关于flashback databse的疑惑

elison

通常情况下：
1.drop table t1 purge;
2.mount
3.flashback database ;可以找回t1

（可能由于受到flashback table to beore drop 的影响，进了思维的死胡同了）

问题1：
如果第一步和第二步期间，t1表空间的其他表有大量insert，t1删除前的所占的块，被其他表填充了能闪回吗？如果能，是什么原理？

问题2：
第一步和第二步之间，我把原t1的表空间和数据文件删除了

Maclean Liu(刘相兵

Oracle Database 10g features Flashback Database, a unique database point-in-time recovery capability, which allows the database to be ‘rewound’ to a prior point-in-time within seconds or minutes. Because only the affected data blocks are restored and recovered, it’s faster than traditional recovery methods. The command is simply ‘flashback database to my point-in-time’

We accomplish this fast recovery by utilizing flashback logs which record old block versions. When writes are issued to disk and flashback database is enabled, the old block version is written to the flashback log while the new block version is written to datafiles. When a flashback database command is issued, only the changed blocks are retrieved from the flashback logs and then recovered with appropriate archived logs to the required point-in-time. You control how far back to retain flashback logging to support your recovery requirements. For example you might enable flashback database for 24 hrs, and then rely on backups for recovery past 24 hrs.

Maclean Liu(刘相兵

"如果第一步和第二步期间，t1表空间的其他表有大量insert，t1删除前的所占的块，被其他表填充了能闪回吗？如果能，是什么原理？"

可以 ， 因为flashback logs闪回日志记录了 旧版本的 数据块。

"第一步和第二步之间，我把原t1的表空间和数据文件删除了"

闪回数据库 无法恢复 物理讹误 或者 介质丢失 ， 因为闪回数据库仅仅可以从闪回日志中获得哪些发生过修改的块的 旧景象(而非数据文件或表空间上的所有数据块都可以得到恢复)，它无法绕过 常规的介质恢复 来解决问题。