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使用oradebug修改数据库scn，使用oradebug修改数据库scn的案例。

这里也做了两个测试,发现该功能确实很巧妙,通过修改内存中的scn值,然后写入控制文件和数据文件,实现修改scn的方法,不过同样该方法的危害性极大,这里仅供测试使用,生产环境切不可乱使用,可能引起很严重后果

1.数据库版本信息

SQL> select * from v$version;

BANNER

----------------------------------------------------------------

Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Prod

PL/SQL Release 10.2.0.4.0 - Production

CORE 10.2.0.4.0 Production

TNS for Linux: Version 10.2.0.4.0 - Production

NLSRTL Version 10.2.0.4.0 - Production

SQL> select '惜分飞' oracleplus FROM DUAL;

XIFENF

------

惜分飞

在open库中修改scn

SQL> oradebug setmypid

Statement processed.

--查看当前scn

SQL> oradebug DUMPvar SGA kcsgscn_

kcslf kcsgscn_ [20009228, 20009248) = 00000000 0007A09F 00000019 00000000 00000000 00000000 00000000 20009034

SQL> select CHECKPOINT_CHANGE# a from v$datafile;

A

----------

499314

499314

499314

499314

SQL> select dbms_flashback.get_system_change_number a from dual;

A

----------

499877

SQL> select to_number('7A09F','xxxxxxxxx') from dual;

TO_NUMBER('7A09F','XXXXXXXXX')

------------------------------

499871

--修改内存中scn值(十进制)

SQL> oradebug poke 0x20009228 4 8

BEFORE: [20009228, 2000922C) = 00000000

AFTER: [20009228, 2000922C) = 00000008

SQL> oradebug DUMPvar SGA kcsgscn_

kcslf kcsgscn_ [20009228, 20009248) = 00000008 0007A0D8 00000052 00000000 00000000 00000000 00000000 20009034

SQL> col a for 999999999999999

SQL> select dbms_flashback.get_system_change_number a from dual;

A

------------

34360238301

SQL> select to_number('8','xx')*4294967296+to_number('0007A0D8','xxxxxxxx') a from dual;

A

----------------

34360238296

--做一个checkpoint为了内存中的scn值写入控制文件和数据文件

SQL> alter system checkpoint;

System altered.

SQL> shutdown immediate

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> startup

ORACLE instance started.

Total System Global Area 318767104 bytes

Fixed Size 1267236 bytes

Variable Size 96471516 bytes

Database Buffers 213909504 bytes

Redo Buffers 7118848 bytes

Database mounted.

Database opened.

SQL> col a for 999999999999999

SQL> select CHECKPOINT_CHANGE# a from v$datafile;

A

----------------

34360238496

34360238496

34360238496

34360238496

SQL> select CHECKPOINT_CHANGE# a from v$datafile_header;

A

----------------

34360238496

34360238496

34360238496

34360238496

2.在mount库中修改scn

SQL> startup mount

ORACLE instance started.

Total System Global Area 318767104 bytes

Fixed Size 1267236 bytes

Variable Size 96471516 bytes

Database Buffers 213909504 bytes

Redo Buffers 7118848 bytes

Database mounted.

SQL> oradebug setmypid

Statement processed.

--因为数据库是mount状态不能看到scn值

SQL> oradebug DUMPvar SGA kcsgscn_

kcslf kcsgscn_ [20009228, 20009248) = 00000000 00000000 00000000 00000000 00000000 00000000 00000000 20009034

SQL> col a for 999999999999999

SQL> select CHECKPOINT_CHANGE# a from v$datafile_header;

A

----------------

34360240739

34360240739

34360240739

34360240739

--求出WRAP SCN值

SQL> select 34360240739/4294967296 from dual;

34360240739/4294967296

----------------------

8.00011697

--修改内存中scn值(十六进制)

SQL> oradebug poke 0x20009228 4 0x0000000a

BEFORE: [20009228, 2000922C) = 00000000

AFTER: [20009228, 2000922C) = 0000000A

SQL> oradebug DUMPvar SGA kcsgscn_

kcslf kcsgscn_ [20009228, 20009248) = 0000000A 00000000 00000000 00000000 00000000 00000000 00000000 20009034

SQL> alter database open;

Database altered.

SQL> select dbms_flashback.get_system_change_number a from dual

2 ;

A

----------------

42949673074

--注意:使用此种方法修改BASE SCN如果不指定,会从0开始计数

SQL> oradebug DUMPvar SGA kcsgscn_

kcslf kcsgscn_ [20009228, 20009248) = 0000000A 00000077 0000001C 00000000 00000000 00000000 00000000 20009034

SQL> select to_number('A','xx')*4294967296+to_number('00000077','xxxxxxxx') a from dual;

A

----------------

42949673079

SQL> alter system checkpoint;

System altered.

SQL> select CHECKPOINT_CHANGE# a from v$datafile_header;

A

----------------

42949673095

42949673095

42949673095

42949673095

SQL> select CHECKPOINT_CHANGE# a from v$datafile;

A

----------------

42949673095

42949673095

42949673095

42949673095

SQL> shutdown immediate;

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> startup

ORACLE instance started.

Total System Global Area 318767104 bytes

Fixed Size 1267236 bytes

Variable Size 96471516 bytes

Database Buffers 213909504 bytes

Redo Buffers 7118848 bytes

Database mounted.

Database opened.

SQL> select CHECKPOINT_CHANGE# a from v$datafile_header;

A

----------------

42949673231

42949673231

42949673231

42949673231

在oradebug推进scn的过程中，需要注意不同平台，不同位数的ORACLE数据库可能推进方式有一定的区别，操作前最好在系统平台位数上进行测试，否则有可能导致恢复后果更加麻烦

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最权威、专业的Oracle案例资源汇总之【学习笔记】Oracle oradebug 使用oradebug修改数据库SCN方法案例