OCP 19c 認證1Z0-083考試題庫（第5題）（2020版）-CUUG內部學員版083題庫解析
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”到過去的某個時間點上的數據庫版本，但是在當前時間和過去的某個時間點上，一個表中的數據可能已經被變更多次，單一版本可能無法滿足恢復的需求。通過 lashback version query 能夠查看指定時間段內 undo 表空間中被提交的記錄的不同版本。
Flashback Transaction Query 也是使用 UNDO 信息來實現。利用這個功能可以查看某個事務執行的所有變化，它需要訪問 flashback_transaction_query 視圖，這個視圖的 XID 列代表事務ID，利用這個 ID 可以區分特定事務發生的所有數據變化。)