从11.2.0.2开始，如果由于某种原因switchover没有成功，可以回滚switchover。

For physical standby databases in situations where an error occurred and it is not
possible to continue with the switchover, it might still be possible to revert the new
physical standby database back to the primary role by using the following steps. (This
functionality is available starting with Oracle Database 11 g Release 2 (11.2.0.2).)
1. Shut down and mount the new standby database (old primary).
2. Start Redo Apply on the new standby database.
3. Verify that the new standby database is ready to be switched back to the primary
role. Query the SWITCHOVER_STATUS column of the V$DATABASE view on the
new standby database. For example:
SQL> SELECT SWITCHOVER_STATUS FROM V$DATABASE;

SWITCHOVER_STATUS
—————–
TO_PRIMARY
1 row selected

A value of TO PRIMARY or SESSIONS ACTIVE indicates that the new standby
database is ready to be switched to the primary role. Continue to query this
column until the value returned is either TO PRIMARY or SESSIONS ACTIVE.
4. Issue the following statement to convert the new standby database back to the
primary role:
SQL> ALTER DATABASE COMMIT TO SWITCHOVER TO PRIMARY WITH SESSION SHUTDOWN;
If this statement is successful, the database will be running in the primary 
database role, and you do not need to perform any more steps.
If this statement is unsuccessful, then continue with Step 5.

一般来说执行到这个步骤，基本就OK了。此时primary database是mount状态，启动到OPEN后，验证同步OK，可以正常工作。

5. When the switchover to change the role from primary to physical standby was
initiated, a trace file was written in the log directory. This trace file contains the
SQL statements required to re-create the original primary control file. Locate the
trace file and extract the SQL statements into a temporary file. Execute the
temporary file from SQL*Plus. This will revert the new standby database back to
the primary role.

6. Shut down the original physical standby database.
7. Create a new standby control file. This is necessary to resynchronize the primary
database and physical standby database. Copy the physical standby control file to
the original physical standby system. Section 3.2.2 describes how to create a
physical standby control file.
8. Restart the original phys ical standby instance.
If this procedure is successful and archive gap management is enabled, the FAL
processes will start and re-archive any missing archived redo log files to the
physical standby database. Force a log switch on the primary database and
examine the alert logs on both the primary database and physical standby
database to ensure the archived redo log file sequence numbers are correct.
See Section 6.4.3.1 for information about archive gap management and
Appendix F for information about locating the trace files.
9. Try the switchover again.
At this point, the Data Guard configuration has been rolled back to its initial state,
and you can try the switchover operation again (after correcting any problems that
might have led to the initial unsuccessful switchover).