升级到Ubuntu 18.04后，需要重新安装Mysql。我在升级之前做了“ sudo service mysqld stop”，然后清理步骤必须删除了mysql服务器。找不到时，我做了：
sudo apt-get remove mysql-server
sudo apt-get install mysql-server
...
Setting up mysql-server-5.7 (5.7.22-0ubuntu18.04.1) ...
update-alternatives: using /etc/mysql/mysql.cnf to provide /etc/mysql/my.cnf (my.cnf) in auto mode
mysqld: [Warning] World-writable config file '/etc/mysql/my.cnf' is ignored.
Please enable --log-error option or set appropriate redirections for standard output and/or standard error in daemon mode.
Warning: Unable to start the server. Please restart MySQL and run mysql_upgrade to ensure the database is ready for use.
mysqld: [Warning] World-writable config file '/etc/mysql/my.cnf' is ignored.
Please enable --log-error option or set appropriate redirections for standard output and/or standard error in daemon mode.
Warning: Unable to start the server.
...
然后我运行了“ systemctl status mysql.service”，报告了此信息：
Jul 19 11:21:49 steve-VAIO mysqld[6106]: 2018-07-19T18:21:49.828124Z 0 [ERROR] InnoDB: Operating system error number 2 in a file oper
Jul 19 11:21:49 steve-VAIO mysqld[6106]: 2018-07-19T18:21:49.828132Z 0 [ERROR] InnoDB: The error means the system cannot find the pat
Jul 19 11:21:49 steve-VAIO mysqld[6106]: 2018-07-19T18:21:49.828140Z 0 [ERROR] InnoDB: Could not find a valid tablespace file for `my
Jul 19 11:21:49 steve-VAIO mysqld[6106]: 2018-07-19T18:21:49.828199Z 0 [Warning] InnoDB: Cannot calculate statistics for table `mysql
Jul 19 11:21:49 steve-VAIO mysqld[6106]: 2018-07-19T18:21:49.828232Z 0 [ERROR] Can't open and lock privilege tables: Tablespace is mi
Jul 19 11:21:49 steve-VAIO mysqld[6106]: 2018-07-19T18:21:49.846964Z 0 [Note] InnoDB: Buffer pool(s) load completed at 180719 11:21:4
Jul 19 11:21:49 steve-VAIO mysqld[6106]: 2018-07-19T18:21:49.934673Z 0 [Note] Event Scheduler: Loaded 0 events
Jul 19 11:21:49 steve-VAIO mysqld[6106]: 2018-07-19T18:21:49.934950Z 0 [Note] /usr/sbin/mysqld: ready for connections.
Jul 19 11:21:49 steve-VAIO mysqld[6106]: Version: '5.7.22-0ubuntu18.04.1' socket: '/var/run/mysqld/mysqld.sock' port: 3306 (Ubuntu
Jul 19 11:21:49 steve-VAIO systemd[1]: Started MySQL Community Server.
mysqld正在运行，并且我的数据库可用，但是我敢肯定，它在更新时会再次叮咬我。 phpMyAdmin显示mysql数据库，看起来很健康。在数据库列表的顶部是一个名为“＃mysql50＃mysql.bkp”的数据库。
有什么清理建议吗？
在建议运行mysql_upgrade后，我做了：
steve@steve-VAIO:~/workspace/JavascriptCourse$ mysql_upgrade -u root -p
mysql_upgrade: [Warning] World-writable config file '/etc/mysql/my.cnf' is ignored.
Enter password:
Checking if update is needed.
Checking server version.
Running queries to upgrade MySQL server.
mysql_upgrade: [ERROR] 1812: Tablespace is missing for table `mysql`.`plugin`.
所以我将寻找该表的表空间的修补程序。