How to find exactly who lock the records

Question

hi:
Session1: lock a record for example
Select e.* from employees e where e.employee_id = 7034 for update;
Session2: try to lock the same record:
Select e.* from employees e where e.employee_id = 7034 for update nowait;
After exception ora-00054 raised query this statement:
Select Vs.Sid,
Vs.Client_Identifier,
Vs.Action,
Vs.Status,
Vs.Machine,
Vs.Terminal,
Vs.Logon_Time,
Dmlock.Name
From Dba_Dml_Locks Dmlock,
V$session Vs
Where Dmlock.Session_Id = Vs.Sid
And Vs.Sid =
(Select Distinct First_Value(Ash.Blocking_Session) Over(Order By Ash.Sample_Time Desc)
From Sys.v_$active_Session_History Ash
Where Ash.Session_Id = Sys_Context('USERENV', 'SID'))

Answer 1

session #1
21:28:14 SQL> Select e.* from employees e where e.employee_id = 206 for update;
EMPLOYEE_ID FIRST_NAME           LAST_NAME
EMAIL                     PHONE_NUMBER         HIRE_DATE JOB_ID         SALARY
COMMISSION_PCT MANAGER_ID DEPARTMENT_ID
        206 William              Gietz
WGIETZ                    515.123.8181         07-JUN-94 AC_ACCOUNT       8300
                      205           110
21:28:39 SQL> session #2
SQL> set time on
21:29:25 SQL>  Select e.* from employees e where e.employee_id = 206 for update nowait;
Select e.* from employees e where e.employee_id = 206 for update nowait
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
21:29:51 SQL> session #3
21:30:18 SQL> SELECT Vs.sid,
       Vs.client_identifier,
       Vs.action,
       Vs.status,
       Vs.machine,
       Vs.terminal,
       Vs.logon_time,
       Dmlock.name
FROM   dba_dml_locks Dmlock,
       v$session Vs
WHERE  Dmlock.session_id = Vs.sid
       AND Vs.sid = (SELECT DISTINCT First_value(Ash.blocking_session)
                                       over(
                                         ORDER BY Ash.sample_time DESC)
                     FROM   sys.v_$active_session_history Ash
                     WHERE  Ash.session_id = Sys_context('USERENV', 'SID')) 21:30:44   2  21:30:44   3  21:30:44   4  21:30:44   5  21:30:44   6  21:30:44   7  21:30:44   8  21:30:44   9  21:30:44  10  21:30:44  11  21:30:44  12  21:30:44  13  21:30:44  14  21:30:44  15  21:30:44  16 
21:30:46  17  /
no rows selected
21:30:49 SQL> Your results are duly UNDERWHELMING!