System change number (SCN) is a logical, internal timestamp used by the Oracle Database. SCNs order events that occur within the database, which is necessary to satisfy the ACID properties of a transaction.
SCN是Oracle数据库内部逻辑上的时钟，是一直持续增长的。数据库中SCN事件，主要是为了满足事务的ACID特性(包括分布式事务)、数据库的恢复等。有时候我们可能需要推进SCN的到一个合适的值去解决一些常见的错误，例如ORA-600 [2662]，ORA-600 [4000]，不同版本有不同的方法，下面是常用的方法：
ACID：原子性(Atomicity)、一致性(Consistency)、隔离性(Isolation)、持久性(Durability)
ORA-600 [2662]
A data block SCN is ahead of the current SCN,occurs when an SCN is compared to the dependent SCN stored in a UGA variable.If the SCN is less than the dependent SCN then we signal the ORA-600 [2662] internal error.
ORA-600 [4000]
It means that Oracle has tried to find an undo segment number in the dictionary cache and failed.
1.通过adjust_scn
有2种方式修改，此方法在9i数据库可用，10g之后版本不可用Open状态下可通过以下方法
alter session set events 'immediate trace name adjust_scn level n';
mount状态下可通过
alter session set events '10015 trace name adjust_scn level n';
n是指把SCN推进到n*1024*1024*1024这个值
2.通过_minimum_giga_scn
_minimum_giga_scn(Minimum SCN to start with in 2^30 units),意思是把SCN往前推进到n*1024*1024*1024，在10g可用，11g大部分版本可用
$ sqlplus / as sysdba SQL*Plus: Release 10.2.0.1.0 - Production on Wed Mar 30 13:11:55 2016 Copyright (c) 1982, 2005, Oracle. All rights reserved. Connected to an idle instance. SQL> startup mount pfile='/tmp/pfile'; ORACLE instance started. Total System Global Area 612368384 bytes Fixed Size 2022800 bytes Variable Size 239075952 bytes Database Buffers 364904448 bytes Redo Buffers 6365184 bytes Database mounted. SQL> show parameter scn NAME TYPE VALUE ------------------------------------ ----------- ---------------------------- _minimum_giga_scn integer 1 SQL> select checkpoint_change# from v$database; CHECKPOINT_CHANGE# ------------------ 736493 SQL> alter database open; Database altered. SQL> select checkpoint_change# from v$database; CHECKPOINT_CHANGE# ------------------ 1073741825 SQL>
3.通过oracdebug
SQL> oradebug help peek
PEEK <addr> <len> [level] Print/Dump memory
SQL> oradebug help poke
POKE <addr> <len> <value> Modify memory
SQL>
oradebug poke <address> <length> <value>
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起始地址 长度 需要设置的值
address：内存地址
length：长度可为1,2,4,8
value：可为10进制，也可以为16进制
**************************************
Oradebug on Linux
*********************************SQL> startup mount
ORACLE instance started.
Total System Global Area 313159680 bytes
Fixed Size 2252824 bytes
Variable Size 167776232 bytes
Database Buffers 138412032 bytes
Redo Buffers 4718592 bytes
Database mounted.
SQL> select checkpoint_change#,current_scn from v$database;
CHECKPOINT_CHANGE# CURRENT_SCN
------------------ -----------
1052507 0
SQL> oradebug setmypid
Statement processed.
SQL> oradebug dumpvar sga kcsgscn
kcslf kcsgscn_ [06001AE70, 06001AEA0) = 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 6001AB50 00000000
SQL> select to_char(1052507,'xxxxxxxx') from dual;
TO_CHAR(1
---------
100f5b
SQL> oradebug peek 0x06001AE70 8
[06001AE70, 06001AE78) = 00000000 00000000
SQL> oradebug poke 0x06001AE70 8 2052507 -->修改SCN为2052507，可以是10进制，也可以是16进制
BEFORE: [06001AE70, 06001AE78) = 00000000 00000000
AFTER: [06001AE70, 06001AE78) = 001F519B 00000000
SQL> select to_char(64425561947,'xxxxxxxxxxxxxxxx') from dual;
TO_CHAR(644255619
-----------------
f00100f5b
SQL> oradebug poke 0x06001AE70 8 64425561947 -->修改SCN为64425561947，0x000f.00100f5b
BEFORE: [06001AE70, 06001AE78) = 001F519B 00000000
AFTER: [06001AE70, 06001AE78) = 00100F5B 0000000F -->Little Endian存储的方式为00100F5B 0000000F，可以看到scn_wrap在后面，scn_base在前
SQL> oradebug peek 0x06001AE70 4 -->查看scn_base的值,地址从0x06001AE70开始
[06001AE70, 06001AE74) = 00100F5B
SQL> oradebug peek 0x06001AE74 4 -->查看scn_wrap的值,地址从0x06001AE74开始
[06001AE74, 06001AE78) = 0000000F
SQL>
SQL>
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Oradebug on AIX
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SQL> startup mount
ORACLE instance started.
Total System Global Area 612368384 bytes
Fixed Size 2022800 bytes
Variable Size 234881648 bytes
Database Buffers 369098752 bytes
Redo Buffers 6365184 bytes
Database mounted.
SQL> select checkpoint_change#,current_scn from v$database;
CHECKPOINT_CHANGE# CURRENT_SCN
------------------ -----------
735435 0
SQL> oradebug setmypid
Statement processed.
SQL> oradebug dumpvar sga kcsgscn
kcslf kcsgscn_ [7000000100120C0, 7000000100120F0) = 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 07000000 ...
SQL> oradebug peek 0x7000000100120C0 8
[7000000100120C0, 7000000100120C8) = 00000000 00000000
SQL> oradebug poke 0x7000000100120C0 8 835435 -->修改SCN为835435
BEFORE: [7000000100120C0, 7000000100120C8) = 00000000 00000000
AFTER: [7000000100120C0, 7000000100120C8) = 00000000 000CBF6B
SQL> oradebug poke 0x7000000100120C0 8 64425561947 -->修改SCN为64425561947
BEFORE: [7000000100120C0, 7000000100120C8) = 00000000 000CBF6B
AFTER: [7000000100120C0, 7000000100120C8) = 0000000F 00100F5B -->Big Endian存储的方式为0000000F 00100F5B，可以看到scn_wrap在前面，scn_base在后
SQL> select to_char(64425561947,'xxxxxxxxxxxx') from dual;
TO_CHAR(64425
-------------
f00100f5b
SQL>
SQL> oradebug peek 0x7000000100120C0 4 -->查看scn_wrap的值,地址从0x7000000100120C0开始
[7000000100120C0, 7000000100120C4) = 0000000F
SQL> oradebug peek 0x7000000100120C4 4 -->查看scn_base的值,地址从0x7000000100120C4开始
[7000000100120C4, 7000000100120C8) = 00100F5B
SQL>
总结：不论AIX,LINUX,HP,Solaris平台，修改SCN都可以用下列方式修改，这样修改最安全
oradebug poke 0x7000000100120C0 8 64425561947
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SCN变量开始地址 长度 需要设置的SCN值，这个值最好用10进制表示， 当然用16进制
也可以，不过用10进制不易混淆更安全
Reference
http://www.orafaq.com/papers/oradebug.pdf
