环境是RAC+RAC的DG的10.2.0.5环境
当时是把主备库打开，想启动MRP进程（实时应用），但是执行语句时出现1093错误：
SQL> alter database recover managed standby database ;
alter database recover managed standby database
*
ERROR at line 1:
ORA-01093: ALTER DATABASE CLOSE only permitted with no sessions connected
SQL> !oerr ora 1093
01093, 00000, "ALTER DATABASE CLOSE only permitted with no sessions connected"
// *Cause: There is at least one more session other than the current one
// logged into the instance. ALTER DATABASE CLOSE is not permitted.
// *Action: Find the other sessions and log them out and resubmit the command
上面说的很清楚：原因是多于一个的会话连接当前数据库实例。ALTER DATABASE CLOSE is not permitted;
解决办法是退出该会话；
字面意思没有很懂，当我查看主备库状态时发现备库处于只读状态：
SQL> select open_mode from gv$database;
OPEN_MODE
----------
READ ONLY
READ ONLY
哎，粗心误大事啊，启动实时应用备库需要在mount状态下的；
解决办法是，把备库启动到mount状态下，再执行：
SQL> shutdown immediate;
Database closed.
Database dismounted.
Oracle instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 327155712 bytes
Fixed Size 1273516 bytes
Variable Size 96469332 bytes
Database Buffers 226492416 bytes
Redo Buffers 2920448 bytes
Database mounted.
SQL> alter database recover managed standby database disconnect from session;
Database altered.
ok，问题解决，
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