You perform a Flashback Query by using a SELECT statementwith an AS OF clause.You use a flashback query to restrieve data as it existed at some time in the past.The query explicitly references a past time by menasof timestamp or SCN.It returns committed data that was current at that point intime.
通過執(zhí)行一個帶as of 子句的select語句進(jìn)行閃回查詢，可以閃回檢索過去某個時間存在的數(shù)據(jù),一個閃回查詢被用來重現(xiàn)過去存在過的數(shù)據(jù)，這個查詢明確的引用了過去的一個時間段或SCN號，閃回查詢返回的數(shù)據(jù)都是過去某時刻已經(jīng)提交的數(shù)據(jù)。
Potential uses of Flashback Query include:
可能使用到閃回查詢的地方：
⊙Recovering lost data or undoing incorrect,committed changes.For example,if you mistakenly delete or update rows,and then commit them,you can immediately undo the mistake.
恢復(fù)丟失的數(shù)據(jù)或撤銷已經(jīng)提交的錯誤。例如如果你不小心刪除或更新了行，并且做了提交操作，你可以立刻撤銷這個錯誤。
⊙Comparing current data with the corresponding data at some time in the past.For example,you might run a daily report that shows the change in data from yesterday.You can compare the individual rows of table data or find intersections or unions of sets of rows.
比較當(dāng)前數(shù)據(jù)和歷史數(shù)據(jù)的一致性。例如，你可能需要生成一份前一天數(shù)據(jù)更新的日報告，你可以分別比較表的每一行或找到行的交集和并集。
⊙Checking the state of transactional data at a particular time.For example,you could verify the account balance of a certain day.
在某個特殊時間檢查事務(wù)型數(shù)據(jù)的狀態(tài)。例如，你可以在某一天驗證賬戶收支。
⊙Simplifying the application design,by removing the need to store some kinds of temporal data.By using a Flashback Query, you can retrieve past data directly from the database.
移除某些因需求儲存的暫時數(shù)據(jù)以簡化應(yīng)用設(shè)計。通過閃回查詢你可以從數(shù)據(jù)庫中直接獲取到過去的數(shù)據(jù)。
⊙Applying the packaged applications such as report generation tools to past data.
使用包裝應(yīng)用（例如報表生成工具）的歷史數(shù)據(jù)
⊙Providing self-service error correction for anapplication,thereby enabling users to undo and correct their errors.
為應(yīng)用提供自服務(wù)錯誤更正，因此可以讓用戶撤銷或更正他們的錯誤。
示例：
SQL> conn /as sysdba;
已連接。
SQL> set pagesize 200
SQL> select * from scott.dept;
DEPTNO DNAME LOC
---------- ---------------------------- -------------------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
現(xiàn)在，我們來增加一條數(shù)據(jù)，并提交：
SQL> insert into scott.dept values(50,'錯誤數(shù)據(jù)','CHINA');
已創(chuàng)建 1 行。
SQL> select * from scott.dept;
DEPTNO DNAME LOC
---------- ---------------------------- -------------------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
50 錯誤數(shù)據(jù) CHINA
SQL> commit;(2011-12-9 10:51:00)
提交完成。
正常情況下，由于已經(jīng)做了commit操作，所以rollback已經(jīng)無效了，要想得到2011-12-9 10:51:00之前的數(shù)據(jù)，怎么辦？
使用timestamp時間點閃回：
SQL> select * from scott.dept as of timestamp to_timestamp('2011-12-09 10:00:00','yyyy-mm-dd hh24:mi:ss');
DEPTNO DNAME LOC
---------- ---------------------------- -------------------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
閃回查詢，主要依靠表空間的undo數(shù)據(jù)，如果想要追溯更久的數(shù)據(jù)，就需要設(shè)置較大的undo_tablespaces大小和undo_retention。
如果想直接更新當(dāng)前的表到歷史的某個時間狀態(tài)，可以直接使用flashback關(guān)鍵字：
SQL> alter table scott.dept enable row movement;
表已更改。
SQL> flashback table scott.dept to timestamp to_timestamp('2011-12-09 10:00:00','yyyy-mm-dd hh24:mi:ss');
閃回完成。
【注意】：
閃回不是萬能的，當(dāng)一個表的數(shù)據(jù)較大或時間過長時，如果沒有設(shè)置較大的閃回空間和時間，閃回操作將會失敗，可能會出現(xiàn)以下錯誤：
