如何删除flashback recovery area中的归档日志？

简单的在操作系统层面rm虽然释放了硬盘空间，但是V$FLASH_RECOVERY_AREA_USAGE中并不知道。

于是，当你希望再次enable flashback database时，会遇到如下错误：

SQL> alter database flashback on;
alter database flashback on
*
ERROR at line 1:
ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38708: not enough space for first flashback database log file

> oerr ora 38708
38708, 00000, "not enough space for first flashback database log file"
// *Cause: An ALTER DATABASE FLASHBACK ON command failed because there
//         was not enough space in the Recovery Area for the first
//         flashback database log file.
// *Action: Make more space in the Recovery Area.  For example, this
//          can be done by increasing the value of
//          DB_RECOVERY_FILE_DEST_SIZE.

很显然，我们需要做的不是简单地增大DB_RECOVERY_FILE_DEST_SIZE，因为通过通过如下知道归档日志占用99%的空间，没被清除。

SQL> select file_type,percent_space_used from V$FLASH_RECOVERY_AREA_USAGE;

FILE_TYPE    PERCENT_SPACE_USED
------------ ------------------
CONTROLFILE                   0
ONLINELOG                     0
ARCHIVELOG                99.81
BACKUPPIECE                   0
IMAGECOPY                     0
FLASHBACKLOG                  0

我们需要做的就是这样简单的两句rman commands：

crosscheck archivelog all;

delete expired archivelog all;