闪回查询（SELECT AS OF）

使用Flashback Query的场景包括如下：

摘自官档

• 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.

• Comparing current data with the corresponding data at an earlier time.

  For example, you can run a daily report that shows the change in data from yesterday. You can compare individual rows of table data or find intersections or unions of sets of rows.

• Checking the state of transactional data at a particular time.

  For example, you can verify the account balance of a certain day.

• Simplifying application design by removing the need to store some kinds of temporal data.

  Oracle Flashback Query lets you retrieve past data directly from the database.

• Applying packaged applications, such as report generation tools, to past data.

• Providing self-service error correction for an application, thereby enabling users to undo and correct their errors.

• 需要授予 FLASHBACK ANY TABLE 权限

假设表personnel在早上4点30的时候发现Smith的记录被删，DBA确定在前天晚上7点30的时候数据是准确的，于是我们可以使用Flashback Query技术找回丢失的数据

查纪录

SELECT * 
  FROM personnel
  AS OF TIMESTAMP
  TO_TIMESTAMP('2012-03-21 07:30:00', 'YYYY-MM-DD HH:MI:SS')
WHERE UPPER(last_name) = 'SMITH';

使用闪回查询恢复数据

INSERT INTO personnel 
 (
  SELECT * 
    FROM personnel
    AS OF TIMESTAMP
    TO_TIMESTAMP('2012-03-21 07:30:00', 'YYYY-MM-DD HH:MI:SS')
   WHERE UPPER(last_name) = 'SMITH'
 );

创建过去表的一个视图

CREATE VIEW hour_ago AS
  SELECT * FROM employees
    AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '60' MINUTE);

• 可以在自连接中使用AS OF子句，或者INTERSECT 和 MINUS提取或者比较两个不同时间点的数据

也可以使用下面的方法恢复丢失的数据

INSERT INTO employees
    (SELECT * FROM employees
     AS OF TIMESTAMP (SYSTIMESTAMP - INTERVAL '60' MINUTE)
    MINUS SELECT * FROM employees;

场景

CREATE TABLE flashback_query_test (
  id  NUMBER(10)
);

SELECT current_scn, TO_CHAR(SYSTIMESTAMP, 'YYYY-MM-DD HH24:MI:SS') FROM v$database;

CURRENT_SCN TO_CHAR(SYSTIMESTAM
----------- -------------------
     722452 2004-03-29 13:34:12

INSERT INTO flashback_query_test (id) VALUES (1);
COMMIT;

SELECT COUNT(*) FROM flashback_query_test;

  COUNT(*)
----------
         1

SELECT COUNT(*)
FROM   flashback_query_test AS OF TIMESTAMP TO_TIMESTAMP('2004-03-29 13:34:12', 'YYYY-MM-DD HH24:MI:SS');

  COUNT(*)
----------
         0

SELECT COUNT(*)
FROM   flashback_query_test AS OF SCN 722452;

  COUNT(*)
----------
         0

使用DBMS_FLASHBACK进程闪回查询

This can as well been done with SCN using ENABLE_AT_SYSTEM_CHANGE_NUMBER procedure or timestamp usingENABLE_AT_TIME procedure:

SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn),TO_CHAR(SYSDATE,'dd-mon-yyyy hh24:mi:ss') AS current_time FROM v$database;

CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)                                               CURRENT_TIME
----------- --------------------------------------------------------------------------- -----------------------------
   28954179 15-AUG-11 11.03.48.000000000 AM                                             15-aug-2011 11:03:48

SQL> EXEC dbms_flashback.enable_at_system_change_number(28954179);

PL/SQL PROCEDURE successfully completed.

SQL> SELECT * FROM test1;

        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three

SQL> EXEC dbms_flashback.disable;

PL/SQL PROCEDURE successfully completed.

SQL> SELECT * FROM test1;

        ID DESCR
---------- ------------------------------
         1 TEMPORARY
         2 TEMPORARY
         3 TEMPORARY

最后,闪回表到过去，（效果和上面的insert into as select一样）

SQL> flashback TABLE test1 TO scn 28954179;

Flashback complete.

SQL> SELECT * FROM test1;

        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three

来自为知笔记(Wiz)