MySQL索引失效之隐式转换的问题

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_num\g
*************************** 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_cou\g
*************************** 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 |    1 |   100.00 | null                                                            |
|  1 | simple      | c     | null       | index | null          | ix_c2 | 32      | null  |    5 |   100.00 | using where; using index; using join buffer (block nested loop) |
+----+-------------+-------+------------+-------+---------------+-------+---------+-------+------+----------+-----------------------------------------------------------------+
# 执行计划分析:
# 1.操作的n表，使用了ix_c1定位到一行数据
# 2.从n表定位到的行数据，拿到c2字段去操作c表，t_num称为驱动表，t_cou称为被驱动表
# 3.同样的n.c2=c.c2，会将c.c2的字符集进行转换，即被驱动表的索引字段上加函数操作，索引失效
# 4.bnl，表join时，驱动表数据读入join buffer，被驱动表连接字段无索引则全表扫，每取一行和join buffer数据对比判断，作为结果集返回

如上，分别对t_num、 t_cou作为驱动表和被驱动表的执行计划分析，总结：

utf8mb4和utf8两种不同字符集（编码）类型的字符串在做比较时，mysql会先把 utf8 字符串转成 utf8mb4 字符集，再做比较。为什么？字符集 utf8mb4 是 utf8 的超集，再做隐式自动类型转换时，为了避免数据在转换过程中由于截断导致数据错误，会“按数据长度增加的方向”进行转换。

表连接过程中，被驱动表的索引字段上加函数操作，会导致对被驱动表做全表扫描。

优化手法：

修改统一join字段的字符集

对驱动表下手，将连接字段的字符集转换成被驱动表连接字段的字符集

root@test 18:09 > explain select n.* from t_num n join t_cou c  on convert(n.c2 using utf8) = 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_c1 | 4       | const |    1 |   100.00 | null                     |
|  1 | simple      | c     | null       | ref  | ix_c2         | ix_c2 | 32      | func  |    1 |   100.00 | using where; using index |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+--------------------------+

二、类型转换

1、字符串转整型

# 字符开头的一律为0
root@test 18:44 > select convert("abc", unsigned integer);
+----------------------------------+
| convert("abc", unsigned integer) |
+----------------------------------+
|                                0 |
+----------------------------------+
# 'abc' = 0是成立的，因此查询时等号右边使用对应的类型很重要，0匹配出字段字符开头数据，'0'只匹配0
root@test 18:44 > select 'abc' = 0;
+-----------+
| 'abc' = 0 |
+-----------+
|         1 |
+-----------+
​
# 数字开头的，直接截取到第一个不是字符的位置
root@test 18:45 > select convert("123abc", unsigned integer);
+-------------------------------------+
| convert("123abc", unsigned integer) |
+-------------------------------------+
|                                 123 |
+-------------------------------------+

2、时间类型转换

root@test 19:11 > show create table time_demo\g
*************************** 1. row ***************************
       table: time_demo
create table: create table `time_demo` (
  `id` int(11) not null auto_increment,
  `c1` datetime default null,
  `c2` date default null,
  primary key (`id`),
  key `ix_c1` (`c1`)
) engine=innodb auto_increment=5 default charset=utf8mb4
​
root@test 19:15 > select count(*) from time_demo;
+----------+
| count(*) |
+----------+
|       11 |
+----------+
​
root@test 19:16 > select * from time_demo limit 4;
+----+---------------------+------------+
| id | c1                  | c2         |
+----+---------------------+------------+
|  1 | 2022-01-08 00:01:01 | 2022-01-08 |
|  2 | 2022-01-06 23:01:01 | 2022-01-06 |
|  3 | 2022-01-06 00:00:00 | 2022-01-06 |
|  4 | 2022-01-08 00:00:00 | 2022-01-08 |
+----+---------------------+------------+
​
# 1.date转datetime：末尾追加 00:00:00
root@test 19:11 > select * from time_demo where c1 between "2022-01-06" and "2022-01-08";
+----+---------------------+------------+
| id | c1                  | c2         |
+----+---------------------+------------+
|  2 | 2022-01-06 23:01:01 | 2022-01-06 |
|  3 | 2022-01-06 00:00:00 | 2022-01-06 |
|  4 | 2022-01-08 00:00:00 | 2022-01-08 |
+----+---------------------+------------+
# 结果分析：c1是datetime类型，进行比较时，between and中的date类型会转换成datetime
# 即 where c1 between "2022-01-06 00:00:00" and "2022-01-08 00:00:00";
# 同 where c1 >= "2022-01-06 00:00:00" and c1 <= "2022-01-08 00:00:00";
root@test 19:42 > explain select * from time_demo where c1 between "2022-01-06" and "2022-01-08";
+----+-------------+-----------+------------+-------+---------------+-------+---------+------+------+----------+-----------------------+
| id | select_type | table     | partitions | type  | possible_keys | key   | key_len | ref  | rows | filtered | extra                 |
+----+-------------+-----------+------------+-------+---------------+-------+---------+------+------+----------+-----------------------+
|  1 | simple      | time_demo | null       | range | ix_c1         | ix_c1 | 6       | null |    3 |   100.00 | using index condition |
+----+-------------+-----------+------------+-------+---------------+-------+---------+------+------+----------+-----------------------+
# 格式化date转datetime
root@test 19:23 > select date_format("2022-01-08","%y-%m-%d %h:%i:%s");
+-----------------------------------------------+
| date_format("2022-01-08","%y-%m-%d %h:%i:%s") |
+-----------------------------------------------+
| 2022-01-06 00:00:00                           |
+-----------------------------------------------+
​
# 2.datetime转date：直接截取date部分
root@test 19:47 > select date(c1) from time_demo limit 1;
+------------+
| date(c1)   |
+------------+
| 2022-01-06 |
+------------+
​
# 3.date转time，没有意义，直接变成 00:00:00

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