What Are SQL Server Deadlocks and How to Monitor Them

An SQL deadlock is one of the worst things that can ever happen to your database. It is sudden, disruptive, and affects database performance. Imagine a situation where two or more processes are stuck in a standoff, with each waiting for the other to release resources, and there is no way forward until SQL Server steps in to break the tie. The result? Aborted transactions, slow performance, and a frantic search for where it all went wrong. These are some of the implications of SQL Server deadlocks.

Whether you are a database developer, DBA, or data analyst, in today's fast-paced technological world, where actively working databases and secured information are key to business growth, you don't want anything messing with your application performance. As a result, understanding what SQL Server deadlocks are, why they occur, and how to monitor and avoid them is crucial.

This guide walks you through what SQL Server deadlocks are and how to guard your database against them properly. Also, you will find out the features that make the dbForge Studio for SQL Server your best bet for handling deadlocks.

Let's get right into it.

What is a deadlock in SQL Server?

A deadlock in SQL Server occurs when two or more tasks permanently block each other by each task having a lock on a resource that the other tasks are trying to lock. In other words, a SQL Server deadlock happens when two or more processes block each other by holding resources that the other needs.

To break the cycle, SQL Server automatically chooses one transaction as the "deadlock victim." Then that transaction is terminated and rolled back, and an error is returned to the application.

The most common SQL deadlock report is the error message error 1205, which appears like this:

Transaction (Process ID xx) was deadlocked on resources with another process  
and has been chosen as the deadlock victim. Rerun the transaction.

Beyond just the error message, other signs that your database is experiencing deadlocks include:

• Unexpected slowdowns in query performance
• Aborted or rolled-back queries
• Interrupted transactions that need to be retried

Causes of SQL Server deadlocks

Although deadlocks in SQL databases often seem like an unpredictable occurrence, there are a few things that can trigger this. Below is a breakdown of the common causes of SQL Server deadlocks and why they happen:

• Poor query design (different order of resource access): When queries access the same tables or indexes but in a different sequence, they can easily lock resources in a conflicting order, setting the stage for a deadlock.
• Long-running transactions: Long-running transactions keep resources locked for a longer period and increase the likelihood that another query will collide with the resources.
• Missing indexes and non-optimal indexing: Queries that are not properly indexed cause SQL Server to conduct full table scans. This action makes the server retain locks on more rows than are required and creates needless contention.
• High isolation levels (blocking readers/writers): Strict isolation levels, such as SERIALIZABLE, can cause processes to hold locks longer, increasing the likelihood of deadlocks.
• Poorly designed foreign keys or cascading actions: If foreign keys aren't properly indexed, cascading deletes or updates may unexpectedly escalate locking behavior.

When you recognize these patterns, you can ensure that your queries are well-optimized, refine indexing strategies, and adjust transaction handling to minimize deadlock risks.

Types of deadlocks in SQL Server

Deadlocks occur in different types and ways depending on how your database transactions interact with resources. Understanding the main types of deadlocks in SQL Server helps you recognize patterns and apply the right solutions. Below are different types of deadlocks.

Update deadlock

When two transactions try to update the same set of resources but lock them in a different sequence, this action triggers an update deadlock. For instance, one transaction updates Table A first and then proceeds to Table B, while another transaction updates Table B first and subsequently attempts to update Table A. Since each transaction holds a lock that the other needs, they end up waiting on each other indefinitely, causing SQL Server to declare a deadlock and terminate one of them.

Reader-writer deadlock

A reader-writer deadlock happens when a SELECT query (the reader) collides with an UPDATE or INSERT query (the writer). The problem arises because:

• The reader acquires a shared lock to ensure data consistency while it's reading.
• The writer needs an exclusive lock on the same resource to make changes.

If both operations touch the same row or page at the same time, the reader can end up waiting for the writer, while the writer is also waiting for the reader to release its lock, resulting in a deadlock.

For example, imagine two operations in a customer database:

• Process 1 (report generator): Runs a SELECT query to read customer details for a dashboard.
• Process 2 (customer update): Runs an UPDATE query to modify the same customer's contact information.

If both touch the same row at the wrong moment, the report query holds a shared lock while the update query requests an exclusive lock. At the same time, the update query may also block the reader if it has already locked related rows, causing a reader-writer deadlock.

Key/index deadlock

Not all deadlocks happen at the table level; some occur within indexes. Key/index deadlocks happen when two queries try to access overlapping key ranges inside an index.

For example,

• Transaction A scans rows where CustomerID BETWEEN 1 AND 100.
• Transaction B scans rows where CustomerID BETWEEN 90 AND 200.
• Because their ranges overlap (90-100), SQL Server may lock those rows or index pages in a way that prevents either transaction from finishing.

This type of deadlock is prevalent when indexes are missing or poorly designed, because SQL Server has to scan larger ranges and lock more rows or pages than necessary. Non-clustered indexes that don't fully cover a query are also frequent culprits, since SQL Server may access both the index and the base table, increasing lock contention.

Example of an SQL deadlock

Here's a simple SQL Server deadlock example that demonstrates how two transactions can block each other.

Assuming you have a simple Accounts table:

CREATE TABLE Accounts ( 
    AccountID INT PRIMARY KEY, 
    Balance DECIMAL(10,2) 
); 

INSERT INTO Accounts (AccountID, Balance) 
VALUES (1, 1000.00), (2, 2000.00);

Now, imagine two different sessions (connections) running the following transactions simultaneously.

Session 1

BEGIN TRANSACTION; 

-- Locks AccountID = 1 
UPDATE Accounts  
SET Balance = Balance - 100  
WHERE AccountID = 1; 

-- Waits here if Session 2 already locked AccountID = 2 
UPDATE Accounts  
SET Balance = Balance + 100  
WHERE AccountID = 2; 

COMMIT TRANSACTION;

Session 2

BEGIN TRANSACTION; 
 
-- Locks AccountID = 2 
UPDATE Accounts  
SET Balance = Balance - 200  
WHERE AccountID = 2; 
 
-- Waits here if Session 1 already locked AccountID = 1 
UPDATE Accounts  
SET Balance = Balance + 200  
WHERE AccountID = 1; 
 
COMMIT TRANSACTION;

What happens:

1. Session 1 updates AccountID = 1 and holds a lock.
2. Session 2 updates AccountID = 2 and holds a lock.
3. Session 1 then tries to update AccountID = 2, but Session 2 is holding that lock.
4. Session 2 then tries to update AccountID = 1, but Session 1 is holding that lock.
5. Both transactions are stuck, waiting on each other.
6. SQL Server resolves the deadlock by terminating one of the transactions and declaring an error.

Now that you understand what a SQL Server deadlock is, its causes, types, and how it happens, let's explore how to find deadlocks in a SQL Server environment so that you can easily prevent this from affecting your database performance.

How to detect and monitor deadlocks in SQL Server

Knowing how to check deadlocks in SQL Server and monitor them is crucial in preventing them from affecting your database performance. Here is how to do this.

Deadlock graphs

A deadlock graph is a visualization tool that shows which processes are involved in a deadlock, the resources they hold, and the ones they are waiting for. Below is how this works.

• XML format: This refers to the raw data SQL Server generates when it captures a deadlock event (via Extended Events, trace flags, or Profiler). It contains detailed technical information in machine-readable form.
• Graphical representation: This refers to the visualization of XML data, typically rendered by tools such as SQL Server Management Studio (SSMS) or third-party solutions. It makes the deadlock easier to interpret by showing processes, resources, and the victim transaction in a diagram.

Deadlock graphs are the foundation for diagnosing locking conflicts in complex environments.

SQL Server tools

SQL Server includes several built-in options for capturing and analyzing deadlock events. These are:

• Extended events: The recommended and modern method for deadlock monitoring. The xml_deadlock_report event, as well as the default system_health session, automatically captures deadlock details in XML format.
• SQL Server Profiler: A legacy tool that can still capture deadlock events in real time. Profiler can also generate deadlock graphs, making it easier to visualize the blocking cycle.
• Trace Flags (1204, 1222): Trace flags can be enabled to log deadlock information directly into the SQL Server error log. This method is lightweight and functional in environments without active monitoring tools.

Detect and monitor deadlocks in SQL Server with dbForge Studio

While native SQL Server tools can be effective for handling SQL Server deadlocks, they often demand extra effort to configure, interpret, and maintain. However, dbForge Studio for SQL Server simplifies the process with built-in deadlock monitoring capabilities that provide clarity at a glance. With dbForge Studio for SQL Server, you enjoy the following:

• Visual deadlock graphs: To instantly identify blocked processes, locking chains, and the victim transaction automatically selected by SQL Server.
• Context-aware query monitoring: To see the exact SQL statements involved in the deadlock, together with their execution context.
• Historical data and reporting: To analyze recurring deadlocks over time, uncover patterns, and apply query or indexing optimizations.
• GUI-driven analysis: To get an intuitive visual environment that accelerates troubleshooting instead of digging into Extended Events XML or error logs.

With dbForge Studio, you get an integrated, developer-friendly environment that helps you monitor SQL Server deadlocks faster and resolve them before they escalate into critical performance issues.

How to resolve SQL Server deadlocks

Resolving deadlocks in SQL Server is a two-stage process. The first stage includes applying immediate remedies to keep your applications running, while the second stage involves implementing long-term solutions to prevent the deadlocks from happening again.

Let's explore these two processes to ensure your database is free from deadlock implications.

Short-term resolution

Generally, when a deadlock occurs, SQL Server automatically chooses a “victim” transaction to terminate. To minimize the disruption that can occur during this termination process, you can apply short-term strategies to fix the deadlock as it appears. These short-term fixes include:

• Retry transactions automatically: If a transaction is terminated due to deadlocks, you can implement retry logic with TRY...CATCH in T-SQL. This retries the query after a short delay.
• Monitor victims and affected queries: You can also monitor your queries to identify which transactions are frequently chosen as victims and analyze them. This helps prioritize which queries to optimize or restructure.

These short-term measures keep your application resilient while you work on permanent fixes.

Long-term fixes

The most effective way to rectify deadlocks in SQL Server is to address their root causes in query design, indexing, and transaction management. Here are some of the ways to do this:

• Consistent resource access order: Ensure that all transactions acquire resources in the same sequence (e.g., always update Table A before Table B). This prevents circular locking.
• Reduce transaction scope and time: Keep transactions as short as possible. Commit quickly, avoid unnecessary locks, and only include essential operations.
• Improve indexing: Use covering indexes and properly indexed foreign keys to minimize locking at the row or page level. Well-designed indexes reduce contention.
• Adjust isolation levels: Where appropriate, consider enabling READ COMMITTED SNAPSHOT or row versioning. These features reduce blocking and can significantly lower deadlock frequency.

By combining short-term fixes with long-term best practices, you can both resolve deadlocks in SQL Server when they occur and build an environment that prevents them from happening in the first place.

How to prevent SQL Server deadlocks

The most effective way to prevent deadlocks in SQL Server is to avoid them altogether. In other words, safeguard your database from falling victim to deadlocks.

How do you achieve this?

Adopting smart design principles, coding strategies, and ongoing monitoring can help you significantly prevent SQL Server deadlocks and reduce the risk of costly interruptions.

Let's break this down.

Query and schema design best practices

• Keep transactions short and specific: The longer a transaction holds locks, the higher the chance of a conflict. Commit quickly and avoid unnecessary operations inside transactions.
• Access objects in consistent order: Always update or query tables in the same sequence across your codebase. Consistency eliminates circular waits that cause deadlocks.
• Apply indexes to frequently accessed columns: Well-placed indexes reduce locking contention by allowing queries to target only the required rows instead of scanning entire tables.

Application-level strategies

• Use retry logic in applications: Implement automatic retries for deadlock victims. A short pause followed by a retry often resolves transient conflicts.
• Batch large operations into smaller chunks: Instead of locking thousands of rows at once, break operations into smaller, manageable sets to reduce contention.

DBA and monitoring practices

• Continuously monitor deadlock frequency: Track how often deadlocks occur and on which queries.
• Analyze historical patterns: Look for recurring queries, tables, or indexes involved in deadlocks to pinpoint areas for optimization.
• Schedule regular health checks with monitoring tools: Leverage solutions like dbForge Studio for SQL Server to visualize deadlock graphs, analyze blocking chains, and ensure your environment remains stable.

By following these practices, you know how to avoid deadlocks in your SQL Server environment and build a more resilient database.

Conclusion

No database developer wishes to experience deadlocks in SQL Server. But the truth is, you cannot completely avoid them, especially in busy systems. However, you can understand, monitor, and prevent them. To achieve this, master the following SQL Server deadlock resolution techniques:

• Identify the causes of SQL Server deadlocks.
• Implement both short-term fixes and long-term strategies.
• Adhere to best practices for prevention.

These strategies will help you maintain stable and responsive databases.

To fast-track the process and ensure that you are always a step ahead of any deadlock trigger, dbForge Studio for SQL Server is your best bet. With its advanced deadlock monitoring, intuitive visual graphs, and query analysis tools, you can detect, resolve, and prevent deadlocks with confidence—all within a single, integrated environment.

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