--- title: "What is a deadlock in SQL Server? How to prevent it?" description: "What is a deadlock in SQL Server? How to prevent it?" author: "Sandra Emily" published: 2023-08-20 updated: 2023-08-21 canonical: https://www.mindstick.com/forum/159601/what-is-a-deadlock-in-sql-server-how-to-prevent-it category: "mssql server" tags: ["sql server", "sql", "error"] reading_time: 3 minutes --- # What is a deadlock in SQL Server? How to prevent it? What is a [deadlock in SQL](https://answers.mindstick.com/blog/99/deadlock-in-sql-server) [Server](https://www.mindstick.com/articles/43769/what-is-serverless-architecture-is-it-worth-switching-over)? How to prevent it? ## Replies ### Reply by Aryan Kumar A [deadlock](https://www.mindstick.com/forum/159421/a-database-update-operation-consistently-throws-a-deadlock-error) in [SQL Server](https://www.mindstick.com/articles/12999/what-is-table-valued-function-in-sql-server) occurs when two or more transactions are blocking each other from continuing because each has locked a database resource that the other transaction needs. This can happen when two transactions are trying to update the same row in a table, or when two transactions are trying to read the same row in a table at the same time. Deadlocks can cause performance problems and can even prevent transactions from completing. To prevent deadlocks, you can use the following techniques: - Use optimistic locking instead of pessimistic locking. Optimistic locking assumes that conflicts will be rare and only locks rows when they are actually updated. Pessimistic locking locks rows as soon as they are read, which can prevent deadlocks but can also lead to performance problems. - Use transaction isolation levels carefully. The isolation level determines how much data each transaction can see. A higher isolation level can prevent deadlocks, but it can also lead to performance problems. - Use the `NOWAIT` hint with `SELECT` statements. The `NOWAIT` hint tells SQL Server to not wait for a lock to be released if it is already held by another transaction. This can help to prevent deadlocks, but it can also lead to performance problems. - Use the `REPEATABLE READ` isolation level with `UPDATE` and `DELETE` statements. The `REPEATABLE READ` isolation level ensures that each transaction sees the same data, which can help to prevent deadlocks. - Use the `SERIALIZABLE` isolation level with `UPDATE` and `DELETE` statements. The `SERIALIZABLE` isolation level is the highest isolation level and is the most likely to prevent deadlocks. However, it can also lead to the most performance problems. By following these techniques, you can help to prevent deadlocks and keep your SQL Server database running smoothly. Here are some additional things you can do to prevent deadlocks in SQL Server: - Use a database design that minimizes the chances of deadlocks. For example, you can avoid using tables with many rows that are frequently updated. - Monitor your database for deadlocks. You can use the SQL Server Management Studio to view deadlocks that have occurred. - Use a deadlock detection tool. There are a number of deadlock detection tools available that can help you to identify and fix deadlocks. By following these tips, you can help to prevent deadlocks and keep your SQL Server database running smoothly. --- Original Source: https://www.mindstick.com/forum/159601/what-is-a-deadlock-in-sql-server-how-to-prevent-it Copyright © MindStick Software Pvt. Ltd. This Markdown version is provided for developers, AI systems, and offline reading.