在Data Guard环境中可以利用数据库的闪回特性将备库以读写模式打开，用于执行对主数据库的某种测试工作，当测试完毕后，再将数据库闪回到之前的状态，作为备份继续提供保护服务。
1 在备库闪回数据库
1）取消备库托管恢复
SQL> alter database recover managed standby database cancel;
Database altered.2）设置备库的闪回恢复区
SQL> show parameter db_recover
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest string /u01/app/oracle/fast_recovery_
area
db_recovery_file_dest_size big integer 4560M3）启动闪回特性
SQL> startup mount
ORACLE instance started.
Total System Global Area 1644167168 bytes
Fixed Size 2925024 bytes
Variable Size 1073745440 bytes
Database Buffers 553648128 bytes
Redo Buffers 13848576 bytes
Database mounted.
SQL> alter database flashback on;
Database altered.
SQL> select flashback_on from v$database;
FLASHBACK_ON
------------------
YES4）创建一个保证还原点
SQL> create restore point before_opem_rw guarantee flashback database;
Restore point created.5）将主库向备库传输日志的目标目的地设置为延迟状态
SQL> show parameter log_archive_dest_2
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest_2 string SERVICE=shhai LGWR ASYNC
VALID_FOR=(ONLINE_LOGFILES,P
RIMARY_ROLE)
DB_UNIQUE_NAME=orcldg
SQL> show parameter log_archive_dest_state_2
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest_state_2 string ENABLE
SQL> alter system set log_archive_dest_state_2='defer';
System altered.6）激活物理备库，将备库转换为独立的数据库运行
SQL> alter database activate physical standby database;
Database altered.
SQL> alter database open;
Database altered.
SQL> select database_role,open_mode,flashback_on from v$database;
DATABASE_ROLE OPEN_MODE FLASHBACK_ON
---------------- -------------------- ------------------
PRIMARY READ WRITE YES7）在激活后的备库执行测试等各种操作
8）测试完毕，将数据库闪回到保证还原点
SQL> shutdown abort
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.
Total System Global Area 1644167168 bytes
Fixed Size 2925024 bytes
Variable Size 1073745440 bytes
Database Buffers 553648128 bytes
Redo Buffers 13848576 bytes
Database mounted.
SQL> flashback database to restore point before_opem_rw;
Flashback complete.
SQL> select database_role,open_mode,flashback_on from v$database;
DATABASE_ROLE OPEN_MODE FLASHBACK_ON
---------------- -------------------- ------------------
PRIMARY MOUNTED YES此时，数据库仍然使独立的数据库，只是状态回到了还原点。
9）将数据库转换为备库
SQL> alter database convert to physical standby ;
Database altered.
SQL> select database_role,open_mode,flashback_on from v$database;
DATABASE_ROLE OPEN_MODE FLASHBACK_ON
---------------- -------------------- ------------------
PHYSICAL STANDBY MOUNTED YES
SQL> alter database open;
Database altered.10）在主库启用向备库的日志传输
SQL> alter system set log_archive_dest_state_2='enable';
System altered.11）启动物理备库的日志应用
SQL> alter database recover managed standby database disconnect from session;
Database altered.
SQL> select database_role,open_mode,flashback_on from v$database;
DATABASE_ROLE OPEN_MODE FLASHBACK_ON
---------------- -------------------- ------------------
PHYSICAL STANDBY READ ONLY WITH APPLY YES上述演示展示了创建还原点将备库激活，以读写模式打开，进行一系列的测试，并利用闪回技术还原，之后再将其转为备库，整个过程比较繁琐，并且再备库以读写模式打开后，实际上是作为一个独立的数据库存在，但这个期间数据库停止接收了来自主库的重做日志，这在一定程度上就失去了对主库的保护，那么有没有一种方法，在使备库处于读写状态时，可以继续接收重做日志，只是不应用？
快照数据库（Snapshot standby database）可以做到，下面将进行演示。
2 快照备用数据库
1）取消备库托管恢复
SQL> alter database recover managed standby database cancel;
Database altered.2）将备库转换为快照数据库
SQL> alter database convert to snapshot standby;
Database altered.
SQL> select database_role,open_mode,flashback_on from v$database;
DATABASE_ROLE OPEN_MODE FLASHBACK_ON
---------------- -------------------- ------------------
SNAPSHOT STANDBY MOUNTED YES3）打开数据库
SQL> alter database open;
Database altered.
SQL> select database_role,open_mode,flashback_on from v$database;
DATABASE_ROLE OPEN_MODE FLASHBACK_ON
---------------- -------------------- ------------------
SNAPSHOT STANDBY READ WRITE YES4）在快照数据库执行一系列的测试等操作；
5）转换快照数据库为物理备库
SQL> shutdown abort
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.
Total System Global Area 1644167168 bytes
Fixed Size 2925024 bytes
Variable Size 1073745440 bytes
Database Buffers 553648128 bytes
Redo Buffers 13848576 bytes
Database mounted.
SQL> alter database convert to physical standby;
Database altered.
SQL> alter database open;
Database altered.6）备库开启托管恢复
SQL> alter database recover managed standby database disconnect from session;
Database altered.7）主库修改数据
SQL> update scott.emp set comm=1000 where empno=7369;
1 row updated.
SQL> commit;
Commit complete.
SQL> select * from scott.emp where empno=7369;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 1000 20
8）备库验证，一致。
SQL> select * from scott.emp where empno=7369;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 1000 20