近期，协助一客户重置MySQL 8.0.11的root密码，碰到了奇怪的问题。
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操作系统版本：rhel 7.6
数据库版本：mysql 8.0.11
根据mysql 8.0官方文档说明，mysql的用户密码有限制，并且被硬编码，没有明确说明密码长度是多少。
Warning
The MySQL user name length limit is hardcoded in MySQL servers and clients, and trying to circumvent it by modifying the definitions of the tables in the
mysql database
does not work.
You should never alter the structure of tables in the
mysql database in any manner whatsoever except by means of the procedure that is described in
Section 2.11, “Upgrading MySQL”. Attempting to redefine MySQL's system tables in any other fashion results in undefined and unsupported behavior. The server is free to ignore rows that become malformed as a result of such modifications.
另外，mysql 8.0不再支持password()函数，因此，不能像mysql5.x那样通过update mysql.user表来修改用户密码。
但是，在mysql 8.0的root密码重置时遇到报错：
[mysql@mysql8~]$ mysql -uroot -p -S /home/mysql/mysql/tmp/mysql.sock
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 7
Server version: 8.0.11 MySQL Community Server - GPL
Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>
mysql> use mysql
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
--update mysql.user重置用户密码失败
mysql> update user set authentication_string = '123456' where user='root' and host='%' ;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> flush privileges;
Query OK, 0 rows affected (0.01 sec)
mysql> select user,host,authentication_string from mysql.user;
+------------------+-----------+------------------------------------------------------------------------+
| user | host | authentication_string |
+------------------+-----------+------------------------------------------------------------------------+
| root | % | 123456 |
| mysql.infoschema | localhost | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| mysql.session | localhost | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| mysql.sys | localhost | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
+------------------+-----------+------------------------------------------------------------------------+
5 rows in set (0.00 sec)
--password函数不被mysql8.0支持
mysql> update user set authentication_string =PASSWORD('123456') where user='root' and host='%' ;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '('
123456') where user='root' and host='%'' at line 1
mysql>
--奇怪的是使用alter命令重置root密码长度为6位时报错，但是不是语法错误
mysql> alter user 'root'@'%' identified by '123456'; ERROR 1396 (HY000): Operation ALTER USER failed for 'root'@'%' --但是，将root密码长度增加到8位时获得成功
mysql> ALTER USER root IDENTIFIED WITH mysql_native_password BY '123456789';
Query OK, 0 rows affected (0.01 sec)
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
mysql> select user,host,authentication_string from mysql.user;
+------------------+-----------+------------------------------------------------------------------------+
| user | host | authentication_string |
+------------------+-----------+------------------------------------------------------------------------+
| root | % | *CC67043C7BCFF5EEA5566BD9B1F3C74FD9A5CF5D |
| mysql.infoschema | localhost | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| mysql.session | localhost | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
| mysql.sys | localhost | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE |
+------------------+-----------+------------------------------------------------------------------------+
5 rows in set (0.00 sec)
但是，我本地的centos 7.6上的mysql 8.0重置root密码缺没有密码长度限制，两套库用的是相同的参数文件。
官方关于用户密码长度没有特别说明，网上也没有类似密码长度问题的资料，很奇怪！
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