![]() The reason this occurs is SQL Server detected two sessions were competing for the same locks which would cause the two sessions to hang indefinitely. The above statement also completes without issue.Īt this point SESSION 63 will hang because it is waiting on SESSION 60 to commit or rollback the transaction on AccountNumber AW00012597.Īt this point SESSION 63 will receive a deadlock error. ![]() The above statement completes without issue. In this example I will show how two records (AccountNumber AW00012597 and AW00012596) are updated by two different sessions which results in a deadlock. This is pretty much the textbook explanation of a deadlock. Two sessions attempting to lock same rows in different order Overly restrictive transaction isolation levels.Two or more sessions are performing full scans and locking pages.Two sessions are attempting to lock the same rows but in different orders.There are several different scenarios I see that cause deadlocks: What is a deadlock?Ī deadlock is most easily described as what occurs when two sessions request a resource that the other session has, but each session is already holding a resource that the other session has a lock on. I’m not going to go into every single detail of reading the deadlock graph however, I am going to show you what you need to look for to quickly get to a fix. I’ll also explain how you can fix the most commonly occurring ones. I will also show the three most common scenarios that cause them. In this post I will explain what a deadlock is. I consider this to be a hack for a real problem that can be addressed in most cases. In a lot of cases the DBA isn’t totally aware of them because applications use retry logic for deadlocked transactions. A common problem I see in systems is deadlocks.
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