主备库切换原因：
1. switch：用户主动切换；
2. failover：主库出现故障，强行切换；
switch切换过程：先主库再备库
主库
1. 查看switchover_status：select database_role, switchover_status from v$database;
如果switchover_status为：to standby 则：
ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY
否则用：ALTER DATABASE COMMIT TO SWITCHOVER TO PHYSICAL STANDBY WITH SESSION SHUTDOWN;
2. 重启数据库：
shutdown immediate;
startup nomount;
3. 以备库mount
alter database mount standby database;
备库
1. 应用redo：alter database recover managed standby database disconnect from session;
2. 查看switchover_status：select database_role, switchover_status from v$database;
如果switchover_status为：to primary 则：ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY
否则用：ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY WITH SESSION SHUTDOWN;
3. 重启数据库
shutdown immediate;
startup;
failover 主备库切换
1.. 停止日志应用
alter database recover managed standby database cancel;
2. 关闭standby日志传输
alter database recover managed standby database finish force;
3. 切换到primary
alter database commit to switchover to primary with session shutdown;
做这一步的时候，若存在gap，则会报ORA-16139:Switchover: Media recovery required - standby not in limbo 错误。
做测试的时候，若先起主库再起备库，且未等待备库相关日志传输完毕，就会出现这个问题。此时需要强制切换
alter database activate physical standby database;
4. 重启数据库到open状态
