Session Management - Oracle 10g/11g Data and Database Management Utilities - page 161

The culprit is discovered as enq:Tx Row Lock Contention. Meanwhile, the other
processes just keep waiting. Now, it is time to correct this situation, either the user
has to commit or rollback the pending transactions or the DBA will have to kill the
user's session.
At the bottom of the page of the Enterprise Manager Performance Tab we find two
links, one named Blocking Sessions , and a second one named Hang Analysis . Both
can be used to monitor the blocking sessions; the first one is in character mode, and
indented to show the session that caused the hang, meanwhile the Hang Analysis
shows the hung sessions in graphic mode.
Looking at the graphical Hang Analysis tool it shows a graphical representation
of the hung sessions. It shows the sessions in three colors; green, yellow, and red.
Green is shown as soon as the blocking event is detected, and this status will remain
for about 30 seconds, if the session remains blocked then it will change the status to
prolonged wait (yellow), and if the event remains for another 30 seconds the status
will be changed to hung (red).