primary库操作：
SQL> select name,open_mode,protection_mode,database_role from v$database;
NAME      OPEN_MODE  PROTECTION_MODE      DATABASE_ROLE
--------- ---------- -------------------- ----------------
YGH       READ WRITE MAXIMUM PERFORMANCE  PRIMARY
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 nomount
ORACLE instance started.
Total System Global Area  264241152 bytes
Fixed Size                  1218868 bytes
Variable Size              88082124 bytes
Database Buffers          171966464 bytes
Redo Buffers                2973696 bytes
SQL> alter database mount standby database;
Database altered.
SQL> alter database recover managed standby database disconnect from session;
Database altered.
SQL> select name,open_mode,protection_mode,database_role from v$database;
NAME      OPEN_MODE  PROTECTION_MODE      DATABASE_ROLE
--------- ---------- -------------------- ----------------
YGH       MOUNTED    MAXIMUM PERFORMANCE  PHYSICAL STANDBY
standby库操作：
SQL> select name,open_mode,protection_mode,database_role from v$database;
NAME      OPEN_MODE  PROTECTION_MODE      DATABASE_ROLE
--------- ---------- -------------------- ----------------
YGH       MOUNTED    MAXIMUM PERFORMANCE  PHYSICAL STANDBY
SQL> alter database commit to switchover to primary;
Database altered.
SQL> shutdown immediate
ORA-01109: database not open

Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area  264241152 bytes
Fixed Size                  1218868 bytes
Variable Size              88082124 bytes
Database Buffers          171966464 bytes
Redo Buffers                2973696 bytes
Database mounted.
Database opened.
SQL> select name,open_mode,protection_mode,database_role from v$database;
NAME      OPEN_MODE  PROTECTION_MODE      DATABASE_ROLE
--------- ---------- -------------------- ----------------
YGH       READ WRITE MAXIMUM PERFORMANCE  PRIMARY