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Select for update

Posted on 2003-02-24
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Last Modified: 2010-05-18
If I've got a web application that is doing a SELECT FOR UPDATE for one user. When subsequent user comes tries to do a SELECT FOR UPDATE, my application just hangs until the lock is released. I've tried using the "NOWAIT" clause at the end of the statement, but that doesn't help.  Is there a test I can do to figure out if there exists a lock on a record? Is an exception thrown when a lock exists and the application does a SELECT FOR UPDATE?
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Question by:marathonburke
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5 Comments
 

Expert Comment

by:tiendh
ID: 8013770
You can use this SQL to determine which is locked

select l.oracle_username "User Name", o.object_name,
z.status, z.machine, z.program, z.type
FROM v$locked_object l, dba_objects o, v$session z
WHERE l.object_id = o.object_id
and l.session_id = z.sid (+)
ORDER by o.object_name

Hope it help
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LVL 2

Expert Comment

by:i014354
ID: 8016839
Using the SQL from tiendh, you can qualify the WHERE clause further by checking for a particular o.object_name.  The SELECT...FOR UPDATE session that is currently waiting will have a status of ACTIVE. The SELECT...FOR UPDATE session that has the object locked will have a status of INACTIVE.

If you use the SELECT...FOR UPDATE NOWAIT, Oracle should throw the error "ORA-00054: resource busy and acquire with NOWAIT specified."
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LVL 15

Expert Comment

by:andrewst
ID: 8016850
If you do SELECT...FOR UPDATE NOWAIT, then if the record is already locked the following Oracle error is raised:

ORA-00054: resource busy and acquire with NOWAIT specified

You can declare your own exception to catch this:

DECLARE
  e_locked EXCEPTION;
  PRAGMA EXCEPTION_INIT( e_locked, -54 );
BEGIN
...
EXCEPTION
  WHEN e_locked THEN ...
END;

It is up to you what to do then - you can just give an error message, or wait a few seconds then try again.
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Accepted Solution

by:
giteshtrivedi earned 600 total points
ID: 8018141
Dear friend,

If your every statment is contain 'SELECT FOR UPDATE NOWAIT' then what happenning Once resource is locked then other statment for same object and row will get that ora -00054 error message.You can trap that error from your PL/SQL block.

From Oracle9i there is new consept is begin it 'SELECT FOR UPDATE WAIT <SEC>' in sec you can give appropriate sec for waiting.

Thanks

Gitesh Trivedi.
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Author Comment

by:marathonburke
ID: 8022501
Thanks for the help!
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