+-------+---------+-------+------+----------+-----------------------------------------------------------------+
| id | select_type | table | partitions | type  | possible_keys | key   | key_len | ref   | rows | filtered | Extra                                                           |
+----+-------------+-------+------------+-------+---------------+-------+---------+-------+------+----------+-----------------------------------------------------------------+
|  1 | SIMPLE      | n     | NULL       | ref   | ix_c1,ix_c2   | ix_c1 | 4       | const |