Zero SQL deadlock by design - any coding patterns?
I am encountering very infrequent yet annoying SQL deadlocks on a .NET 2.0 webapp running on top of MS SQL Server 2005. In the past, we have been dealing with the SQL deadlocks in the very empirical way - basically tweaking the queries until it work.
Yet, I found this approach very unsatisfactory: time consuming and unreliable. I would highly prefer to follow deterministic query patterns that would ensure by design that no SQL deadlock will be encountered - ever.
For example, in C# multithreaded programming, a simple design rule such as the locks must be taken following their lexicographical order ensures that no deadlock will ever happen.
Are there any SQL coding patterns guaranteed to be deadlock-proof?
Answer
Writing deadlock-proof code is really hard. Even when you access the tables in the same order you may still get deadlocks [1]. I wrote a post on my blog that elaborates through some approaches that will help you avoid and resolve deadlock situations.
If you want to ensure two statements/transactions will never deadlock you may be able to achieve it by observing which locks each statement consumes using the sp_lock system stored procedure. To do this you have to either be very fast or use an open transaction with a holdlock hint.
Notes:
- Any SELECT statement that needs more than one lock at once can deadlock against an intelligently designed transaction which grabs the locks in reverse order.