Oracle Flashback Technology【闪回技术】
-------------------------与其他数据库相比,Oracle的闪回让开发者多了一条选择的路。
Flashback的目的
Oracle Flashback Technology is a group of Oracle Database features that that let you view past states of database objects or to return database objects to a previous state without using point-in-time media recovery. |
- Flashback Query (引入自Oracle 9i)
- Flashback Table (引入自 Oracle 10g)
- Flashback Drop (引入自 Oracle 10g)
- Flashback Version Query (引入自Oracle 10g)
- Flashback Transaction Query (引入自Oracle 10g)
- Flashback Database (引入自Oracle 10g)
- Flashback Data Archive (引入自Oracle 11g)
- Flashback Transaction (引入自Oracle 11g)
Flashback Query useful to view the data at a point-in-time in the past. This can be used (only) to view and reconstruct lost data that was deleted or changed by accident.
Flashback Table useful to recover a table to a point-in-time in the past without restoring a backup. Flashback Table is a push button solution to restore the contents of a table to a given point-in-time. An application on top of
Flashback Query can achieve this, but with less efficiency.
Flashback Drop provides a way to restore accidentally dropped tables. This will be done with the help of Recyclebin feature.
Flashback Version Query uses undo data stored in the database to view the changes to one or more rows along with all the metadata of the changes.
Flashback Transaction Query useful to examine changes to the database at the transaction level. As a result, we can diagnose problems, perform analysis and audit transactions.
Flashback Database useful to bring database to a prior point in time by undoing all the changes that have taken place since that time. This operation is fast, because we do not need to restore the backups. This in turn results
in much less downtime following data corruption or human error. Flashback Database applies to the entire database. It requires configuration and resources, but it provides a fast alternative to performing incomplete database recovery.
Flashback Data Archive - from Oracle 11g, flashback will make use of flashback logs, explicitly created for that table, in FRA (Flash/Fast Recovery Area), will not use undo. Flashback data archives can be defined on any table/tablespace.
Flashback data archives are written by a dedicated background process called FBDA so there is less impact on performance. Can be purged at regular intervals automatically.
SQL> alter session set nls_date_format='yyyy-mm-dd:hh24:mi:ss';
Session altered SQL> select * from scott.emp;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
----- ---------- --------- ----- ----------- --------- --------- ------
7369 SMITH CLERK 7902 1980/12/17 800.00 20
7499 ALLEN SALESMAN 7698 1981/2/20 1600.00 300.00 30
7521 WARD SALESMAN 7698 1981/2/22 1250.00 500.00 30
7566 JONES MANAGER 7839 1981/4/2 2975.00 20
7654 MARTIN SALESMAN 7698 1981/9/28 1250.00 1400.00 30
7698 BLAKE MANAGER 7839 1981/5/1 2850.00 30
7782 CLARK MANAGER 7839 1981/6/9 2450.00 10
7788 SCOTT ANALYST 7566 1987/4/19 3000.00 20
7839 KING PRESIDENT 1981/11/17 5000.00 10
7844 TURNER SALESMAN 7698 1981/9/8 1500.00 0.00 30
7876 ADAMS CLERK 7788 1987/5/23 1100.00 20
7900 JAMES CLERK 7698 1981/12/3 950.00 30
7902 FORD ANALYST 7566 1981/12/3 3000.00 20
7934 MILLER CLERK 7782 1982/1/23 1300.00 10
14 rows selected SQL> create table test1 as (select * from scott.emp);
Table created SQL> select sysdate from dual;
SYSDATE
-----------
2015/7/19 15:22:17 --这里DBA误操作,误删除了test1业务表,并且执行了commit
SQL> delete from test1;
14 rows deleted SQL> commit;
Commit complete
此时test1表中已经没有数据了
SQL> select * from test1;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
----- ---------- --------- ----- ----------- --------- --------- ------ SQL>
但我们依然可以通过Flashback Query查询之前被删除的数据:
SQL> select * from test1 as of timestamp to_timestamp('2015-07-19:15:22:17','yyyy-mm-dd:hh24:mi:ss');
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
----- ---------- --------- ----- ----------- --------- --------- ------
7369 SMITH CLERK 7902 1980/12/17 800.00 20
7499 ALLEN SALESMAN 7698 1981/2/20 1600.00 300.00 30
7521 WARD SALESMAN 7698 1981/2/22 1250.00 500.00 30
7566 JONES MANAGER 7839 1981/4/2 2975.00 20
7654 MARTIN SALESMAN 7698 1981/9/28 1250.00 1400.00 30
7698 BLAKE MANAGER 7839 1981/5/1 2850.00 30
7782 CLARK MANAGER 7839 1981/6/9 2450.00 10
7788 SCOTT ANALYST 7566 1987/4/19 3000.00 20
7839 KING PRESIDENT 1981/11/17 5000.00 10
7844 TURNER SALESMAN 7698 1981/9/8 1500.00 0.00 30
7876 ADAMS CLERK 7788 1987/5/23 1100.00 20
7900 JAMES CLERK 7698 1981/12/3 950.00 30
7902 FORD ANALYST 7566 1981/12/3 3000.00 20
7934 MILLER CLERK 7782 1982/1/23 1300.00 10
14 rows selected SQL>
如果需要,还可以恢复数据:
SQL> insert into test1 select * from test1 as of timestamp to_timestamp('2015-07-19:15:22:17','yyyy-mm-dd:hh24:mi:ss');
14 rows inserted SQL> commit;
而借助Flashback Database特性,可以进行这样的恢复
其实Flashback Logs并不神秘,与所有的日志文件功能相同,它也是用来记录日志。只不过这个日志保存的信息比较特别,并不简单记录做过什么的操作,而是记录下操作执行前要修改的数据,即数据块的前映像。这些信息被写入一个专用存储区叫Flash Recovery Area,简称FRA。当需要Flashback Database时,通过Flashback Log中保存的数据,就可以快速将Oracle数据库恢复到指定时间点块的状态,然后再通过应用重做日志,将数据库恢复到一致性状态。
Flashback Log的创建、删除、修改都由Oracle自动进行,完全无须DBA手工干预,唯一需要DBA介入的,就是给Flash Recovery Area分配适当的空间。
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