MySQL索引失效之隐式转换的问题 MySQL索引失效之隐式转换的问题

常见索引失效：
一、常见索引失效场景

1、条件字段函数操作
2、条件字段运算操作
3、隐式类型转换
4、隐式字符编码转换

二、类型转换

1、字符串转整型
2、时间类型转换

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_num\G*************************** 1. row ***************************Table: t_numCreate 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=utf8root@test 14:02 > show create table t_cou\G*************************** 1. row ***************************Table: t_couCreate 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 con 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 con 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：
【MySQL索引失效之隐式转换的问题】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 con 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0root@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0root@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_demoCreate 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 00:00:00root@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datetimeroot@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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