要求如下：
1.开启闪回数据库功能；
2.使用闪回数据库恢复HR用户的数据；


操作如下：
（1） 启用闪回数据库功能；
SYS@GMDBA> select FLASHBACK_ON from v$database;
FLASHBACK_ON
------------------
NO
SYS@GMDBA> show parameter db_recovery_
NAME TYPE VALUE
------------------------------- -------- ----------------------------------
db_recovery_file_dest string /u01/app/oracle/fast_recovery_area
db_recovery_file_dest_size big integer 4977M
SYS@GMDBA> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SYS@GMDBA> startup mount
ORACLE instance started.
Total System Global Area 422670336 bytes
Fixed Size 1345380 bytes
Variable Size 327157916 bytes
Database Buffers 88080384 bytes
Redo Buffers 6086656 bytes
Database mounted.
SYS@GMDBA> alter database flashback on;
Database altered.


SYS@GMDBA>
SYS@GMDBA> alter database open;
Database altered.
SYS@GMDBA> select FLASHBACK_ON from v$database;
FLASHBACK_ON
------------------
YES
（2） 确实闪回需要的SCN或时间；
SYS@GMDBA> select dbms_flashback.GET_SYSTEM_CHANGE_NUMBER from dual;
GET_SYSTEM_CHANGE_NUMBER
------------------------
1186295
SYS@GMDBA> alter session set nls_date_format='yyyy-mm-dd hh24:mi:ss';
Session altered.
SYS@GMDBA> select sysdate from dual;
SYSDATE
-------------------
2013-12-03 15:57:03
（3） 删除HR用户；
SYS@GMDBA> drop user hr cascade;
User dropped.
SYS@GMDBA>
（4） 利用闪回功能回到HR用户被删除前的状态；
SYS@GMDBA> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SYS@GMDBA> startup mount;
ORACLE instance started.


Total System Global Area 422670336 bytes
Fixed Size 1345380 bytes
Variable Size 327157916 bytes
Database Buffers 88080384 bytes
Redo Buffers 6086656 bytes
Database mounted.
SYS@GMDBA> flashback database to scn 1186295;
Flashback complete.
SYS@GMDBA> select checkpoint_change# from v$database;
CHECKPOINT_CHANGE#
------------------
1187757
SYS@GMDBA> select checkpoint_change# from v$datafile;
CHECKPOINT_CHANGE#
------------------
1186296
1186296
1186296
1186296
1186296
1186296
1186296
7 rows selected.
SYS@GMDBA> select checkpoint_change# from v$datafile_header;
CHECKPOINT_CHANGE#
------------------
1186296
1186296
1186296
1186296
1186296
1186296
1186296
7 rows selected.


SYS@GMDBA> alter database open read only;
Database altered.
SYS@GMDBA> conn hr/oracle
Connected.
HR@GMDBA> select * from tab;
TNAME TABTYPE CLUSTERID
------------------------------ ------- ----------
COUNTRIES TABLE
DEPARTMENTS TABLE
EMPLOYEES TABLE
EMPLOYEES_TEST TABLE
EMP_DETAILS_VIEW VIEW
JOBS TABLE
JOB_HISTORY TABLE
LOCATIONS TABLE
REGIONS TABLE
9 rows selected.
HR@GMDBA> conn sys/oracle as sysdba
Connected.
SYS@GMDBA> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SYS@GMDBA> alter database open resetlogs;
Database altered.