为测试部门搭建一台Oracle DB，需满足可以经常将整个数据库回退到某时间点的需求。比较简单的方法是使用Oracle的闪回特性之一"闪回数据库"。
预备知识：闪回数据库需要使用两种日志：闪回日志和重做日志。重做日志的概念不需多谈，所谓的闪回日志的记录正好与重做日志的记录相反。可以简单的这样认为：比如重做日志记录了一条insert命令，那么闪回日志就记录这条记录相关的delete命令，当然实际上这是非常复杂的过程。
这里并不会介绍“闪回数据库”的原理，因为已经有太多的资料来阐明它，也不会介绍“闪回数据库”的操作方法，因为它非常简单（在进入MOUNT状态后执行"flashback database to scn …"或"flashback database to timestamp …"）。
1. 开启DB闪回之前需确保启用了归档模式
SQL> archive log list
Database log mode Archive Mode
Automatic archival Enabled
……
2. 查看闪回日志是否已启用
SQL> select LOG_MODE,FLASHBACK_ON from v$database;
LOG_MODE FLASHBACK_ON
---------- ---------------
ARCHIVELOG NO
3. 启用闪回日志，即闪回数据库功能
SQL> alter database flashback on;
alter database flashback on
*
ERROR at line 1:
ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38709: Recovery Area is not enabled.
报错，无法开启闪回功能。到这里本文的主角--"oerr"命令君隆重登场。
4. 根据上文中提示的错误号使用oerr命令排错
$ oerr ORA 38709
38709, 00000, "Recovery Area is not enabled."
// *Cause: An ALTER DATABASE FLASHBACK ON command failed because the
// Recovery Area was not enabled.
// *Action: Set DB_RECOVERY_FILE_DEST to a location and retry.
5. 从上述的提示中找到错误原因:DB_RECOVERY_FILE_DEST 没有预设
SQL> show parameter DB_RECOVERY_FILE_DEST
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest string
db_recovery_file_dest_size big integer 0
6. 设置DB_RECOVERY_FILE_DEST
SQL> alter system set db_recovery_file_dest='/opt/oracle/fast_recovery_area';
alter system set db_recovery_file_dest='/opt/oracle/fast_recovery_area'
*
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
7. 使用oerr命令排错
$ oerr ORA 19802
19802, 00000, "cannot use DB_RECOVERY_FILE_DEST without DB_RECOVERY_FILE_DEST_SIZE"
// *Cause: There are two possible cause for this error:
// 1) The DB_RECOVERY_FILE_DEST parameter was in use when no
// DB_RECOVERY_FILE_DEST_SIZE parameter was encountered while
// fetching initialization parameter.
// 2) An attempt was made to set DB_RECOVERY_FILE_DEST with the
// ALTER SYSTEM command when no DB_RECOVERY_FILE_DEST_SIZE
// was in use.
// *Action: Correct the dependency parameter definitions and retry the command.
8. 从上述错误原因与解决方案提示中得知需要先设置DB_RECOVERY_FILE_DEST_SIZE
SQL> alter system set DB_RECOVERY_FILE_DEST_SIZE=20G scope=both;
System altered.
SQL> show parameter db_recovery
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest string
db_recovery_file_dest_size big integer 20G
9. 设置DB_RECOVERY_FILE_DEST
SQL> alter system set db_recovery_file_dest='/opt/oracle/fast_recovery_area' scope=both;
System altered.
10. 启用闪回日志，即闪回数据库功能
SQL> alter database flashback on;
Database altered.
此时可以在'db_recovery_file_dest'设定的目录中看到扩展名为.flb的文件，它们就是闪回日志。
$ ls
o1_mf_98mpkdl6_.flb o1_mf_98okkcs9_.flb