QUERY DEADLOCK SQL SERVER PATCH

We are going to use the following two tables to understand deadlock. Let us understand the Deadlock in SQL Server with Examples. Understanding Deadlock in SQL Server with Examples. The process that is chosen as the deadlock victim will give the following error. As a result, other processes can move forward. When deadlocks occur in SQL Server, then SQL Server chooses one of the processes (transactions) as the deadlock victim and then rolls back that process. In such cases, neither of the transactions (processes) can move forward resulting in a deadlock. At the same time, Process B needs the resource Table A which is already locked by Process A. Now to complete its execution, process A needs the resource Table B which is already locked by Process B. Process A started and it acquires a lock on table A and in the meantime, Process B started and it acquired a lock on Table B. We have two tables such as Table A and Table B in the database. when a deadlock occurs in a database.Īs shown in the above diagram, we have two processes (process means transactions) such as Process A and Process B.

If you are confused, then just have a look at the following diagram which explains the above points i.e. In such cases, neither of the processes can move forward, as each process is waiting for the other process to release the lock resulting in a deadlock in SQL Server. When a deadlock occurs in SQL Server?Ī deadlock occurs in a database when two or more processes have already a resource locked, and then each process wants to acquire a lock on the resource that the other process has already locked.

Before going to understand what is Deadlock in SQL Server, let’s first discuss when a deadlock occurs in SQL Server. Please read our previous article where we discussed Concurrency Problems in SQL Server with examples. In this article, I am going to discuss Deadlock in SQL Server with examples. Data Structures and Algorithms Tutorialsīack to: SQL Server Tutorial For Beginners and Professionals Deadlock in SQL Server with Examples.

QUERY DEADLOCK SQL SERVER PATCH

So why didn't it complete?īefore I suggest something like adding WITH(NOLOCK) to every query in Excel files, which is just a patch and not a solution I'd like to find out why this happened so that we can avoid this in the future. If I can trust the results of sp_whoisactive it wasn't waiting on anything else. And since running the query separately only took a few seconds I don't understand how it was running for over 12 hours. Now as far as my limited understanding of locks goes if the query executed by session id 598 would actually lock the table it should only do so for the duration of the query. When googling I'm finding a lot of resources about Excel actually locking tables. Obviously this is extremely bad practice as the connection string can be found inside, but as with everything legacy that's just the way it is now until it is fixed. Apparently we have lots of Excel files floating around which run queries against our SQL database. I asked around and this query is executed by an Excel file. Afterwards, executing that same query that was keeping the table in a deadlock in a separate window in SSMS only took 2 seconds. I checked sp_whoisactive for answers and found the following.Ī simple SELECT on this table with session id 172, was waiting for a DELETE query on this table with session id 478, which was waiting for an INDEX REORGANIZE on this table with session id 207, which was waiting for a simple SELECT on this table with session id 598.Īfter killing session 598 everything completed immediately. I ran into an issue where no queries could be executed at all against a single table in our database.

Query deadlock sql server