The difference is that a lock conflict happens before the transaction is committed, whereas an update conflict happens during the commit, as the database engine checks the rows in the version store against the committed rows and finds a mismatch. The Lock waits per second counter tracks the number of times per second that SQL Server is not able to immediately retain a lock for a resource. Using row versioning, you can get an update conflict, as I did. With transactional isolation, a deadlock is detected when two or more transactions have a lock conflict since they want to acquire one or more incompatible locks. This is similar in some ways to a deadlock that can occur under pessimistic, transactional isolation. Since there is no way for the database engine to know who should win (that is actually a business decision) it had to kill one transaction. When trying to commit the version of the row in the second update, SQL Server noticed that the first transaction was also trying to update the same row. “Transaction aborted due to update conflict.” The problem is that both queries tried to update the same row. In this article, Gail Shaw looks at how you can identify common types of deadlock, the difference between a deadlock and severe blocking, and how to avoid and fix the most common deadlock types. The most important part is the first sentence. When a SQL Server instance deadlocks, it can be anything from minor irritation to something far more severe. Retry the transaction or change the isolation level for the update/delete statement Orders’ directly or indirectly in database ‘MyOrders’ to update, delete, or insert the row that has been modified or deleted by another transaction. You cannot use snapshot isolation to access table ‘Orders. Snapshot isolation transaction aborted due to update conflict. The first one completed normally, but look at what happened to the second one! The full text of the error message is: to-simulate-a-deadlock-in-sql-server-in-a-single-process) - spsimulatedeadlock.sql.
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