How to use prelim option in sqlplus....  

what is sqlplus -prelim : This is the backdoor entry method to access database (only SGA) in a hung situation 
 where no other methods are allowing to create new sessions in database.
 This will not create any session in DB rather connect to SGA directly ...

1) 

   sqlplus -prelim / as sysdba
   sqlplus system/mypass -prelim /nolog 

2) 

SQL> set _prelim on
SQL> conn / as sysdba
Prelim connection established

SQL> oradebug setmypid
Statement processed.

SQL> oradebug unlimit
Statement processed.

SQL> oradebug dump hanganalyze 1
Statement processed.

SQL> oradebug dump systemstate 10
Statement processed.

SQL> oradebug tracefile_name
/oracle/admin/diag/rdbms/demo11g/demo11g/trace/demo11g_ora_25010.trc


sqlplus -prelim / as sysdba
oradebug setmypid
oradebug unlimit;
oradebug dump systemstate 10


Note: 215858.1 – "Interpreting HANGANALYZE trace files to diagnose hanging and performance problems"

Note: 310830.1 – "How to Use Oradebug to Get Trace File Name and Location"

Note: 417879.1 – "-prelim Option When Running System State Dump"