查看角色、状态和保护模式

select database_role,open_mode from v$database;
主库处于read write状态，备库处于mounted状态

物理备库 应用日志

alter database recover managed standby database disconnect;

物理备库在处于日志应用状态时，是无法开库进行数据查询的。直接尝试开库会报错。

直接尝试open备库报错
select instance_name,status from v$instance;
alter database open;

物理备库取消日志应用：

alter database recover managed standby database cancel;

物理备库开库（read only）

alter database open;

select database_role,open_mode from v$database;

如果物理备库从read only状态回到日志应用工状态，要先关掉物理备库，再将库启到mount状态，最后重新应用日志。

shutdown immediate;

mount startup mount;

alter database recover managed standby database disconnect from session; 

这种日志应用方式无法实现备库上日志应用和数据查询同时进行。
如果想实现Active Standby，即日志实时应用 同时备库可以进行数据查询，备库上需使用Real-Time Apply来应用日志。

【Oracle官方文档中对Real-Time Apply介绍】

If the real-time apply feature is enabled, apply services can apply redo data as it is received, without waiting for the current standby redo log file to be archived. This results in faster switchover and failover times because the standby redo log files have been applied already to the standby database by the time the failover or switchover begins.

Use the ALTER DATABASE statement to enable the real-time apply feature, as follows:

• For physical standby databases, issue the ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE statement.

• For logical standby databases, issue the ALTER DATABASE START LOGICAL STANDBY APPLY IMMEDIATE statement.

Real-time apply requires a standby database that is configured with a standby redo log and that is in ARCHIVELOG mode.

Figure 7-1 shows a Data Guard configuration with a local destination and a standby destination. As the remote file server (RFS) process writes the redo data to standby redo log files on the standby database, apply services can recover redo from standby redo log files as they are being filled.