In addition, a deadlock that resembles the following will occur when there are two sessions concurrently insert their rows to a Clustered Columnstore Index (CCI). How can I handle a PC wanting to be a "twist" villain? experts to answer whatever question you can come up with. User contributions are licensed under, Mythbusting: Concurrent Update/Insert Solutions, https://dev.mysql.com/doc/refman/8.0/en/innodb-locks-set.html, If the key exists and two sessions try to update simultaneously, they'll. Stack Exchange network consists of 176 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Recently I have been encountering deadlocks occasionally when I try to insert data concurrently on two tables. I have no idea if this will work for you, because SQL Server is closed source, so I cannot see the source code, and as such I … Podcast 285: Turning your coding career into an RPG, Creating new Help Center documents for Review queues: Project overview, Feature Preview: New Review Suspensions Mod UX, 2020 Community Moderator Election Results. What does Isaiah 43:10 mean by there being no other gods before or after the Lord? First, let us start with a scenario where we always get a considerable amount of deadlocks. By using our site, you acknowledge that you have read and understand our Cookie Policy, Privacy Policy, and our Terms of Service. Making statements based on opinion; back them up with references or personal experience. Phantom rows can't exist here, so why use an isolation level that exists to prevent them? Alter both procedures, rerun the loop, and see that you no longer have deadlocks: Instead of invoking sp_getapplock, we can modify the following table: Once we have this table created and populated, we can replace the following line. Asking for help, clarification, or responding to other answers. Bulk insert will be converted to trickle insert under memory pressure. How to prove episodes of academic misconduct? Transaction 1 will lock TableA (SaveTableA), Transaction 2 will lock TableB (SaveTableB), Transaction 1 waits for Lock on TableB to be release to do the update/insert, Transaction 2 waits for Lock on TableA to be released to do the update/insert. It's a merge: an insert or an update. If you get less than 20, increase the number of iterations. Given an 'orrible mess that might take time to pick apart, it's a handy trick to serialise a process that's deadlocking. On a single transaction I am trying to complete data insertion on tableA and Table B by calling respective stored procedure (SaveTableA and SaveTableB). If I understand correctly; without it, transaction won't be collision and/or deadlock proof. Database Administrators Stack Exchange is a question and answer site for database professionals who wish to improve their database skills and learn from others in the community. How do scientists know the min limit of temperature is -273 degree celsius?
Here is a variation of Mark's answer - because you are returning the new ID value as a recordset, you can do away with the scalar variable entirely, no explicit transaction should be necessary either, and I would agree that messing around with isolation levels is unnecessary as well.
Anyway, this index IS used by the actual execution plan: I've taken the advice that @AaronBertrand gave and modified it slightly. You can profile it with SQL profiler or just add actuall execution plan and SET STATISTICS TIME ON. Will you have a very big system, with hundreds of users coming through a web interface and all adding thousands of rows, all at the same time? Mike Defehr showed me an elegant way to accomplish this in a very lightweight way: (For completeness, here is the table associated with the stored proc). Should indeed work but you're using SERIALIZABLE where it's not applicable. it is very easy to have sp_getapplock become a deadlock victim, but not if you begin transaction, call sp_getapplock once to acquire an exclusive lock, and go ahead with your modification. How to automatically use typewriter font in arrays? We do not have any deadlocks in our system at all, which is great for our work-life balance. Can a landlord dictate what I put online? We've got lots of great SQL Server From this reference I understand that the lock here is on the Btree of the Index based on the reference https://technet.microsoft.com/en-us/library/ms189849(v=sql.105).aspx and the deadlock graph above. But, should it be the first choice for a case like this which is easily understood and can (perhaps should) be dealt with by standard locking mechanisms? Without UPDLOCK/SERIALIZABLE: empty table, mostly inserts | mostly updates, With UPDLOCK/SERIALIZABLE: empty table, mostly inserts | mostly updates. The database itself is configured with READ_COMMITTED_SNAPSHOT = 1. to consider including application design, database size/configuration and server specification. In the best case, if you are updating a row that already exists, it will be more efficient to only locate that row once. From the above graph I infer that the SaveTableB from one thread is victim while other instance of same procedure on different thread was holding lock on primary key of TableA.
Recently I have been encountering deadlocks occasionally when I try to insert data concurrently on two tables. You can rerun the stress test, and see for yourself that we have no deadlocks. Occasionally the stored proc gets a deadlock - I believe I've built an appropriate error handler; however I'm interested to see if this methodology works as I think it does, or if I'm barking up the wrong tree here. And, hey, another shoutout to Paul White (sql.kiwi | @SQK_Kiwi), because he is so excellent at making hard concepts easy to understand and, in turn, explain. Using our current understanding of science, is it possible for biologically plausible animals to be resistant to our current military weapons? If running two querys that is in the opposite order can cause deadlock. You're not just inserting, as you say in your question. So, unless the overall load on your server gets tremendous, which introduces its own problems, the multithreaded operations should be just fine. So about same time as the whole procedure runs. Curious to understand that if it is possible to get deadlocks in this kind of scenario mentioned, trying to understand on how to approach it so that I will prepare myself to solve it. Modern IDEs are magic. — direct from https://dev.mysql.com/doc/refman/8.0/en/innodb-locks-set.html, Design and content © 2012-2020 SQL Sentry, LLC. Combining this logic with the high-level operations that would have to happen in the database, it is slightly simpler: In this case, one path only incurs a single index operation. Viewed 6k times 6. Given two known statements, how can I replicate deadlock? Thanks for your quick response. The UPDATE performs essentially the same check. Make only one stored procedure call, and increment the counter by 20. What does Dumbledore mean when he talks about Merope's magical powers? Is there a Linear Programming Library that natively supports fractions instead of floating point arithmetic? Msg 1205, Level 13, State 134, Line LineNumber. To learn more, see our tips on writing great answers. That can get messy. Of course, this can significantly slow down modifications. KB4506912 - FIX: Concurrent inserts into a CCI can cause deadlock under memory pressure in SQL Server 2016 and 2017 Symptoms. How did German unification affect existing sentences for criminal convicts? Thanks for response from @Colin'tHart. What's the red, white and blue (with stars) banner that Trump was using on the stage in his election campaign? I got fired on my last day; is that possible? I tried several ways to make it fail by injecting artificial delays in between the two statements for two competing sessions, but couldn't – and didn't see any material difference between applying the hint and not, except that without the hint there are 1 or 2 additional key RangeX-X locks. I believe that's automatic in mysql/mariadb: @user824910 Well its lock from the MERGE statement starts until it ends. SQL Server supports a range of concurrency control. Sure, but like MERGE, if that syntax existed in SQL Server, I expect you would still need the right locking/isolation semantics because that is still fundamentally two distinct operations (even though the syntax makes it seem like one). The best way to create a deadlock is to do the same thing in a different order. Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. building an execution plan takes too long on sql server. More importantly, performance aside, unless you both use an explicit transaction and elevate isolation level, multiple things could go wrong when the row doesn't already exist: That last one is the worst, IMHO, because it's the one that potentially corrupts data. on further analyzing the graph, two statements were issued to execute SaveTableB (while one of them is victim) procedure on concurrent threads, hope you could help me understand why there is a lock on PK of TableA? Then reccommend "create. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. In both cases, the writer who won the race loses their data to anything the "loser" updated after them. How can I overcome this? Ask me how I know. How to Resolve deadlock on concurrent MERGE on two tables, https://technet.microsoft.com/en-us/library/ms189849(v=sql.105).aspx, technet.microsoft.com/en-us/library/ms178104(v=sql.105).aspx, https://blogs.msdn.microsoft.com/dbrowne/2013/02/25/why-is-tsql-merge-failing-with-a-primary-key-violation-isnt-it-atomic/. I will point to this post every time I spot the IF EXISTS pattern in the wild. I'd have to think about any possible ways this could fail as written, and I think the transaction wrapper protects the insert case without the hints (unless you also have concurrent sessions that process in the reverse order, inserts first; but you should never do this with multiple rows, as I explained in the post). This issue is fixed in the following cumulative updates for SQL Server: Cumulative Update 8 for SQL Server 2016 SP2.
Assuming you are sending a set of rows in using something like a table-valued parameter, you would update using a join, and then insert using NOT EXISTS, but the pattern would still be equivalent to the first approach above: If you're getting multiple rows together in some other way than a TVP (XML, comma-separated list, voodoo), put them into a table form first, and join to whatever that is. Michael Swart also treated this subject several years ago in Mythbusting: Concurrent Update/Insert Solutions, including the fact that leaving the initial logic in place and only elevating the isolation level just changed key violations to deadlocks.
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