What is an Oracle deadlock?

Whenever you have competing DML running against the same data, you run the risk of a deadlock.  This deadlock condition is an age-old issue known as the "perpetual embrace"!  The doc note that a retry may work:

ORA-00060: deadlock detected while waiting for resource

Cause: Transactions deadlocked one another while waiting for resources.

Action: Look at the trace file to see the transactions and resources involved. Retry if necessary.

Deadlocks in Oracle result in this error:

ORA-00060: deadlock detected while waiting for resource

If your ORA-00060 is caused by competing resources, the perpetual embrace happens when the aborted task (Task B) attempts to lock a row which is being held by another task (Task A), which, in-turn, is waiting for task B to release a lock.  To prevent a perpetual wait, Oracle aborts the transaction that caused the deadlock.

See my notes here on resolving the deadlock detected error.

Resolving Oracle deadlocks

There are several remedies for resolving aborted tasks from deadlocks:

• Tune the application - Single-threading related updates and other application changes can often remove deadlocks.  Re-scheduling batch update jobs to low-update times an also help.
   
• Add INITRANS - In certain conditions, increasing INITRANS for the target tables and indexes(adding slots to the ITL) can relieve deadlocks.
   
• Use smaller blocks with less data - Since the deadlock contention is at the block-level, consider moving these tables and indexes to a super-small blocksize (create a db2k_cache_size), and using a high PCTFREE to space-out the data over MORE blocks.

Inside Oracle deadlock machinations

Deadlock Detection

Transaction deadlocks occur when two or more transactions are attempting to access an object with incompatible lock modes. The following script can be used to identify deadlocks in the database. The query depends upon objects that are created by the script $ORACLE_HOME/rdbms/admin/dbmslock.sql. Log on as SYS or with SYSDBA authority and run this script in all databases. You may have to run the deadlock monitoring script below numerous times before you identify the transaction that is causing the problem.

The Enqueue Deadlock Per Sec Oracle metric is the number of times per second that a process detected a potential deadlock when exchanging two buffers and raised an internal, restartable error.

Avoiding Deadlock Conditions

A deadlock can occur whenever multiple users are in a waiting pattern for data locked by each other. Deadlocks prevent some transactions from continuing to work. In the event of deadlock, Oracle writes the message and error in the form of an ORA-60 error to the Oracle alert.log file. The following diagram illustrates the perfect storm condition that causes a deadlock or deadly embrace to occur within Oracle .
 
Deadlock problems have a similar root cause as that found with basic locking issues with Oracle which is the result of poor database application design. To resolve deadlock conditions with Oracle, the DBA needs to work together with the developer and software engineering team to modify or rewrite the database application code so that such deadlocks do not reoccur.

Lock Contention Issues and Solutions

After the database administrator has exhausted possibilities to visit the design of the database application with the development team, the next step is to perform further analysis to solve lock contention issues.

Oftentimes, the lock issue is the result of a zombie batch process or hung database session which has placed an exclusive lock on a specific row or table, thereby blocking access to the data from other users.

The simple solution to this type of problem is to identify the particular user and session causing the blocking condition and then to contact the user so that the session can be killed using the alter system kill session 'sid,serial#' immediate command from within SQL*Plus. In the previous section, numerous locking scenarios and potential solutions were covered.

See my related notes on deadlocks here: