Here’s what our MERGE statement looks like in the execution plan. With Read Committed, locks are placed on objects that you modify, in order to prevent other processes from reading something that you may or may not choose to commit later on.īut Serializable places locks even on things you look at – because if you’ve looked at it, SQL Server makes sure that it looks the same if you come back and look at it later on. To maintain an isolation level’s promise, and certainly that of the Serializable isolation level, SQL Server needs to place locks. If you want to dig a little deeper, check out Klaus Aschenbrenner’s posts on Lock Escalation, Update Locks, and Intent lock. “I promise that what you saw will still be true if and when you look again.”ĭisclaimer: Simplifications ahead. The “ Serializable” isolation level is the strictest of them all.It roughly translates to “To somebody, this may have been accurate at the time, but we don’t know if that transaction will even be committed.” You may have played around with “ Read Uncommitted” (the NOLOCK hint).The default “ Read Committed” isolation level, basically means: “I promise that what I showed you was accurate and committed at the time.”.I have a whole post on isolation levels, but you can think of isolation levels as the server’s “SLA”, a promise if you will, to the client application. The HOLDLOCK hint tells SQL Server to use the Serializable transaction isolation level. One of the generally accepted recommendations to avoid most of these issues is to add a WITH (HOLDLOCK) hint to the target table. For a very simple use-case like the one we’re working on here, we’ll probably be fine. My own takeaway from this post is that if you’re on an older SQL Server version, or if you do funky things like filtered indexes or indexed views, you should carefully review the list if there are issues that might impact you. Years ago, Aaron Bertrand compiled an excellent blog post with all the reported issues with the MERGE statement (there are many), and a surprising amount of them are marked as “Won’t fix” by Microsoft – either because it’s “By design” or because it’s just plain-old not important enough for Microsoft to fix them. You’ll notice that there’s a HOLDLOCK hint on the target table. Thanks to all for the attention.MERGE INTO dbo.ApplicationState WITH (HOLDLOCK) AS t Monday, Novem12:11:02 PM - Viacheslav Maliutin Plerase share If a Table Has More Than One Foreign Key? how did you fix all these. It is going to be published soon.įriday, Ma1:40:01 PM - Pradeep Kumar Giri Here is the second one explaining the issue with multiple foreing keys. Pradeem Kumar Giri, thank you for the reading my article. Saturday, Ma8:25:51 AM - Viacheslav Maliutin i.e reteriving data from more tables concurrently please go for a little bit more complex case. I would be interested how you resolved a table with multiple foreign keys. The execution plan for both steps 7 and 8 will now look like this. To get only one city it is better to use a seek in the index for the row id and These dummy cities create statistics that tells SQL Server that if a client wants Script 10 below will show us all the locks have been acquired in the database. In order to understand the reason of the deadlock we need to take a look at what I will explain this in detail and how this solved this problem. To solve the deadlock you just need to cluster the foreign key “FK_Country”. Understanding and Solving the SQL Server Deadlock Issue Now let me solve the problem with a good problem explanation and a good solution. To tell our developers, but unfortunately they can’t change the way the SQL Why we have the problem I am talking about. Actually, it never writes the “*”, but it picks up everyĬolumn and that’s why my scripts 8 and 9 are written that way and that’s Yes, this will work inĪ perfect world, but when using an ORM it writes the SQL like “SELECT * FROM With the “CityName” column in scripts 8 and 9. So, to not have a deadlock just replace the “*” symbol in the SELECT queries On the “CountryID” column and included the “CityName” column. You might say: “Hey, you created an index “ IX_City_Not_Covering_Index” In query window #1, select data from the City table for the USA records. This willġ9980000, -As Google just said and that's cool! Transaction, but not committing or rolling back the transaction. In query window #1 run the following code. The same T-SQL command with different data to show how the deadlock will be reproduced. In the scope of each transaction I will run Wanted to show that our system had many useful indexes and even with useful indexesĪs usual, to demonstrate a deadlock we need two connections to be opened. You might have noticed that I created an index on the “CountryID”Ĭolumn and included the “CityName” column. script 1 - Creation of County and City tables
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