Lock + Insert..

Question

Hi,
I have a job in my Oracle Forms 10g/ Oracle DB 10g application that runs every minute to identify if there is a Blocking Lock in the database.
I have been getting an INSERT statement as the cause of the blocking lock every day! How can an insert statement create a blocking?
I have not used LOCK TABLE tab1 IN EXCLUSIVE MODE;
The query I use in my job to get the Blocking SQL text is:
SELECT vs.SID, vs.username, vs.logon_time, vs.machine, vs.sql_hash_value,
vsql.piece, vsql.sql_text, vs.prev_hash_value
FROM v$session vs, v$sqltext vsql
WHERE SID IN (SELECT LPAD (' ', DECODE (vl.request, 0, 0, 1)) || SID
FROM v$lock vl
WHERE id1 IN (SELECT id1
FROM v$lock vl2
WHERE vl2.lmode = 0))
AND vs.sql_hash_value = vsql.hash_value
ORDER BY 2, 5, 6;
Please advice
Warm Regards
Sajan

Answer 1

DROP TABLE T1;
CREATE TABLE T1 (C1 NUMBER PRIMARY KEY);
-- Session 1:
INSERT INTO T1 VALUES (0);
-- Session 2:
INSERT INTO T1 VALUES (0);
-- The statement "hangs"
-- Session 3 (SYSTEM)
SELECT vs.SID, vs.username, vs.logon_time, vs.machine, vs.sql_hash_value,
     vsql.piece, vsql.sql_text, vs.prev_hash_value
     FROM v$session vs, v$sqltext vsql
     WHERE SID IN (SELECT LPAD (' ', DECODE (vl.request, 0, 0, 1)) || SID
     FROM v$lock vl
     WHERE id1 IN (SELECT id1
     FROM v$lock vl2
     WHERE vl2.lmode = 0))
     AND vs.sql_hash_value = vsql.hash_value
     ORDER BY 2, 5, 6;
SID USERNAME   LOGON_TIME          ..  SQL_TEXT                    
51 <......>   23.08.2006 13:15:50 ..  INSERT INTO T1 VALUES (0)
and
SELECT
           (SELECT USERNAME FROM V$SESSION WHERE SID = A.SID) Blocker
          ,A.SID
          ,' is blocking '
          ,(SELECT USERNAME FROM V$SESSION WHERE SID = B.SID) Blockee
          ,B.SID
     FROM V$LOCK A, V$LOCK B
     WHERE A.BLOCK = 1
          AND B.REQUEST > 1
          AND A.ID1 = B.ID1
          AND A.ID2 = B.ID2
BLOCKER               SID 'ISBLOCKING'  BLOCKEE               SID
<......>               31  is blocking  <......>                51