我们实际感受一下使用Flashback Database功能找回被TRUNCATE表的快感。

1.Oracle数据库版本信息
sys@secooler> select * from v$version;

BANNER
---------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0      Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

2.验证是否启用了flashback database
sys@secooler> select flashback_on,force_logging from v$database;

FLASHBACK_ON       FOR
------------------ ---
NO                 NO

这里显示没有开启闪回功能，同时force_logging也没有开启。

3.开启闪回功能和force logging
开启过程可以详细参考《【Flashback】启用Flashback闪回功能》http://space.itpub.net/?uid-519536-action-viewspace-itemid-590636
1）关闭数据库，启动到mount状态
sys@secooler> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.

NotConnected@> startup mount;
ORACLE instance started.

Total System Global Area 726540288 bytes
Fixed Size                  2216904 bytes
Variable Size             541068344 bytes
Database Buffers          176160768 bytes
Redo Buffers                7094272 bytes
Database mounted.

2）设置db_recovery_file_dest和db_recovery_file_dest_size参数
NotConnected@> alter system set db_recovery_file_dest='/oracle/ora11gR2/flash_recovery_area';

System altered.

NotConnected@> alter system set db_recovery_file_dest_size=4g scope=spfile;

System altered.

NotConnected@> show parameter db_recovery_file_dest

NAME                                     TYPE                 VALUE
---------------------------------------- -------------------- ------------------------------------------------------------
db_recovery_file_dest                    string               /oracle/ora11gR2/flash_recovery_area
db_recovery_file_dest_size               big integer          3882M

3）重启数据库到mount状态
NotConnected@> shutdown immediate;
ORA-01109: database not open

Database dismounted.
ORACLE instance shut down.
NotConnected@> startup mount;
ORACLE instance started.

Total System Global Area 726540288 bytes
Fixed Size                  2216904 bytes
Variable Size             541068344 bytes
Database Buffers          176160768 bytes
Redo Buffers                7094272 bytes
Database mounted.

4）开启闪回功能
（1）第一次尝试
NotConnected@> alter database flashback on;
alter database flashback on
*
ERROR at line 1:
ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38707: Media recovery is not enabled.

之所以会报此错误，是因为这个数据库没有运行在归档模式下。

（2）将数据库修改为归档模式
NotConnected@> alter database archivelog;

Database altered.

（3）再次尝试开启闪回功能，成功。
NotConnected@> alter database flashback on;

Database altered.

5）OPEN数据库，
NotConnected@> alter database open;

Database altered.

6）开启force logging功能
sys@secooler> alter database force logging;

Database altered.

7）最后的验证
sys@secooler> select flashback_on,force_logging from v$database;

FLASHBACK_ON       FOR
------------------ ---
YES                YES

此时，数据库已经启用闪回功能，同时force_logging也被开启。

4.在sec用户下创建一个T表并初始化一条数据
sys@secooler> conn sec/sec
Connected.
sec@secooler> select * from tab;

no rows selected

sec@secooler> create table t (x varchar2(8));

Table created.

sec@secooler> insert into t values ('secooler');

1 row created.

sec@secooler> commit;

Commit complete.

sec@secooler> select * from t;

X
--------
secooler

5.查看当前数据库可以闪回到的时间
sec@secooler> alter session set nls_date_format='yyyy-mm-dd hh24:mi:ss';

Session altered.

sec@secooler> select oldest_flashback_scn,oldest_flashback_time from v$flashback_database_log;

OLDEST_FLASHBACK_SCN OLDEST_FLASHBACK_TI
-------------------- -------------------
             8443212 2010-03-17 19:26:45

6.模拟表T被误TRUNCATE
sec@secooler> select sysdate from dual;

SYSDATE
-------------------
2010-03-17 19:47:14

sec@secooler> truncate table t;

Table truncated.

7.此时数据已被删除
sec@secooler> select * from t;

no rows selected

8.使用Flash Database功能
1）关闭数据库
sys@secooler> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.

2）开启数据库到mount exclusive状态
sys@secooler> startup mount exclusive;
ORACLE instance started.

Total System Global Area 726540288 bytes
Fixed Size                  2216904 bytes
Variable Size             541068344 bytes
Database Buffers          176160768 bytes
Redo Buffers                7094272 bytes
Database mounted.

3）恢复到truncate表T之前的时间，注意这里时间格式的写法
sys@secooler> Flashback Database to timestamp(to_date('2010-03-17 19:47:14','yyyy-mm-dd hh24:mi:ss'));

Flashback complete.

4）打开数据库到read only状态
sys@secooler> alter database open read only;

Database altered.

5）到sec用户中确认，数据是否被恢复
sys@secooler> conn sec/sec
Connected.
sec@secooler> select * from t;

X
--------
secooler

“激动人心”的消息：“我secooler又回来啦！”

此时也可以以“resetlogs”的方式开启数据库（alter database open resetlogs;），不推荐这样使用，因为这样打开数据库后，恢复到的时间点之后的数据将会丢失。

9.小结
我们真实完整的操练了一遍Flash Database功能，它可以完美、便捷和快速的恢复被误TRUNCATE的表。在“特定场合”下可以考虑使用这个优秀的功能。
“特定场合”：数据库的数据量不大，确认数据库确实可以恢复到指定的时间点。
受这个技术本身的

Good luck.

secooler
10.03.17

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【Flashback】使用闪回功能恢复被TRUNCATE表的内容