字符集问题的初步探讨(七)-字符集更改的内部操作
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archives/2004/09/nls_
character_set_07.html
这部分并未包含于itpub技术丛书《Oracle数据库DBA专题技术精粹》中，是后来补充的内容.
前面我们提到，通过修改props$的方式更改字符集在Oracle7之后是一种极其危险的方式，应该尽量避免。
我们又知道，通过ALTER DATABASE CHARACTER SET更改字符集虽然安全可靠，但是有严格的子集和超集的约束，实际上我们很少能够
用到这种方法。
实际上Oracle还存在另外一种更改字符集的方式.
如果你注意过的话，在Oracle的alert<sid>.log文件中，你可能看到过这样的日志信息:
alter database character set INTERNAL_CONVERT ZHS16GBK
Updating character set in controlfile to ZHS16GBK
SYS.SNAP$ (REL_QUERY) - CLOB representation altered
SYS.METASTYLESHEET (STYLESHEET) - CLOB representation altered
SYS.EXTERNAL_TAB$ (PARAM_CLOB) - CLOB representation altered
XDB.XDB$RESOURCE (SYS_NC00027$) - CLOB representation altered
ODM.ODM_PMML_DTD (DTD) - CLOB representation altered
OE.WAREHOUSES (SYS_NC00003$) - CLOB representation altered
PM.ONLINE_MEDIA (SYS_NC00042$) - CLOB representation altered
PM.ONLINE_MEDIA (SYS_NC00062$) - CLOB representation altered
PM.ONLINE_MEDIA (PRODUCT_TEXT) - CLOB representation altered
PM.ONLINE_MEDIA (SYS_NC00080$) - CLOB representation altered
PM.PRINT_MEDIA (AD_SOURCETEXT) - CLOB representation altered
PM.PRINT_MEDIA (AD_FINALTEXT) - CLOB representation altered
Completed: alter database character set INTERNAL_CONVERT ZHS1
|
在这里面，我们看到这样一条重要的，Oracle非公开的命令:
这个命令是当你选择了使用典型方式创建了种子数据库以后，Oracle会根据你选择的字符集设置，把当前种子数据库的字符集更改为期望字符
集，这就是这条命令的作用.
在使用这个命令时，Oracle会跳过所有子集及超集的检查，在任意字符集之间进行强制转换，所以，使用这个命令时你必须十分小心，你必须
清楚这一操作会带来的风险.
我们之前讲过的内容仍然有效，你可以使用csscan扫描整个数据库，如果在转换的字符集之间确认没有严重的数据损坏，或者你可以使用有效
的方式更改，你就可以使用这种方式进行转换.
我们来看一下具体的操作过程及Oracle的内部操作:
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.
Total System Global Area 135337420 bytes
Fixed Size 452044 bytes
Variable Size 109051904 bytes
Database Buffers 25165824 bytes
Redo Buffers 667648 bytes
Database mounted.
SQL> ALTER SYSTEM ENABLE RESTRICTED SESSION;
System altered.
SQL> ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0;
System altered.
SQL> ALTER SYSTEM SET AQ_TM_PROCESSES=0;
System altered.
SQL> ALTER DATABASE OPEN;
Database altered.
SQL> alter session set events '10046 trace name context forever,level 12';
Session altered.
SQL> alter database character set INTERNAL_USE ZHS16CGB231280
Database altered.
SQL>
|
这是alert.log文件中的记录信息:
| Tue Oct 19 16:26:30 2004
Database Characterset is ZHS16GBK
replication_dependency_tracking turned off (no async multimaster replication found)
Completed: ALTER DATABASE OPEN
Tue Oct 19 16:27:07 2004
alter database character set INTERNAL_USE ZHS16CGB231280
Updating character set in controlfile to ZHS16CGB231280
Tue Oct 19 16:27:15 2004
Thread 1 advanced to log sequence 118
Current log# 2 seq# 118 mem# 0: /opt/oracle/oradata/primary/redo02.log
Tue Oct 19 16:27:15 2004
ARC0: Evaluating archive log 3 thread 1 sequence 117
ARC0: Beginning to archive log 3 thread 1 sequence 117
Creating archive destination LOG_ARCHIVE_DEST_1: '/opt/oracle/oradata/primary/archive/1_117.dbf'
ARC0: Completed archiving log 3 thread 1 sequence 117
Tue Oct 19 16:27:20 2004
Completed: alter database character set INTERNAL_USE ZHS16CGB231280
Shutting down instance: further logons disabled
Shutting down instance (immediate)
License high water mark = 1
Tue Oct 19 16:29:06 2004
ALTER DATABASE CLOSE NORMAL
...
|
格式化10046跟踪文件,得到以下信息(摘要):
alter session set events '10046 trace name context forever,level 12'
alter database character set INTERNAL_USE ZHS16CGB231280
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 4.88 6.04 910 16825 18099 0
Fetch 0 0.00 0.00 0 0 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 2 4.88 6.04 910 16825 18099 0
Misses in library cache during parse: 1
Optimizer goal: CHOOSE
Parsing user id: SYS
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
control file sequential read 4 0.00 0.00
control file parallel write 2 0.05 0.08
log file sync 2 0.08 0.08
SQL*Net message to client 1 0.00 0.00
SQL*Net message from client 1 18.06 18.06
********************************************************************************
....
update col$ set charsetid = :1
where
charsetform = :2
....
update argument$ set charsetid = :1
where
charsetform = :2
....
update collection$ set charsetid = :1
where
charsetform = :2
....
update attribute$ set charsetid = :1
where
charsetform = :2
....
update parameter$ set charsetid = :1
where
charsetform = :2
....
update result$ set charsetid = :1
where
charsetform = :2
....
update partcol$ set spare1 = :1
where
charsetform = :2
....
update subpartcol$ set spare1 = :1
where
charsetform = :2
....
update props$ set value$ = :1
where
name = :2
....
update "SYS"."KOTAD___FCKpd___4quot; set SYS_NC_ROWINFO$ = :1
where
SYS_NC_OID$ = :2
....
update seq$ set increment$=:2,minvalue=:3,maxvalue=:4,cycle#=:5,order$=:6,
cache=:7,highwater=:8,audit$=:9,flags=:10
where
obj#=:1
....
update kopm$ set metadata = :1, length
= :2
where
name='DB_FDO'
....
ALTER DATABASE CLOSE NORMAL
|
此处生成的日志你可以在这里下载(供参考):
http://www.eygle.com/special/primary_ora_13730.zip
http://www.eygle.com/special/primary_ora_13730.tkf.log
我们看到这个过程和之前ALTER DATABASE CHARACTER SET操作的内部过程是完全相同的，也就是说INTERNAL_USE提供的帮助就是使
Oracle数据库绕过了子集与超集的校验.
这一方法在某些方面是有用处的，比如测试；应用于产品环境大家应该格外小心，除了你以外，没有人会为此带来的后果负责:
结语(我们不妨再说一次):
对于DBA来说，有一个很重要的原则就是:不要把你的数据库置于危险的境地！
这就要求我们，在进行任何可能对数据库结构发生改变的操作之前，先做有效的备份，很多DBA没有备份的操作中得到了惨痛的教训。
