![]() ![]() ![]() The above result shows that the transaction in connection 1 first gets into a deadlock situation however it runs in the second try because the transaction in connection 2 has already finished execution. Replace the query in connection 2 with the one specified above and re run the queries in connection 1 and 2 in parallel. In their, if the error number is 1205 which specifies a deadlock the transaction will run again else if it’s some other error stop the loop, output the error and rollback the transaction. If the transaction errors out, it will get into the catch statement. If the transactions runs successfully first time without any error, set = 0 and get out of the loop. Begin a transaction and put in the deadlock code. At first we specify number of retries needed and start a while loop for the retries. SELECT = ERROR_MESSAGE() = ERROR_SEVERITY() = ERROR_STATE() Print 'Deadlock Occured - Retrying the transaction!!!' it's a deadlock exception - 1205 specifies deadlock exception. run the code untill all retries are done Let’s now rewrite one of the transactions in a way that when a deadlock is detected, the transaction is rerun. UPDATE Person.Person SET Suffix='Mr.' WHERE BusinessEntityID=1 Why does SQL Server chose a clustered index scan just for this particular SQL (steps 7 and 8), this is because with this small amount of data the index IXCityNotCoveringIndex is not selective enough and that’s why SQL Server chose to do the scan. UPDATE Person.Person SET Suffix='Dr.' WHERE BusinessEntityID=1 It creates an infinite wait - and then a deadlock. The below transactions when run in parallel in two different connections create a deadlock situation. ![]() Let’s apply the above approach on the deadlock example I shared in my earlier blog on cyclic deadlock Setting Deadlock Priority As stated previously, SQL Server normally. The number of retries can be specified in the code. Another frequent cause of deadlock is transactions that run at a higher isolation. Transaction retry takes advantage of this and reruns the deadlocked transaction. The locks are being acquired and released so fast that same locking situation might not exist the other minute. The Deadlock occurs because of incompatible locks. The “victim” session is chosen according to the session’s deadlock priority.Transaction retry is one of the easiest way to get control of SQL Server Deadlock Resolution, however it might get tedious depending on number of deadlock an application is experiencing. The value LOW corresponds to ?5, NORMAL (the default value) corresponds to 0, and HIGH corresponds to 5. There are 21 different priority levels, from ?10 to 10. You can affect which transaction the system chooses as the “victim” by using the DEADLOCK_PRIORITY option of the SET statement. (The other transaction is executed after that.) A programmer can handle a deadlock by implementing the conditional statement that tests for the returned error number (1205) and then executes the rolled-back transaction again. If both transactions in Example 13.5 are executed at the same time, the deadlock appears and the system returns the following output:Īs the output of Example 13.5 shows, the database system handles a deadlock by choosing one of the transactions as a “victim” (actually, the one that closed the loop in lock requests) and rolling it back. Therefore, Example 1 below uses the WAITFOR statement to pause both transactions for ten seconds to simulate the deadlock. The parallelism of processes cannot be achieved naturally using the smallsampledatabase, because every transaction in it is executed very quickly. (In general, several transactions can cause a deadlock by building a circle of dependencies.)Įxample 1 below shows the deadlock situation between two transactions. The first transaction has a lock on some database object that the other transaction wants to access, and vice versa. A SQL Server deadlock is a special concurrency problem in which two transactions block the progress of each other. ![]()
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