打開數據庫flashback
1. 確認一個數據庫是否打開了flashback，有兩個簡單的方法：
（1）直接查數據庫
SQL> select name,log_mode,flashback_on from v$database;
NAME LOG_MODE FLASHBACK_ON
--------- ------------ ------------------
ORA11G ARCHIVELOG NO --> flashback未打開
（2）查看後臺進程
Oracle@bmp1a:~> ps -ef | grep rvw |grep -v grep
oracle 9288 1 0 10:45 ? 00:00:00 ora_rvwr_ora11g --> flashback已經打開www.linuxidc.com
2. 如果數據庫沒有打開flashback，那麼將flashback打開：
（1）確定flashback的存儲路徑
SQL> show parameter recovery
NAME TYPE VALUE
----------------------------- ----------- ------------------------------
db_recovery_file_dest string
db_recovery_file_dest_size big integer 0
recovery_parallelism integer 0
修改db_recovery_file_dest：
SQL> alter system set db_recovery_file_dest = '/home/oracle/flashback' scope = both;
alter system set db_recovery_file_dest = '/home/oracle/flashback' scope = both
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-19802: cannot use DB_RECOVERY_FILE_DEST without DB_RECOVERY_FILE_DEST_SIZE
上面的錯誤，是因爲在設置db_recovery_file_dest之前，一定要先設置db_recovery_file_dest_size，於是：
SQL>alter system set db_recovery_file_dest_size = 20G scope =both;
System altered.
SQL> show parameter recovery
NAME TYPE VALUE
------------------------------- ----------- ------------------------------
db_recovery_file_dest string
db_recovery_file_dest_size big integer 20G
recovery_parallelism integer 0
SQL> alter system set db_recovery_file_dest = '/home/oracle/flashback' scope = both;
System altered.
SQL> show parameter recovery
NAME TYPE VALUE
------------------------------- ----------- ------------------------------
db_recovery_file_dest string /home/oracle/flashback
db_recovery_file_dest_size big integer 20G
recovery_parallelism integer 0
（2）打開flashback
SQL> alter database flashback on;
alter database flashback on
*
ERROR at line 1:
ORA-38759: Database must be mounted by only one instance and not open.
上面的錯誤，是因爲在打開數據庫的flashback必須要mount狀態下方可以，於是：
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 5344731136 bytes
Fixed Size 2169104 bytes
Variable Size 2130708208 bytes
Database Buffers 3204448256 bytes
Redo Buffers 7405568 bytes
Database mounted.
SQL> alter database flashback on;
Database altered.
SQL> alter database open;
Database altered.
SQL> select name,log_mode,flashback_on from v$database;
NAME LOG_MODE FLASHBACK_ON
--------- ------------------ ------------
ORA11G ARCHIVELOG YES