紧急情况下将dataguard备库切换为可读写数据库

不到万不得已，千万别使用该方法，否则又要重新搭建DG。
1.shutdown immediate;
2.stratup mount;
3.alter system set dg_broker_start=false scope=spfile;
4.alter database recover managed standby database finish force;
5.alter database commit to switchover to primary;
6.不用再次关闭数据库，直接：alter database open;
7.查看数据库状态：select open_mode, database_role from v$database