在某些情况下，数据库失去响应，sqlplus也无法连接，此时通常只能通过杀掉进程来解决。

但是我们仍然希望能够获得此时的数据库状态信息，以便用于事后诊断。
从Oracle10g开始，sqlplus提供了一个参数选项-prelim，可以在通常sqlplus无法连接的情况下进行连接。
通过以下步骤可以获取系统信息：

sqlplus -prelim / as sysdba    #注意 /和as之间要有空格，否则出错！
oradebug setmypid
oradebug unlimit;
oradebug dump systemstate 10

这种方法非常有用：

$ sqlplus -prelim / as sysdba

SQL*Plus: Release 10.2.0.1.0 - Production on Thu Oct 25 09:42:20 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.

SQL>

但是需要注意的是，在Oracle 10.2.0.1中，使用如上方法会出现一个错误：

$ sqlplus -prelim / as sysdba

SQL*Plus: Release 10.2.0.1.0 - Production on Thu Oct 25 09:38:14 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.

SQL> oradebug setmypid
Statement processed.
SQL> oradebug unlimit
Statement processed.
SQL> oradebug dump systemstate 10
ORA-03113: end-of-file on communication channel
ORA-24323: value not allowed

此时在alert文件中会记录如下错误提示：

Thu Oct 25 09:38:32 2007
System State dumped to trace file
Thu Oct 25 09:38:32 2007
Errors in file /opt/oracle/admin/test201/udump/test201_ora_1402.trc:
ORA-07445: exception encountered: core dump [kgldmp()+1360] [SIGSEGV] [Address not mapped to object] [0x000000030] [] []

这是由于一个Bug导致的，Bug号为5730231，该Bug在10.2.0.3中修正。

对于9i也有变通的方法使用

-prelim is feature of Sql*Plus 10g and latter.

So as long you have any Sql*Plus 10g or latter version (ex. from client installation) and a valid net service name to connect to your 9i database you should be able to establish a "backdoor" connection.

You can do the following:

$sqlplus -prelim /nolog

and once you are in, just connect to the database using SYSDBA account

SQL> connect sys/password@net_service_name as sysdba
Prelim connection established