一，问题描述
背景说明，Dg备库需要恢复到三天前，但是期间没有开闪回，临时开始，出现了这个错误。原因分析：
To enable Flashback Database, you configure a fast recovery area and set a flashback retention target. This retention target specifies how far back you can rewind a database with Flashback Database.
From that time onwards, at regular intervals, the database copies images of each altered block in every data file into the flashback logs. These block images can later be reused to reconstruct the data file contents for any moment at which logs were captured.
When you use Flashback Database to rewind a database to a past target time, the command determines which blocks changed after the target time and restores them from the flashback logs. The database restores the version of each block that is immediately before the target time. The database then uses redo logs to reapply changes that were made after these blocks were written to the flashback logs.
Redo logs on disk or tape must be available for the entire time period spanned by the flashback logs. For example, if the flashback retention target is 1 week, then you must ensure that online and archived redo logs that contain all changes for the past week are accessible. In practice, redo logs are typically needed much longer than the flashback retention target to support point-in-time recovery.
During the flashback operation, RMAN may need to restore archived redo logs from backup. Enter the following command to see whether channels are configured (sample output is included):
这个是官方文档说明了如果要恢复在指定时间点确保期间的重做日志存在，例如，如果闪回保留目标为1周，则必须确保可以访问包含过去一周所有更改的联机和归档重做日志。实际上，重做日志通常比闪回保留目标更长，以支持时间点恢复。所以出现的问题可能为归档丢失，或者闪回是临时开启，更本就没有存放任何恢复操作。
