[2020年]OCP 19c 认证1Z0-083考试题库（第5题）-CUUG内部学员版083题库解析
5、Which three are true about requirements for various FLASHBACK operations? (Choose three.)
A. FLASHBACK transaction query requires undo to retrieve all versions of a row that existed between two points in time.
B. FLASHBACK drop requires that the RECYCLEBIN parameter be set to ON.
C. FLASHBACK version query requires that the RECYCLEBIN parameter be set to ON.
D. FLASHBACK DATA ARCHIVE requires undo to store all versions of all rows of a table being tracked.
E. FLASHBACK drop requires undo to retrieve all versions of a row that existed between two points in time.
F. FLASHBACK version query requires undo to retrieve all versions of a row that existed between two points in time.
Answer: ABF
(解析：flashback query 只能“穿越”到过去的某个时间点上的数据库版本，但是在当前时间和过去的某个时间点上，一个表中的数据可能已经被变更多次，单一版本可能无法满足恢复的需求。通过 lashback version query 能够查看指定时间段内 undo 表空间中被提交的记录的不同版本。
Flashback Transaction Query 也是使用 UNDO 信息来实现。利用这个功能可以查看某个事务执行的所有变化，它需要访问 flashback_transaction_query 视图，这个视图的 XID 列代表事务ID，利用这个 ID 可以区分特定事务发生的所有数据变化。)