Flashback Database uses its own logging mechanism, creating flashback logs which are stored in the flash recovery area. You can only use Flashback Database if flashback logs are available. Therefore, you must set up your database in advance to create flashback logs if you want to take advantage of this feature.
To enable Flashback Database, you set up a flash recovery area, and set a flashback retention target, to specify how far back into the past you want to be able to restore your database with Flashback Database.
From that time on, at regular intervals, the database copies images of each altered block in every datafile into the flashback logs. These block images can later be reused to reconstruct the datafile contents as of any moment at which logs were captured.
When a database is restored to its state at some past target time using Flashback Database, each block changed since that time is restored from the copy of the block in the flashback logs most immediately prior to the desired target time. The redo log is then used to re-apply changes since the time that block was copied to the flashback logs.
Redo logs must be available for the entire time period spanned by the flashback logs, whether on tape or on disk. (In practice, however, redo logs are generally needed much longer than the flashback retention target to support point-in-time recovery.)
rvwr进程一段时间将block image写入flashback log中（not every change）
flashback database是怎么恢复的呢，根据flashback log中的at some past target time的block进行restore，然后根据redo log进行recover到target time，哈哈就是这样简单
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