在Oracle DG Standby库上启用flashback database功能
曾经发生的一次故障，让想起了在Oracle Data Guard环境中配置flashback databae功能的必要性；这样在primary库中用户不小心做truncate或其他不便回退操作时，也可以在standbay上做一下flashback database 后只读模式打开standby库，将丢失数据导出或dblink方式传输到其他数据库中。
1、首先要坚持standby库环境及参数：
-bash-3.00$ sqlplus / as sysdba
SQL> archive log list
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /backup/archivelog
Oldest online log sequence 6799
Next log sequence to archive 0
Current log sequence 6802
SQL> show parameter db_flashback_retention_target
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_flashback_retention_target integer 1440
SQL> show parameter DB_RECOVERY_FILE_DEST_SIZE
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest_size big integer 0
SQL> show parameter DB_RECOVERY_FILE_DEST
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest string
db_recovery_file_dest_size big integer 0
SQL> show parameter flashback
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_flashback_retention_target integer 1440
也就是在备库还没有配置过闪回区域相关的两个如下参数：
db_recovery_file_dest
db_recovery_file_dest_size
2、检查standby 数据库服务器存储空间使用情况：
-bash-3.00$ df -h
Filesystem size used avail capacity Mounted on
/dev/md/dsk/d10 119G 31G 87G 27% /
………..
/dev/dsk/c1t0d0s6 652G 368G 277G 58% /oradata
/dev/dsk/c1t0d1s6 808G 225G 574G 29% /backup
可以看出有足够Flashback Logs空间；
3、具体配置过程
Step 1. Shut down the database and start up mount. The database must be mounted but not open. （关闭standby数据库并打开为mount状态）
SQL>shutdown immediate;
SQL>startup mount;
SQL> select status from v$instance;
STATUS
------------
MOUNTED
SQL> archive log list;
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /backup/archivelog
Oldest online log sequence 6799
Next log sequence to archive 0
Current log sequence 6802
Step 2. Set the flashback retention target to your desired value. We will use 24 hours as the window. （配置 db_flashback_retention_target参数，单位为分钟）
alter system set db_flashback_retention_target=1440 SCOPE=BOTH;
Step 3. Set the values for db_recovery_file_dest and db_recovery_file_dest_size (flash recovery area parameters).（配置闪回区域参数）
Flashback logs increase FRA usage significantly. It would behoove you to at least double the given size of the FRA.
因此具体参数带该如下：
SQL> ALTER SYSTEM SET DB_RECOVERY_FILE_DEST_SIZE = 600151336 SCOPE=BOTH ;
SQL> ALTER SYSTEM SET DB_RECOVERY_FILE_DEST = '/backup/fra/' SCOPE=BOTH;
Step 4. Turn flashback logging on. This is done in the same fashion as turning archivelog on-with
an ALTER DATABASE command when the database is mounted but not open.（启用flashback功能）
SQL> alter database flashback on;
Step 5. 因standby库，因此不能open而进入管理模式：
SQL>alter database recover managed standby database disconnect from session;