Find the blocking session

Command

SELECT bl.sid       AS blocker_sid,
       bl.serial#   AS blocker_serial,
       bl.username  AS blocker_user,
       w.sid        AS waiter_sid,
       w.serial#    AS waiter_serial,
       w.username   AS waiter_user,
       w.seconds_in_wait AS wait_sec,
       w.event
FROM   v$session w
  JOIN v$session bl ON bl.sid = w.blocking_session
WHERE  w.blocking_session IS NOT NULL
ORDER BY w.seconds_in_wait DESC;

What it does

Lists blocker and waiter session pairs with the wait time and event.

When to use it

To find the culprit in lock problems such as "application hung / enq: TX wait".

Caution

Confirm with the business owner before killing the blocker; it may be a critical operation. Query only; no risk.