起因：
在工作中可能遇到这样一种情况：升级时发现备份的sql表列太宽，以至于出现以下错误：
Row size too large (> 8126). Changing some columns to TEXT or BLOB may help. In current row format, BLOB prefix of 0 bytes is stored inline.
这时候就要进行版本回退，将宽表修改后重新备份，然后再升级。
降级步骤：
0、查看已有yum源：
[root@decoder ~]# yum repolist all 已加载插件：fastestmirror, refresh-packagekit Loading mirror speeds from cached hostfile 仓库标识 仓库名称 状态 cdrom cdrom 启用: 6,575 repolist: 6,575
1、将yum源移除：
[root@localhost mysql]# yum list mariadb Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile Available Packages mariadb.x86_64 1:5.5.60-1.el7_5 local [root@localhost mysql]# cd /etc/yum.repos.d/ [root@localhost yum.repos.d]# ls backup local.repo MariaDB.repo nginx.repo [root@localhost yum.repos.d]# mv local.repo backup/ [root@localhost yum.repos.d]# mv nginx.repo backup/
2、清除yum源缓存后生成cache：
[root@localhost yum.repos.d]# yum clean all Loaded plugins: fastestmirror Cleaning repos: mariadb Cleaning up list of fastest mirrors Other repos take up 101 M of disk space (use --verbose for details) [root@localhost yum.repos.d]# yum makecache Loaded plugins: fastestmirror Determining fastest mirrors mariadb | 2.9 kB 00:00:00 (1/3): mariadb/primary_db | 54 kB 00:00:01 (2/3): mariadb/other_db | 9.2 kB 00:00:00 (3/3): mariadb/filelists_db | 281 kB 00:00:13 Metadata Cache Created
3、查看已安装的mariadb：
[root@localhost yum.repos.d]# yum list mariadb* Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile Installed Packages MariaDB-client.x86_64 10.3.15-1.el7.centos @mariadb MariaDB-common.x86_64 10.3.15-1.el7.centos @mariadb MariaDB-compat.x86_64 10.3.15-1.el7.centos @mariadb MariaDB-server.x86_64 10.3.15-1.el7.centos @mariadb Available Packages MariaDB-backup.x86_64 10.3.15-1.el7.centos mariadb MariaDB-backup-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-cassandra-engine.x86_64 10.3.15-1.el7.centos mariadb MariaDB-cassandra-engine-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-client-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-common-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-connect-engine.x86_64 10.3.15-1.el7.centos mariadb MariaDB-connect-engine-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-cracklib-password-check.x86_64 10.3.15-1.el7.centos mariadb MariaDB-cracklib-password-check-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-devel.x86_64 10.3.15-1.el7.centos mariadb MariaDB-devel-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-gssapi-server.x86_64 10.3.15-1.el7.centos mariadb MariaDB-gssapi-server-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-oqgraph-engine.x86_64 10.3.15-1.el7.centos mariadb MariaDB-oqgraph-engine-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-rocksdb-engine.x86_64 10.3.15-1.el7.centos mariadb MariaDB-rocksdb-engine-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-server-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-shared.x86_64 10.3.15-1.el7.centos mariadb MariaDB-shared-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-test.x86_64 10.3.15-1.el7.centos mariadb MariaDB-test-debuginfo.x86_64 10.3.15-1.el7.centos mariadb MariaDB-tokudb-engine.x86_64 10.3.15-1.el7.centos mariadb MariaDB-tokudb-engine-debuginfo.x86_64 10.3.15-1.el7.centos mariadb
4、卸载已安装的mariadb：
[root@localhost yum.repos.d]# yum remove MariaDB-client MariaDB-common.x86_64 MariaDB-compat.x86_64 MariaDB-server.x86_64 -y
5、查看是否卸载完全：
[root@localhost yum.repos.d]# systemctl status mariadb Unit mariadb.service could not be found. [root@localhost yum.repos.d]# systemctl status mysql Unit mysql.service could not be found. [root@localhost yum.repos.d]# systemctl status mysqld Unit mysqld.service could not be found.
6、安装低版本的yum源：
[root@localhost yum.repos.d]# mv MariaDB.repo backup/ [root@localhost yum.repos.d]# mv backup/local.repo .
7、更新yum源：
[root@localhost yum.repos.d]# yum clean all [root@localhost yum.repos.d]# yum makecache
8、安装低版本mariadb：
[root@localhost yum.repos.d]# yum install -y mariadb mariadb-server
9、新建用户：
MariaDB [(none)]> create user ajie@localhost identified by 'ajie123'; #创建一个数据库用户ajie，密码为ajie123 Query OK, 0 rows affected (0.00 sec)
10、普通用户提权：
MariaDB [(none)]> grant select,update,delete,insert ON mysql.* to ajie@localhost; #授予ajie用户查询、更新、删除、插入权限。 # mysql.*代表授予ajie的权限对数据库mysql下的所有表单有效 Query OK, 0 rows affected (0.00 sec)
11、取消普通用户授权：
MariaDB [(none)]> revoke select,update,delete,insert on mysql.* from ajie@localhost; #移除刚才授予ajie的权限 Query OK, 0 rows affected (0.00 sec) MariaDB [(none)]> show grants for ajie@localhost; #查看用户ajie的权限，已经没有了之前授予的权限 +-------------------------------------------------------------------------------------------------------------+ | Grants for ajie@localhost | +-------------------------------------------------------------------------------------------------------------+ | GRANT USAGE ON *.* TO 'ajie'@'localhost' IDENTIFIED BY PASSWORD '*A65FBC245EF7CC4346000B7CFA058E0D5A234219' | +-------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)