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Oracle kernel expert explains Oracle savepoints

Scott Martin, (www.tlingua.com) an MIT graduate and former member of the Oracle kernel group, recently clarified the internals of Oracle row locks and savepoints:

 

Row locks are NOT released by SETTING a savepoint.  Row locks are release by one of three events - commit, rollback, or rollback to savepoint.  My argument is that Oracle does not handle the latter well.

 

1.      If transaction A updates row 1, sets a savepoint, and updates row 2 (but does not as yet commit).  Then transaction B wishes to update row 2.

 

2.      Transaction B will correctly block on the commit or rollback of transaction A. 

 

3.      However, if transaction A does a rollback to savepoint, it will continue to have hold a lock on row 1 (but not row 2). 

 

4.      In fact a third transaction can now update row 2 (as it's not locked by

5.      transaction A). 

 

6.      However, our poor transaction B, is still waiting (incorrectly) for transaction A to commit or rollback.

 

The problem is that Oracle provides no way of waiting on a row - you can only wait on a transaction - and sometimes transactions (through the use of rollback to savepoint) release rows WITHOUT committing or aborting. 

 

Imagine what could happen when by issuing a savepoint the lock of an standing transaction would be released, and another transaction would change 'my' row, and then I do a full rollback. After my rollback the record is not in the state as I found it when opening my transaction - and that seems to be against the nature of an atomic transaction.

 

In my opinion there is no such thing as the false deadlock.
 

 

 
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