Example of how to trace a session and use tkprof

Enable trace for a user session:

SQLPLUS> exec sys.dbms_system.set_sql_trace_in_session(SID, SERIAL, true);
Use tkprof to analyze the trace file:
tkprof [TRACE FILE] [OUTPUT FILE] explain=/ sort=prsela,exeela,fchela
table=ops\$oracle.plan_table 

identify trace file 
SELECT s.sid,
       s.serial#,
       pa.value || '/' || LOWER(SYS_CONTEXT('userenv','instance_name')) ||    
       '_ora_' || p.spid || '.trc' AS trace_file
FROM   v$session s,
       v$process p,
       v$parameter pa
WHERE  pa.name = 'user_dump_dest'
AND    s.paddr = p.addr
AND    s.audsid = SYS_CONTEXT('USERENV', 'SESSIONID'); 

logon trigger sample 

CREATE OR REPLACE TRIGGER db_logon
AFTER LOGON ON DATABASE
declare
  my_exception exception;
begin
    execute immediate 'begin ; end;';
  if user='' then
    execute immediate 'ALTER SESSION SET EVENTS ''10046 TRACE NAME CONTEXT FOREVER, LEVEL 12''';
    --raise my_exception;
  end if;
exception 
  when my_exception then
       RAISE_APPLICATION_ERROR(-20001,'!!! ERROR');
  when OTHERS then
       null;
end;

Levels 

Sql trace = true is equal to tracing for event 10046 at level 1
SQL> alter system set events='10046 trace name context forever, level 1';
SQL> alter system set events='10046 trace name context off' ;
Level 4 is level 1 + bind variables (debugging)
SQL> alter system set events='10046 trace name context forever, level 4' ;
SQL> alter system set events='10046 trace name context off' ;
Level 8 is level 1 + wait info (performance tuning)
SQL> alter system set events='10046 trace name context forever, level 8' ;
Level 12 is level 1 + bind variables & wait info (debugging + performance tuning)
SQL> alter system set events='10046 trace name context forever, level 12' ;

In order to stop the 10046 session trace use this one
SQL> alter system set events='10046 trace name context off' ;