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Thursday, 10 November 2011

Identify blocking session for an UPDATE

Simulate a blocking session first:

Open 2 session windows, either in SQL Plus or any other cilent tool
you may be using.
In session 1 create a table and insert some data

create table test_lock (a varchar2(2), bb varchar2(2));
insert into test_lock values ('i','1');
insert into test_lock values ('ii', '2');
select * from test_lock;

Before starting the simulation, obtain the unique session ID's
for both sessions by using the following statement;

select distinct sid from v$mystat;

Obtain an exclusive lock in session 1 as follows;

select * from test_lock for update;

Now, in session 2, run these queries to identify blocking sessions;

update test_lock set a='i' where bb='1';

This statement will simply hang, since session 1 holds an exclusive lock on all rows within the table. Do not issue a rollback or commit;

Now query V$LOCK in session 1 using the following 3 statements;

set pagesize 3000
set linesize 1000
select * from v$lock;

select l1.sid, ' IS BLOCKING ', l2.sid, l1.lmode, l2.request
from   v$lock l1, v$lock l2
where   l1.block    = 1
and     l2.request  > 0
and     l1.id1      =l2.id1
and    l1.id2       =l2.id2;

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;

Finally, use the next few statements to identify the object ID and
blocked row information.

Replace the session id in the where clause with the
waiting session id, session 2.

select row_wait_obj#, row_wait_file#, row_wait_block#, row_wait_row#
from v$session
where sid=&sid;

Replace the SID in the where clause with that of session 2
to obtain the Object name and ROWID

select do.object_name
      ,dbms_rowid.rowid_create ( 1, ROW_WAIT_OBJ#
                                   ,ROW_WAIT_ROW# )
from   v$session       s
      ,dba_objects     do
where  sid             = &sid -- this is the SID from session2
and    s.ROW_WAIT_OBJ# = do.OBJECT_ID;

Replace the ROWID from the previous result set to obtain the
actual row data

select *
from test_lock
where rowid='AAAVnHAAQAAAp0tAAA';

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