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Oracle Deadlocks

Oracle Database Tips by Burleson Consulting

June 17, 2015

Question:  I understand that Oracle has locks and that it is possible to get an Oracle deadlock, where one task aborts.  I also understand that there are transient deadlocks, where a task waits for a resource to become free.  What are the different types of Oracle deadlocks, and how do I find the source objects for an Oracle deadlock?

Answer:  Oracle has a row level locking mechanism that generates a lock on a resource when a transaction starts.  Any transaction beginning after that point that requires the locked resource will have to wait for the completion of the transaction that generated the lock.  When the transaction commits or rollbacks, it will release the exclusive lock.  Once the lock is released, the next transaction requests a lock so that it may proceed.

There are several kinds of deadlock waits in Oracle:

  • The deadly embrace Oracle deadlock occurs when a second transaction asks for resources that another transaction is currently holding, and this other transaction is waiting for a block held by the second transactions.
  • In a buffer busy wait deadlock, a transaction deadlocks while waiting for a buffer to become available.  This occurs when an Oracle session needs to access a block in the buffer cache but cannot because the data block in the buffer is locked.   This can happen if the block is being read into the buffer by another session, or if another session has the buffer block locked in a mode that is incompatible with the waiting sessions request. For more information on identifying the cause of buffer busy waits, you can read more in this helpful article.
  • SQL has a lock for update clause that locks a set of rows exclusively for the session that creates the lock.  This type of lock is especially dangerous when a transaction aborts and a zombie process continues to hold row locks.  There are better locking strategies available.


 
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