在測試備份還原時，使用XtraBackup還原數據庫后，創建一個測試賬號時遇到了下面錯誤：
mysql> grant all on house.* to test@'192.168.%' identified by 'test1249';
ERROR 3009 (HY000): Column count of mysql.user is wrong. Expected 45, found 43. Created with MySQL 50620, now running 50721. Please use mysql_upgrade to fix this error.
mysql>
因為備份的數據庫版本為MySQL 5.6.20，目標數據庫的MySQL版本為5.7.21，因為兩個數據庫版本不一致，在還原過后，忘記升級數據庫（還原后沒有使用mysql_upgrade升級數據結構）。其實從錯誤信息也能看出一二，需要執行mysql_upgrade命令會檢查所有數據庫中的所有表與當前版本的MySQL服務器是否不兼容。 mysql_upgrade還會升級系統表，以便你可以使用一些新特性或功能。
# mysql_upgrade -u root -p
Enter password:
Checking if update is needed.
Checking server version.
Running queries to upgrade MySQL server.
Checking system database.
mysql.columns_priv OK
mysql.db OK
mysql.db_database_info OK
mysql.db_instance_info OK
mysql.db_server_info OK
mysql.engine_cost OK
......................................................
......................................................
Upgrade process completed successfully.
Checking if update is needed.
關於mysql_upgrade的官方介紹如下，具體細節參考“4.4.7 mysql_upgrade — Check and Upgrade MySQL Tables”：
mysql_upgrade examines all tables in all databases for incompatibilities with the current version of MySQL Server. mysql_upgrade also upgrades the system tables so that you can take advantage of new privileges or capabilities that might have been added.
If mysql_upgrade finds that a table has a possible incompatibility, it performs a table check and, if problems are found, attempts a table repair. If the table cannot be repaired, see Section 2.11.3, “Rebuilding or Repairing Tables or Indexes” for manual table repair strategies.
You should execute mysql_upgrade each time you upgrade MySQL.
參考資料：
https://dev.mysql.com/doc/refman/5.7/en/mysql-upgrade.html