Check the Lock, Blocking & deadlock in Oracle database

Quick way to find out the blocking at instance level:

select * from V$lock where block > 0;


How long a blocking session is blocking by other sessions:

select blocking_session, sid, serial#, wait_class, seconds_in_wait
from  v$session
where blocking_session is not NULL
order by blocking_session;

Query shows the blocking session as well as all the blocked sessions

select s1.username || '@' || s1.machine|| ' ( SID=' || s1.sid || ' )  is blocking '|| s2.username ||'@' || s2.machine || ' ( SID=' || s2.sid || ' ) ' AS blocking_status from v$lock l1, v$session s1, v$lock l2,v$session s2 where s1.sid=l1.sid and s2.sid=l2.sid and l1.BLOCK=1 and l2.request > 0 and l1.id1 = l2.id1 and l2.id2 = l2.id2;

Check the Status of blocker or waiter

Select sid , decode(block,0,'NO','YES') Blocker , decode (request ,0,'NO','YES')WAITER from v$lock where request>0 or block>0 order by block desc;
 

Check the Query involved causing blocking

select sid, sql_text
from v$session s, v$sql q
where sid in (68,81)
and ( q.sql_id = s.sql_id or q.sql_id = s.prev_sql_id );

Check the status of session is active or inactive

SQL> select sid,serial#,USERNAME,status from v$session where sid in (4284);
SID   SERIAL# USERNAME STATUS
----- ------- -------- ----------
1470  7233 APPS     INACTIVE
1895  48684 APPS     ACTIVE


Kill the inactive session with following command

alter system kill session '1470,7233' immediate;

Note: If you got the following message : session-marked-for-kill. Follow the link to check the transaction status:
ORA-00031 Session Marked For Kill

If multiple session is caused blocking then need to kill multiple session use the following scripts:
Script to kill multiple sessions

select 'alter system kill session '''|| sid ||','||serial# ||''' immediate;' from v$session where sid in (1645, 1757,2100,827,1678,1609,1653,1640,1730);


For RAC and 12c Pluggable Database Go to link: 12c Pluggable and RAC Locking