【oracle】flashback query

今天无意间有个同事问 SELECT employee_id,salary FROM employees AS OF TIMESTAMP(SYSTIMESTAMP-INTERVAL'10' minute).
表达的意思。看到 as of ，之前还真没见过，遂查之。
原来这句话是想查 employees 十分钟之前的数据。
随后就引出了oracle的 flashback 技术。
本文通过实验说明其中的 flashback query 部分。

查看数据库版本：
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Prod
PL/SQL Release 10.2.0.3.0 - Production
CORE    10.2.0.3.0    Production
TNS for 32-bit Windows: Version 10.2.0.3.0 - Production
NLSRTL Version 10.2.0.3.0 - Production

A表为实验数据
SQL> select * from a;
       NUM NAME
---------- ----
        12 d
        12 d
        12 d
        12 d
        12 d
        12 d
6 rows selected

查看当前 SCN：
SQL> select dbms_flashback.get_system_change_number from dual;
GET_SYSTEM_CHANGE_NUMBER
------------------------
          11274291850925

删除A表数据：
SQL> delete from a;
6 rows deleted
SQL> commit;
Commit complete

A表为空
SQL> select * from a;
       NUM NAME
---------- ----
恢复到删除前的那个点
SQL> flashback table a to scn 11274291850925;
Done

查看A表中数据，已经恢复。
SQL> select * from a;
       NUM NAME
---------- ----
        12 d
        12 d
        12 d
        12 d
        12 d
        12 d
6 rows selected

而在实际中，开发人员几乎不会去记录DML操作前的SCN。这时候就用另外的方法。

B表为测试表。
SQL> create table b as select * from a;
Table created
SQL> select * from b;
       NUM NAME
---------- ----
        12 d
        12 d
        12 d
        12 d
        12 d
        12 d
6 rows selected

删除并提交
SQL> delete from b;
6 rows deleted
SQL> commit;
Commit complete

从flashback_transaction_query 查询我们需要的信息
SQL> select * from flashback_transaction_query where table_name='B';
XID               START_SCN START_TIMESTAMP COMMIT_SCN COMMIT_TIMESTAMP LOGON_USER  UNDO_CHANGE# OPERATION  TABLE_NAME TABLE_OWNERROW_ID              UNDO_SQL
---------------- ---------- --------------- ---------- ---------------- ----------- ------------ ---------- ----------------------------------------- ---------------------------------------------------------
02001900EC5B0000 1127429185 2012/3/19 21:26 1127429185 2012/3/19 21:27: SCOTT                  1 DELETE     B          SCOTT      AAAOkQAAGAABBTEAAF  insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
02001900EC5B0000 1127429185 2012/3/19 21:26 1127429185 2012/3/19 21:27: SCOTT                  2 DELETE     B          SCOTT      AAAOkQAAGAABBTEAAE  insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
02001900EC5B0000 1127429185 2012/3/19 21:26 1127429185 2012/3/19 21:27: SCOTT                  3 DELETE     B          SCOTT      AAAOkQAAGAABBTEAAD  insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
02001900EC5B0000 1127429185 2012/3/19 21:26 1127429185 2012/3/19 21:27: SCOTT                  4 DELETE     B          SCOTT      AAAOkQAAGAABBTEAAC  insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
02001900EC5B0000 1127429185 2012/3/19 21:26 1127429185 2012/3/19 21:27: SCOTT                  5 DELETE     B          SCOTT      AAAOkQAAGAABBTEAAB  insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
02001900EC5B0000 1127429185 2012/3/19 21:26 1127429185 2012/3/19 21:27: SCOTT                  6 DELETE     B          SCOTT      AAAOkQAAGAABBTEAAA  insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
6 rows selected

在最后可以看到 undo_sql列。利用此列的内容，就可以恢复之前的数据。

insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');
insert into "SCOTT"."B"("NUM","NAME") values ('12','d ');

SQL> commit;
Commit complete
查询B表
SQL> select * from b;
       NUM NAME
---------- ----
        12 d
        12 d
        12 d
        12 d
        12 d
        12 d
6 rows selected