Blocking vs. Deadlocks in Microsoft SQL Server (2024)

FAQs

How to check blocking and deadlock in SQL Server? ›

Resolving locking issues (and the related blocking) requires looking at the database query causing the locking, not looking at the query being blocked. Monitoring the SQL Server database reveals queries that cause excessively long locking and blocking.

To view deadlock information, the SQL Server Database Engine provides monitoring tools in the form of the system_health xEvent session, two trace flags, and the deadlock graph event in SQL Profiler.

To check blocking in SQL Server, you can use a query to identify the blocking SPID. All you need to do is use SQL Server DMVs and execute a query using the sys. dm_exec_requests DMV. This will return a list of the blocking SPID, any blocked SPIDs, and the wait type and duration.

Killing a Blocking Process

After you have connected, right click on the instance name and select 'Activity Monitor' from the menu. Once Activity Monitor has loaded, expand the 'Processes' section. Scroll down to the SPID of the process you would like to kill. Right click on that line and select 'Kill Process'.

However, you may be able to solve them by setting the query's MAXDOP to 1. This forces the SQL Server to execute the query in a serial plan, preventing a parallel deadlock. Alternatively, you can use OPTION (FORCE ORDER), which forces the query optimizer to use your specified order of joins.

A deadlock is a more severe form of blocking where two or more processes are each waiting for the other to release a resource, creating an endless loop that can't be resolved without external intervention. Deadlocks can halt operations, causing significant business impact.

A deadlock is resolved by aborting and restarting a process, relinquishing all the resources that the process held. This technique does not limit resource access or restrict process action.

Which best practice minimizes deadlocks in a database? ›

Use SQL Server Profiler to identify the cause of a deadlock. A deadlock occurs when there is a cyclic dependency between two or more threads, or processes, for some set of resources within SQL Server. Using SQL Server Profiler, you can create a trace that records, replays, and displays deadlock events for analysis.

To obtain information about locks in the SQL Server Database Engine, use the sys. dm_tran_locks dynamic management view.

In a database, a deadlock is a situation in which two or more transactions are waiting for one another to give up locks. For example, Transaction A might hold a lock on some rows in the Accounts table and needs to update some rows in the Orders table to finish.

You can check the status of the trace flag using the DBCC TRACESTATUS (1222, -1) command. You can see by the following results that the trace flag is enabled, and that it is enabled globally. You can turn off the trace flag any time by simply issuing the DBCC TRACEOFF (1222,-1) command.

Deadlock detection algorithms, such as the Wait-For Graph, are used to identify deadlocks, and recovery algorithms, such as the Rollback and Abort algorithm, are used to resolve them. The recovery algorithm releases the resources held by one or more processes, allowing the system to continue to make progress.

Open SQL server profiler > click on file > Open trace file > change the file type to XML > Select the XML trace file. Click on the deadlock graph event in the profiler displayed traces.