Explicitly rollback when you encounter a deadlock.

Posted by Pramod Sadalage on Tuesday, May 26, 2009

Dead lock is caused in the database when you have resources (connections) waiting for other connections to release locks on the rows that are needed by the session, resulting in all session being blocked. Oracle automatically detects deadlocks are resolves the deadlock by rolling back the statement in the transaction that detected the deadlock. Thing to remember is that last statement is rolled back and not the whole transaction, which means that if you had other modifications, those rows are still locked and the application should make sure that it does a explicit rollback on the connection.

For example. Lets assume there are two tables Parent(ParentID) and Child(ChildID)

SESSION_A >create table parent (parentId number(10));
Table created.
SESSION_A >create table child (childId number(10));
Table created.
SESSION_A >insert into parent values (100);
1 row created.
SESSION_A >insert into child values (200);
1 row created.
SESSION_A >commit;
Commit complete.
SESSION_A >select * from parent;

SESSION_A >select * from child;

Now lets create a situation where a deadlock happens. There are two sessions connected to the same database and same user, SESSION_A and SESSION_B are the two sessions in question.

SESSION_A >update parent set parentid = 1000 where parentid=100;
1 row updated.
SESSION_B >update child set childid = 2000 where childid = 200;
1 row updated.
SESSION_B >update parent set parentid = 2001 where parentid=100;
--Waiting For Lock on Row in Parent Table, held by SESSION_A
SESSION_A >update child set childid = 1001 where childid = 200;
update child set childid = 1001 where childid = 200
ERROR at line 1:
ORA-00060: deadlock detected while waiting for resource
--SESSION_A requesting lock on row, held by SESSION_B causing deadlock.

After you get the ORA-00060 error the statement update child set childid = 1001 where childid = 200; is rolled back.. but SESSION_B is still waiting for the lock on the Parent table to be released.

So when your application get the ORA-00060 or any deadlock exception in any other database, explicitly rollback your transaction (not just the current statement) so that all the changes made in the transaction and all the locks held by the transaction are released.