flashback versions query 和 flashback transaction query 特性详解

前面提到了oracle 自9i以来的flashback query 特性，借助该特性，可以看到某个过去某个时间点的某张表的数据，这为我们修正dml 误操作等提供了很大的方便。
但是对于该特性来说，如果只是想回退某个时间点以来的部分数据，那么纯粹依靠前后的数据对比来判断，就会显的非常吃力，或许根本就是无法办到的。比如，你某时刻同时跑起了多个job作业，
如果一个job有问题，而其他的job正常完成的，此时，如果你仅仅是想回复错误job处理过的数据，而要保留正常job的处理结果，但是或许你无法判断后面变化的数据哪些是正常job的，哪些是错误job的，因此，很难进行部分数据的修复工作。

   10g提供了flashback versions query 和 flashback transaction query 特性，它强化了9i 的flashback query 特性，正是解决上面类似问题的利器。
  
1.flashback versions query
   flashback versions query 能够得到某个时间段内，某些数据行的所有不同版本。这里的版本以事务为单位，事务中的每次数据变化就是一个版本。
举例说明，下面的会话2次update id=1 时的 name 值，因此分别提交过2次，因此是2个不同的事务，在flashback versions query 的结果中，关于
id=1 ，我们看到了3个不同的版本，一个是原来的值，一个是第一次修改的值，一个是第二次修改的值；通过flashback versions query 中的versions_startscn,versions_starttime
versions_endscn, versions_endtime,versions_xid, versions_operation 等伪列，我们也获得了相关版本的开始scn，结束scn，开始时间和结束时间，以及做dml操作的具体transaction id (xid)，以及操作类型等；
同时，我们也看到对于id=1的数据行，三个版本的VERSIONS_ENDSCN,VERSIONS_ENDtime 和 VERSIONS_startscn，VERSIONS_starttime 是承上启下的，一个版本的结束，也就是下一个版本的开始。

15:37:22 SQL> select *  from test;

        ID NAME
---------- --------------------
         1 kkk
         2 fff
         4 kkk

Executed in 0.031 seconds

15:37:34 SQL> update   test set name='TEST' where id=1;

1 row updated

Executed in 0.015 seconds

15:37:54 SQL> commit;

Commit complete

Executed in 0 seconds

15:37:56 SQL> update   test set name='TEST_new' where id=1;

1 row updated

Executed in 0.015 seconds

15:38:04 SQL> commit;

Commit complete

Executed in 0 seconds

15:38:06 SQL>
15:39:12 SQL> SELECT versions_startscn, versions_starttime,
           2                      versions_endscn, versions_endtime,
           3                      versions_xid, versions_operation,
           4                      id,name
           5                 FROM test
           6               VERSIONS BETWEEN TIMESTAMP
           7                     TO_TIMESTAMP('2008-04-27 15:37:30', 'YYYY-MM-DD HH24:MI:SS')
           8               AND TO_TIMESTAMP('2008-04-27 15:38:30', 'YYYY-MM-DD HH24:MI:SS')
           9  ;

VERSIONS_STARTSCN VERSIONS_STARTTIME                                                               VERSIONS_ENDSCN VERSIONS_ENDTIME                                                                 VERSIONS_XID     VERSIONS_OPERATION         ID NAME
----------------- -------------------------------------------------------------------------------- --------------- -------------------------------------------------------------------------------- ---------------- ------------------ ---------- --------------------
           819393 27-4月 -08 03.38.06 下午                                                                                                                                                          040015006F010000 U                           1 TEST_new
           819387 27-4月 -08 03.37.54 下午                                                                  819393 27-4月 -08 03.38.06 下午                                                         0500150088010000 U                           1 TEST
                                                                                                            819387 27-4月 -08 03.37.54 下午                                                                                                      1 kkk
                                                                                                                                                                                                                                                 2 fff
                                                                                                                                                                                                                                                 4 kkk

Executed in 0.031 seconds

15:39:13 SQL> select *  from test;

        ID NAME
---------- --------------------
         1 TEST_new
         2 fff
         4 kkk

Executed in 0 seconds

15:43:52 SQL>

2. flashback transaction query
    flashback transaction query 其实就是查询历史的事务信息，通过查询，我们可以得到过去某个事务操作信息，包括改变的数据行rowid，事务开始和结束时间，事务对应的loggon user，以及用来撤销某个数据行改变的undo sql等。
flashback transaction query 其实就是查询 FLASHBACK_TRANSACTION_QUERY 视图。

比如，通过上面 versions query 得到的xid，我们可以得到这些xid 对应的信息。

15:43:52 SQL> SELECT xid, operation, start_scn,commit_scn, logon_user, undo_sql  FROM flashback_transaction_query  WHERE xid = HEXTORAW('&1');

XID              OPERATION                         START_SCN COMMIT_SCN LOGON_USER                     UNDO_SQL
---------------- -------------------------------- ---------- ---------- ------------------------------ --------------------------------------------------------------------------------
040015006F010000 UPDATE                               819387     819393 TEST1                          update "TEST1"."TEST" set "NAME" = 'TEST' where ROWID = 'AAAM1/AAEAAAAGsAAA';
040015006F010000 BEGIN                                819387     819393 TEST1                         

Executed in 0.297 seconds

15:57:34 SQL> SELECT xid, operation, start_scn,commit_scn, logon_user, undo_sql  FROM flashback_transaction_query  WHERE xid = HEXTORAW('&1');

XID              OPERATION                         START_SCN COMMIT_SCN LOGON_USER                     UNDO_SQL
---------------- -------------------------------- ---------- ---------- ------------------------------ --------------------------------------------------------------------------------
0500150088010000 UPDATE                               819184     819387 TEST1                          update "TEST1"."TEST" set "NAME" = 'kkk' where ROWID = 'AAAM1/AAEAAAAGsAAA';
0500150088010000 BEGIN                                819184     819387 TEST1                         

Executed in 0.032 seconds

15:58:07 SQL>

3.一个例子
flashback versions query 和flashback transaction query 一般都是结合起来用的，通过前者得到某个时间段的某些数据的多个版本的xid，然后通过后者得到具体的xid的操作信息及undo_sql，最后根据需要做适当的数据修复操作。

SQL> create table  test  (id number ,name varchar2(20));

Table created

SQL> set time on
14:55:14 SQL> set timing on
14:55:18 SQL> insert into test values ( 1,'kkk');

1 row inserted

Executed in 0.016 seconds

14:55:27 SQL> insert into test values ( 2,'fff');

1 row inserted

Executed in 0.016 seconds

14:55:34 SQL> commit;

Commit complete

Executed in 0 seconds

14:55:39 SQL>

此时打开第二个会话窗口，执行如下：
SQL> insert into test1.test values ( 4,'kkk');

1 row inserted

SQL> commit;

Commit complete

SQL>

--再回到第一个窗口，进行 flashback versions query

15:00:20 SQL> SELECT versions_startscn, versions_starttime,
           2         versions_endscn, versions_endtime,
           3         versions_xid, versions_operation,
           4         id,name
           5    FROM test
           6  VERSIONS BETWEEN TIMESTAMP
           7        TO_TIMESTAMP('2008-04-27 14:55:20', 'YYYY-MM-DD HH24:MI:SS')
           8  AND TO_TIMESTAMP('2008-04-27 14:57:00', 'YYYY-MM-DD HH24:MI:SS')
           9  ;

VERSIONS_STARTSCN VERSIONS_STARTTIME                                                               VERSIONS_ENDSCN VERSIONS_ENDTIME                                                                 VERSIONS_XID     VERSIONS_OPERATION         ID NAME
----------------- -------------------------------------------------------------------------------- --------------- -------------------------------------------------------------------------------- ---------------- ------------------ ---------- --------------------
           817371 27-4月 -08 02.55.39 下午                                                                                                                                                          0900070081010000 I                           2 fff
           817371 27-4月 -08 02.55.39 下午                                                                                                                                                          0900070081010000 I                           1 kkk
           817386 27-4月 -08 02.56.03 下午                                                                                                                                                          0700280072010000 I                           4 kkk

Executed in 0.016 seconds

15:00:21 SQL>

-- flashback  transaction query
15:00:21 SQL> SELECT xid, operation, start_scn,commit_scn, logon_user, undo_sql  FROM flashback_transaction_query  WHERE xid = HEXTORAW('&1');

XID              OPERATION                         START_SCN COMMIT_SCN LOGON_USER                     UNDO_SQL
---------------- -------------------------------- ---------- ---------- ------------------------------ --------------------------------------------------------------------------------
0900070081010000 INSERT                               817359     817371 TEST1                          delete from "TEST1"."TEST" where ROWID = 'AAAM1/AAEAAAAGsAAB';
0900070081010000 INSERT                               817359     817371 TEST1                          delete from "TEST1"."TEST" where ROWID = 'AAAM1/AAEAAAAGsAAA';
0900070081010000 BEGIN                                817359     817371 TEST1                         

Executed in 0.593 seconds

15:03:54 SQL>
15:11:27 SQL>
15:11:27 SQL>
15:11:27 SQL> SELECT xid, operation, start_scn,commit_scn, logon_user, undo_sql  FROM flashback_transaction_query  WHERE xid = HEXTORAW('&1');

XID              OPERATION                         START_SCN COMMIT_SCN LOGON_USER                     UNDO_SQL
---------------- -------------------------------- ---------- ---------- ------------------------------ --------------------------------------------------------------------------------
0700280072010000 INSERT                                    0     817386 TEST2                          delete from "TEST1"."TEST" where ROWID = 'AAAM1/AAEAAAAGwAAA';
0700280072010000 BEGIN                                     0     817386 TEST2                         

Executed in 0.031 seconds

15:11:47 SQL>

 

 

4.undo_rentention

和9i flashback query 一样，flashback  versions query和flashback  transaction query 也都是依赖于undo tablespace，依赖于undo_retention 的。换句话说，对于已经不存在于undo tablespace 的数据版本信息，
你当然是无法通过 这些特性把它查出来的。因此设置好undo tablespace 的大小，设置好undo_rentention 的大小就尤为重要(10g 虽然有自动tuning undo_retention 的功能，不过考量并设置合适的undo_retention 最小值也是很有必要的）

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