![]() ![]() You can find out which tables and queries where involved the deadlock process, which process was killed off, and which locks caused the deadlock to occur. It takes some skill to read the XML from a deadlock graph, but it contains a great deal of information about what happened. Once you start seeing deadlocks, you’ll want to start gathering more information about them. If you don’t have any other monitoring tools in place, Extended Events are a great place to start. Yesterday's deadlocks are tomorrow's news!Just like newspapers help us find out what happened yesterday, Extended Events provide a great way to investigate deadlocks that have already occurred. It’s possible that to build a set of monitoring tools that log all deadlocks to a file and then analyze that file after the events happen. Event data can be held in memory (which is the default) or written out to a file. Although they’re a relatively new feature to SQL Server, they first appeared in SQL Server 2008, Extended Events already provide an incredibly rich set of tools for monitoring SQL Server. Extended Events are a great source of data for analysis. ![]() Once deadlocks show up, your database administrator might reach for a script to pull deadlocks out of Extended Events. You can enable several trace flags to detect deadlocks (trace flag2 12), but they output the deadlock to the SQL Server error log and produce output that is difficult to read and analyze. Unfortunately, if the cause of the deadlock is still running, odds are that your transaction will fail. SQL Server is telling you exactly how to solve the problem – re-run your transaction. Another sign of a deadlock is an error (error 1205 to be precise) and a very helpful error message: Transaction (Process ID %d) was deadlocked on resources with another process and has been chosen as the deadlock victim. That’s the first sign of a deadlock, but that’s also the first sign of a lot of other problems. What is the first sign of a deadlock? Queries that should be fast start taking a long time to respond. The query that’s eliminated is called the deadlock victim. In order for the database to keep responding, one of these queries has to go. In order for either query to finish, they need access to the other query’s resources. ![]() Q2 won’t release its lock on tB until it can get a lock on tA. Q1 won’t release its lock on tA until it can get a lock on tB. Q1 must wait for Q2 to release its lock before Q1 can finish. So far, there’s nothing out of the ordinary happening. The first query, Q1, takes an exclusive lock on tA at the same time that the second query, Q2, takes an exclusive lock on tB. Assume that there are two tables, tA and tB. What is a Deadlock?Ī deadlock occurs when two queries need exclusive access to different tables and each query is waiting for the other to finish. The next thing you know, the office looks like Lord of the Flies. Developers will ask the DBA to fix the problem, DBAs will push the problem back on developers. Users will complain about the app being slow or broken. Deadlocks can kill an application’s performance. ![]()
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