作者：徐耀荣

爱可生南区交付服务部 DBA 团队成员，次要负责MySQL故障解决以及相干技术支持。喜好电影，游戏，游览以及桌球。

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一、问题背景

有客户之前遇到一个 mysql8.0.21 实例中排序规定的报错，是在调用视图时抛出，报错信息如下：

ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation '='

二、问题模仿

mysql> show create table t1\G;*************************** 1. row ***************************       Table: t1Create Table: CREATE TABLE `t1` (  `name1` varchar(12) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci1 row in set (0.00 sec)mysql> show create table t2\G;*************************** 1. row ***************************       Table: t2Create Table: CREATE TABLE `t2` (  `name2` varchar(12) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=latin11 row in set (0.00 sec)mysql> CREATE VIEW t3 as select * from t1,t2 where `t1`.`name1`= `t2`.`name2`;Query OK, 0 rows affected (0.06 sec)mysql> select * from t3;ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation '='

三、问题剖析

通过查看视图定义，能够发现因为视图中波及到的两张表字符集不同，所以创立视图时 MySQL 会主动应用 convert 函数转换字符集。

mysql> show create view t3\G;*************************** 1. row ***************************                View: t3         Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `t3` AS select `t1`.`name1` AS `name1`,`t2`.`name2` AS `name2` from (`t1` join `t2`) where (`t1`.`name1` = convert(`t2`.`name2` using utf8mb4))character_set_client: utf8mb4collation_connection: utf8mb4_general_ci1 row in set (0.00 sec)

在 MySQL 8.0 中 utf8mb4 的默认排序规定为 utf8mb4_0900_ai_ci ，而在 t1 表的排序规定为 utf8mb4_general_ci ，那么咱们试着将排序规定相干的参数批改后再执行 SQL 看看，批改后的环境参数如下

mysql> show variables like '%collat%';+-------------------------------+--------------------+| Variable_name                 | Value              |+-------------------------------+--------------------+| collation_connection          | utf8mb4_general_ci || collation_database            | utf8mb4_bin        || collation_server              | utf8mb4_bin        || default_collation_for_utf8mb4 | utf8mb4_general_ci |+-------------------------------+--------------------+

再次执行 sql 发现还是会报一样的错。

mysql> select * from t1,t2 where `t1`.`name1`=convert(`t2`.`name2` using utf8mb4);ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation '='

通过 show collation 来查看 utf8mb4 字符集对应的默认排序规定，输入显示默认规定为 utf8mb4_general_ci ，并不是 utf8mb4_0900_ai_ci 。

mysql> show collation like '%utf8mb4%';+----------------------------+---------+-----+---------+----------+---------+---------------+| Collation                  | Charset | Id  | Default | Compiled | Sortlen | Pad_attribute |+----------------------------+---------+-----+---------+----------+---------+---------------+| utf8mb4_general_ci         | utf8mb4 |  45 | Yes     | Yes      |       1 | PAD SPACE     |+----------------------------+---------+-----+---------+----------+---------+---------------+mysql> show character set like '%utf8mb4%';+---------+---------------+--------------------+--------+| Charset | Description   | Default collation  | Maxlen |+---------+---------------+--------------------+--------+| utf8mb4 | UTF-8 Unicode | utf8mb4_general_ci |      4 |+---------+---------------+--------------------+--------+1 row in set (0.00 sec)

持续排查发现元数据中的字符集默认排序规定如下，默认规定为 utf8mb4_0900_ai_ci 。

mysql>  select * from INFORMATION_SCHEMA.COLLATIONS where IS_DEFAULT='Yes' and CHARACTER_SET_NAME='utf8mb4'\G;*************************** 1. row ***************************    COLLATION_NAME: utf8mb4_0900_ai_ciCHARACTER_SET_NAME: utf8mb4                ID: 255        IS_DEFAULT: Yes       IS_COMPILED: Yes           SORTLEN: 0     PAD_ATTRIBUTE: NO PAD1 row in set (0.00 sec)

查看参数发现，元数据信息中 utf8mb4 字符集默认排序规定是 utf8mb4_0900_ai_ci ，show collation/show character 输入的都是 utf8mb4_general_ci 。为什么 show 显示的后果和 INFORMATION_SCHEMA.COLLATIONS 表查到的信息还不一样呢？此处咱们暂且按下不表，咱们先看看官网文档中 convert 函数用法，其中有上面这段原文：

If you specify CHARACTER SET charset_name as just shown, the character set and collation of the result are charset_name and the default collation of charset_name. If you omit CHARACTER SET charset_name, the character set and collation of the result are defined by the character_set_connection and collation_connection system variables that determine the default connection character set and collation (see Section 10.4, “Connection Character Sets and Collations”).

从上述原文可知如果 convert 只指定了字符集，那么该后果的排序规定就是所指定字符集的默认规定，由之前的测试状况可知，convert 应用的是 INFORMATION_SCHEMA.COLLATIONS 的排序规定，而不是 default_collation_for_utf8mb4 指定的 utf8mb4_general_ci ，那咱们来看看 default_collation_for_utf8mb4 参数次要作用场景：

  1. SHOW COLLATION and SHOW CHARACTER SET.
  2. CREATE TABLE and ALTER TABLE having a CHARACTER SET utf8mb4 clause without a COLLATION clause, either for the table character set or for a column character set.
  3. CREATE DATABASE and ALTER DATABASE having a CHARACTER SET utf8mb4 clause without a COLLATION clause.
  4. Any statement containing a string literal of the form _utf8mb4'some text' without a COLLATE clause.

其中，第一点解释了为什么 show 查到的信息和元数据中信息不一样，default_collation_for_utf8mb4 批改后影响 show COLLATION and SHOW CHARACTER SET 的查问后果，并不会扭转字符集的默认排序规定，所以utf8mb4 的默认规定还是 utf8mb4_0900_ai_ci ，sql 执行仍然会报错。

将 convert 函数指定为 t1.name1 字段的排序规定后，sql 执行失常。

mysql> select * from t1,t2 where `t1`.`name1` = convert(`t2`.`name2` using utf8mb4) collate utf8mb4_general_ci;+-------+-------+| name1 | name2 |+-------+-------+| jack  | jack  |+-------+-------+1 row in set (0.00 sec)

另外，上面测试能够验证 default_collation_for_utf8mb4 的第四个场景。

mysql> select * from INFORMATION_SCHEMA.COLLATIONS where IS_DEFAULT='Yes' and CHARACTER_SET_NAME='utf8mb4'\G;*************************** 1. row ***************************    COLLATION_NAME: utf8mb4_0900_ai_ciCHARACTER_SET_NAME: utf8mb4                ID: 255        IS_DEFAULT: Yes       IS_COMPILED: Yes           SORTLEN: 0     PAD_ATTRIBUTE: NO PAD1 row in set (0.00 sec)mysql> show variables like '%default_collation%';+-------------------------------+--------------------+| Variable_name                 | Value              |+-------------------------------+--------------------+| default_collation_for_utf8mb4 | utf8mb4_general_ci |+-------------------------------+--------------------+1 row in set (0.01 sec)mysql> set @s1 = _utf8mb4 'jack',@s2 = _utf8mb4 'jack';Query OK, 0 rows affected (0.00 sec)mysql> SELECT @s1 = @s2;+-----------+| @s1 = @s2 |+-----------+|         1 |+-----------+1 row in set (0.00 sec)_utf8mb4申明的@s1和@s2排序规定是default_collation_for_utf8mb4参数值，为utf8mb4_general_cimysql> SELECT @s1 = CONVERT(@s2 USING utf8mb4);ERROR 1267 (HY000): Illegal mix of collations (utf8mb4_general_ci,IMPLICIT) and (utf8mb4_0900_ai_ci,IMPLICIT) for operation '='此时，通过CONVERT函数解决的@s2排序规定是utf8mb4_0900_ai_ci，所以会报错mysql> SELECT @s1 = CONVERT(@s2 USING utf8mb4) collate utf8mb4_general_ci;+-------------------------------------------------------------+| @s1 = CONVERT(@s2 USING utf8mb4) collate utf8mb4_general_ci |+-------------------------------------------------------------+|                                                                 1 |+-------------------------------------------------------------+1 row in set (0.00 sec)

四、问题总结

运维中为防止字符集引起的报错问题，有如下倡议可供参考：（具体参数值依据业务需要抉择）

  • 创立数据库实例时需指定参数 character_set_database（默认值：utf8mb4），character_set_server（默认值：utf8mb4）。
  • 当须要创立非默认字符集 database / table 时，须要在 sql 中明确指定字符集和排序规定。
  • 应用convert函数转换字符集时，当字段排序规定不是转换后字符集的默认排序规定，须要指定具体的排序规定。SELECT @s1 = CONVERT(@s2 USING utf8mb4) collate utf8mb4_general_ci
  • MySQL 5.7迁徙至MySQL 8.0时，需注意MySQL 5.7版本中utf8mb4默认排序规定是 utf8mb4_general_ci，MySQL 8.0中 utf8mb4 默认排序规定是 utf8mb4_0900_ai_ci 。

参考

https://dev.mysql.com/doc/ref...

https://dev.mysql.com/doc/ref...

https://dev.mysql.com/doc/ref...