今天一朋友和我说他的数据库不能open,open过程提示ORA-00900错误,通过分析alert日志和props$表,发现他们修改了一个无效的NLS_CHARACTERSET值，导致数据库无法正常启动(准确的说，因为数据库里面该值无效，当数据库open的过程中，检测到控制文件指定的编码和该值不一致，然后修改控制文件的编码，修改之后，数据库一到mount状态执行任何语句都报ORA-00900错误),通过一些工具修改NLS_CHARACTERSET为正确值该故障解决

重现ORA-00900故障

SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - Production
PL/SQL Release 11.2.0.3.0 - Production
CORE    11.2.0.3.0      Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production
SQL> select value$ from props$ where name='NLS_CHARACTERSET';
VALUE$
-------------------------------------------------------
ZHS16GBK
SQL> update props$ set value$='AL16UTF16' where name='NLS_CHARACTERSET';
1 row updated.
SQL> commit;
Commit complete.
SQL> alter database backup controlfile to trace as '/tmp/ora11g.ctl';
Database altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area  175775744 bytes
Fixed Size                  1343668 bytes
Variable Size             117444428 bytes
Database Buffers           50331648 bytes
Redo Buffers                6656000 bytes
Database mounted.
ORA-01092: ORACLE instance terminated. Disconnection forced
ORA-00604: error occurred at recursive SQL level 1
ORA-00900: invalid SQL statement
Process ID: 5277
Session ID: 125 Serial number: 5
SQL> startup nomount;
ORACLE instance started.
Total System Global Area  175775744 bytes
Fixed Size                  1343668 bytes
Variable Size             117444428 bytes
Database Buffers           50331648 bytes
Redo Buffers                6656000 bytes
SQL> alter database mount;
Database altered.
SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-00900: invalid SQL statement
SQL>select * from dual;
select * from dual
*
ERROR at line 1:
ORA-00900: invalid SQL statement
SQL> shutdown abort
ORACLE instance shut down.

第一次startup(open)过程报错

SMON: enabling tx recovery
Updating character set in controlfile to AL16UTF16
Errors in file /u01/oracle/diag/rdbms/ora11g/ora11g/trace/ora11g_ora_5277.trc:
ORA-00604: error occurred at recursive SQL level %s
ORA-00900: invalid SQL statementursive SQL level %s
Errors in file /u01/oracle/diag/rdbms/ora11g/ora11g/trace/ora11g_ora_5277.trc:
ORA-00604: error occurred at recursive SQL level %s
ORA-00900: invalid SQL statementursive SQL level %s
Error 604 happened during db open, shutting down database
USER (ospid: 5277): terminating the instance due to error 604
Errors in file /u01/oracle/diag/rdbms/ora11g/ora11g/trace/ora11g_smon_5243.trc:
ORA-00604: error occurred at recursive SQL level %s
ORA-00900: invalid SQL statementursive SQL level %s
Instance terminated by USER, pid = 5277
ORA-1092 signalled during: ALTER DATABASE OPEN...
opiodr aborting process unknown ospid (5277) as a result of ORA-1092
Sat May 18 00:44:27 2013
ORA-1092 : opitsk aborting process

这里比较明显的看到有一条(Updating character set in controlfile to AL16UTF16),正是由于这个操作，更新控制文件的编码为一个无效的编码，从而导致在后面数据库mount(加载控制文件)之后，就不能再进行其他任何操作

解决思路
使用odu找出来block位置，或者在同版本库中查询
使用dul或者bbed修改props$的NLS_CHARACTERSET值
重建控制文件(noresetlogs方式)

处理过程

SQL> shutdown abort
ORACLE instance shut down.
odu找出来block位置
dul或者bbed修改block值
重建控制文件(noresetlogs方式)
SQL> startup
ORACLE instance started.
Total System Global Area  175775744 bytes
Fixed Size                  1343668 bytes
Variable Size             117444428 bytes
Database Buffers           50331648 bytes
Redo Buffers                6656000 bytes
Database mounted.
ORA-01113: file 1 needs media recovery
ORA-01110: data file 1: '/u01/oracle/oradata/ora11g/system01.dbf'
SQL> recover database;
Media recovery complete.
SQL> alter database open;
Database altered.

从oracle 9i开始，修改数据库的编码，直接使用alter database character set internal_use方式进行,而不要使用直接修改props$基表,更不能修改一个实际中不存在的编码值(本文中数据库编码和国家编码搞混淆从而出现该故障)