What is a deadlock and Explain

A deadlock is a condition where two or more users are waiting for data locked by each other. Oracle automatically detects a deadlock and resolves them by rolling back one of the statements involved in the deadlock, thus releasing one set of data locked by that statement. Statement rolled back is usually the one which detects the deadlock. Deadlocks are mostly caused by explicit locking because oracle does not do lock escalation and does not use read locks. Multitable deadlocks can be avoided by locking the tables in same order in all the applications, thus precluding a deadlock.
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shiju k daniel

  • Nov 21st, 2005
 

System waiting for an event that may or may not happens and can be rectified using round robin algorithm which implemented internally by operating systems

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satyam_Ora

  • Aug 17th, 2007
 

 Deadlock occurs when two or more users are waiting for data locked by each other. When this happens, these users are stuck (deadly embraced) and cannot continue processing.

Oracle automatically detects deadlocks and resolves them by rolling back one of the statements involved in the deadlock, thus releasing one set of data locked by that statement. The session that is rolled back will observe Oracle error:
 ORA-00060: deadlock detected while waiting for resource. Oracle will also write out a trace file with detailed information to the database's UDUMP directory.

Multi-table deadlocks can be avoided by locking tables in same order (in all applications), thus preventing a deadlock condition.
For example, session1 lock table: emp then dept; session2: emp then dept. If this is not possible, your application should check for ORA-60 errors and restart the rolled back transactions.

Regards,
Satyam

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