DG物理standby主备库切换（swithover）

swithover一般用于数据库升级或硬件升级，这时只需要较短时间中断数据库访问，主备库的角色切换完成后，即可打开primay角色的备库来提供数据库访问。

主库上面：

SQL> select database_role,switchover_status from v$database;

DATABASE_ROLE    SWITCHOVER_STATUS
---------------- --------------------
PRIMARY          SESSIONS ACTIVE

SQL> alter database commit to switchover to physical standby with session shutdown;

Database altered.

SQL> shutdown immediate;
ORA-01507: database not mounted

ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.

Total System Global Area 1073741824 bytes
Fixed Size                  1223540 bytes
Variable Size             864027788 bytes
Database Buffers          205520896 bytes
Redo Buffers                2969600 bytes
Database mounted.
SQL> alter database recover managed standby database disconnect from session;

Database altered.

SQL> select database_role,switchover_status from v$database;

DATABASE_ROLE    SWITCHOVER_STATUS
---------------- --------------------
PHYSICAL STANDBY TO PRIMARY

SQL> select max(sequence#) from v$archived_log;

MAX(SEQUENCE#)
--------------
            39

备库上面操作：

SQL> select database_role,switchover_status from v$database;

DATABASE_ROLE    SWITCHOVER_STATUS
---------------- --------------------
PHYSICAL STANDBY TO PRIMARY

如果swithover_status状态为recovery needed或swithover latent,需要应用完所有归档日志后才能切换（alter database recover managed standby database disconnect from session;)

SQL> alter database commit to switchover to primary with session shutdown;

Database altered.

SQL> alter database open;

Database altered.

SQL> select database_role,switchover_status from v$database;

DATABASE_ROLE    SWITCHOVER_STATUS
---------------- --------------------
PRIMARY          SESSIONS ACTIVE

SQL> select max(sequence#) from v$archived_log;

MAX(SEQUENCE#)
--------------
            39

SQL> alter system switch logfile;

进主库：

SQL> select max(sequence#) from v$archived_log;

MAX(SEQUENCE#)
--------------
            40