在做switchover的时候，备库打开报如下报错
@>alter database open read only;
alter database open read only
*
ERROR at line 1:
ORA-10456: cannot open standby database; media recovery session may be in progress

这个的原因如下 ： 恢复模式不可打开数据库   所以先要cancel日志应用，在开启日志应用
操作流程如下:
1)alter database recover managed standby database cancel;
2)alter database open read only;
3)alter database recover managed standby database using current logfile disconnect from session;

测试情况如下：
@>archive log list
Database log mode              Archive Mode
Automatic archival             Enabled
Archive destination            +DATA
Oldest online log sequence     38
Next log sequence to archive   0
Current log sequence           39
@>select name,open_mode from v$database;

NAME               OPEN_MODE
------------------ ----------------------------------------
RAC                MOUNTED

@>alter database recover managed standby database cancel;

Database altered.

@>alter database open read only;

Database altered.

@>alter database recover managed standby database using current logfile disconnect from session;

Database altered.

@>select name,open_mode from v$database;

NAME               OPEN_MODE
------------------ ----------------------------------------
RAC                READ ONLY WITH APPLY