Answer:

Use SYS.DBMS_SYSTEM.SET_EV() procedure. Here is the specification for this procedure:

 
PROCEDURE SET_EV
Argument Name                  Type                    In/Out Default?
------------------------------ ----------------------- ------ --------
SI                             BINARY_INTEGER          IN
SE                             BINARY_INTEGER          IN
EV                             BINARY_INTEGER          IN
LE                             BINARY_INTEGER          IN
NM                             VARCHAR2                IN

• SID: V$SESSION.SID
• SE: V$SESSION.SERIAL#
• EV: Event number. For example:
  • 10046: SQL traces.
  • 10053: Optimizer traces.
  • NNN : ORA-NNN errors.
  • 65535: IMMEDIATE traces.
• LE: Event level. For Event 10046 events:
  • 0: Disable event.
  • 1: PARSE, FETCH, EXEC, EXECUTION PLAN
  • 4: Level 1 + BINDS
  • 8: Level 1 + WAITS
  • 12: Level 4 + Level 8
• NM: Event name. For example:
  • ERRORSTACK.......: For error stack traces.
  • PROCESSSTATE...: For process states
  • SYSTEMSTATE.......: For System states.
  • ''..................................: For CONTEXT FOREVER.

Sample:

Dumps PROCESSSTATE trace IMMEDIATELY in LEVEL 10:

 
SQL> exec dbms_system.set_ev(8,1056,65535,10,'PROCESSSTATE');

Dumps ERRORSTACK trace in LEVEL 3 on ORA-942 error:

 
SQL> exec dbms_system.set_ev(8,1060,942,3,'ERRORSTACK');

Dumps Event 10046 trace in LEVEL 8 for CONTEXT FOREVER:

 
SQL> exec dbms_system.set_ev(8,1060,10046,8,'');