How can you optimize transactions and locking in T-SQL stored procedures for better performance?

1 Choose the right isolation level

The isolation level of a transaction determines how much data is locked and for how long, as well as how the transaction handles concurrent changes by other users. The default isolation level in SQL Server is READ COMMITTED, which means that a transaction only reads data that has been committed by other transactions, and locks the data it modifies until the transaction ends. However, this can cause blocking and concurrency issues, such as lost updates, non-repeatable reads, and phantom reads. To avoid these problems, you can use a higher isolation level, such as REPEATABLE READ, SERIALIZABLE, or SNAPSHOT, which provide more consistency and isolation, but also lock more data and consume more resources. Alternatively, you can use a lower isolation level, such as READ UNCOMMITTED, which allows a transaction to read uncommitted data from other transactions, but also introduces the risk of dirty reads, inconsistent data, and missing rows. Therefore, you should choose the isolation level that suits your business logic and performance requirements, and use it consistently throughout your stored procedures.

2 Use explicit transactions

An explicit transaction is a transaction that you start and end with the BEGIN TRANSACTION and COMMIT TRANSACTION (or ROLLBACK TRANSACTION) statements. Explicit transactions give you more control over the scope and duration of your transactions, as well as the ability to handle errors and rollbacks. Explicit transactions also allow you to use the SET TRANSACTION ISOLATION LEVEL statement to change the isolation level of a specific transaction, without affecting the session or database level settings. Using explicit transactions can help you optimize transactions and locking in T-SQL stored procedures by reducing the number of locks, minimizing the locking time, and avoiding deadlocks.

3 Avoid long-running transactions

Long-running transactions can cause performance issues like blocking other transactions, consuming resources, and increasing the chances of deadlocks. To avoid this, you should break down large transactions into smaller ones and commit them frequently. Additionally, avoid user interaction or external dependencies within a transaction, and use error handling and retry logic instead. Furthermore, unnecessary or redundant operations such as cursors, triggers, or nested transactions should be avoided. Finally, use the appropriate indexes, statistics, and query hints to optimize the query execution plan and reduce the locking overhead.

4 Use lock hints

Lock hints are keywords that you can use in the FROM clause of a query to specify how the query acquires and releases locks on the data, allowing you to optimize transactions and locking in T-SQL stored procedures. For example, you can use lock hints to reduce the locking level of a query, such as using NOLOCK or READUNCOMMITTED to read uncommitted data without acquiring any locks, or increase the locking level of a query, such as using HOLDLOCK or SERIALIZABLE to hold locks until the end of the transaction. Additionally, you can control the locking scope of a query with ROWLOCK, PAGLOCK, or TABLOCK to force row-level, page-level, or table-level locking. However, be aware that lock hints can introduce side effects such as data inconsistency, deadlocks, or reduced concurrency; and so you should test and monitor their impact on your queries before using them.

5 Monitor and troubleshoot locking issues

Even when you adhere to the best practices for optimizing transactions and locking in T-SQL stored procedures, you may still encounter blocking, deadlocks, or lock contention issues. To monitor and troubleshoot these issues, you can use a variety of tools and methods. The sys.dm_tran_locks dynamic management view shows the current locks held by transactions and sessions. The sys.dm_exec_requests and sys.dm_exec_sessions dynamic management views indicate the current requests and sessions running on the server, as well as their blocking and deadlock status. Additionally, sp_who2 and sp_lock system stored procedures display active processes and locks on the server. Moreover, SQL Server Profiler and Extended Events enable you to capture and analyze events related to transactions and locking. Finally, the Database Engine Tuning Advisor can help you optimize indexes and statistics of tables and queries, reducing locking overhead.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?