How to Check and Resolve a Deadlock in SQL Server

By Staff Contributor on March 3, 2023

The dreaded ‘Event ID 1205’ – your transaction has been deadlocked. But what does this mean exactly, and what steps can you take to prevent this from happening again in the future?

To begin, let’s look at how a deadlock occurs. When SQL Server tasks are performed simultaneously and block each other. As a result, it’s known as a deadlock. In the simplest terms, a SQL Server deadlock occurs when there’s a competition for resources, such that when you send a query, the process of retrieving data may be blocked.

Resolving these blocks is key to improving both SQL Server performance and the performance of your entire system. To resolve deadlocks effectively, you will need a detection tool that allows you to identify problems before they affect your overall performance.

Because a SQL Server deadlock prevents queries from retrieving data, you will typically see a drop in function and efficiency. Fortunately, there are plenty of tools that will help you check deadlocks in SQL Server and help you to identify the most logical ways to resolve them.

Let’s look at how a SQL Server deadlock can happen and how you can identify and resolve them as quickly as possible.

What Is a Deadlock in SQL Server? 

A SQL Server deadlock isn’t inherently bad. By freezing a “victim” process, deadlocks allow your server to continue operating during resource scarcity. Query completion is possible even when two queries compete. Plenty of database servers operate on a lock-based system, and the SQL Server is no exception.

Despite the practical intentions of a deadlock in SQL Server, deadlocks can significantly harm your productivity if left unaddressed. Protecting your data with a deadlock requires the SQL Server to identify a “victim” process, which will stall until the running process is completed. SQL Server uses locking mechanisms to prevent the victim process from continuing until the active process is complete.

Once the victim process is designated, it is “killed,” a function much less serious than it sounds. Killing a victim process prevents it from being completed, and your SQL Server will display an error message that calls out the deadlock. Quickly identifying a deadlock in SQL Server and then effectively addressing it is crucial for maintaining an efficient SQL Server. As a database admin, the specific types of deadlocks you’ll encounter include:

Lookup deadlocks

Seeking a range of queries can lead to a lookup deadlock, which can be avoided with the proper techniques. By scanning a range of queries or by reading a snapshot of the queries, you can find a query without risking a lookup deadlock.

Order of operations deadlocks

This is the most basic type of SQL Server deadlock, which involves a competition for resources during a process in the SQL Server. Avoiding order of operations deadlocks requires you to identify which processes need which resources before one of the processes becomes a victim to the competition.

Parallelism deadlocks

Parallelism deadlocks occur when your query plan allows for parallel processes within the server. When parallel processes run on different threads, the same deadlock can occur simultaneously in both threads.

Fortunately for server admins, all three types of deadlocks are preventable with the correct server resource apportionment, and they can usually be fixed quickly once they’re identified. To avoid downtime and stalled operations, investing in a database management tool that explicitly addresses SQL Server deadlocks can be worth your time and money.

How Can Resolving Deadlocks Help Your Business?

Even though deadlocks help to prevent major resource shortages in your SQL Server, they can often delay IT performance once a process has become a victim, which can potentially cause efficiency issues for your business. By “reviving” a deadlock victim promptly, you can get your server up and running and help avoid business slowdowns.

Before addressing a deadlock, it’s essential to understand why fixing it is worth your time. Some SQL management software includes specific reports that detail the impact of a deadlock on your server performance. This allows you to calculate resource costs associated with deadlocks and downtime due to resource competition.

SQL management software that includes deadlock summary reporting is a valuable way to identify your business savings from eliminating deadlocks. Deadlock reports, combined with effective alerting and troubleshooting features, allow you to streamline SQL Server management and understand precisely how your business benefits.

Best Tools for Resolving a Deadlock in SQL Server

With the right software, deadlocks in SQL Server don’t have to take a toll on your business. There are a few SQL management software options that I find to be incredibly effective for identifying and addressing deadlocks.

First is SolarWinds® Database Performance Analyzer (DPA), which offers deadlock analysis within its broader suite of SQL Server monitoring capabilities. With DPA agentless monitoring, you can identify victims and analyze key deadlock metrics. You can view a wide range of system metrics from the DPA dashboard, analyze performance tuning advisors, and highlight queries with higher execution waits than expected with its anomaly detection engine.

Quest Spotlight is another popular program for SQL Server management. Spotlight generates workflows to help you visualize SQL processes and to recognize deadlocks when they occur. Like SolarWinds, Quest offers helpful notifications when resource competition affects server performance.

For a complete approach to SQL Server management, there is SolarWinds® SQL Sentry® — with consistently high customer ratings and a focus specifically on SQL Server management. A significant advantage of SolarWinds products is the user-friendly interface, with graphics that visualize resource consumption and nodes. With color-coded labels, SQL Sentry allows you to view deadlock victims for easy identification. SQL Sentry is also helpful because it helps you to identify deadlocks based on query analysis without the need to install agents. These are some of the most effective options for identifying a SQL Server deadlock, rolling back a SQL deadlock query, and saving money. Deadlocks are a natural part of lock-based databases like SQL Server — and addressing deadlocks is central to effective database management.

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