{"id":242331,"date":"2024-06-27T15:59:11","date_gmt":"2024-06-27T22:59:11","guid":{"rendered":"https:\/\/virtual-dba.com\/?p=242331"},"modified":"2024-07-03T16:00:48","modified_gmt":"2024-07-03T23:00:48","slug":"understanding-and-preventing-sql-server-deadlocks","status":"publish","type":"post","link":"https:\/\/virtual-dba.com\/blog\/understanding-and-preventing-sql-server-deadlocks\/","title":{"rendered":"Understanding and Preventing SQL Server Deadlocks"},"content":{"rendered":"\n

SQL server deadlock<\/strong> is a common issue that database administrators often encounter, causing significant disruptions in database operations. This article aims to demystify the concept of SQL server deadlocks, particularly focusing on the nuances between traditional deadlocks and parallelism deadlocks. By understanding their causes and learning how to identify and prevent them, you can maintain a more efficient and reliable SQL Server environment.<\/p>\n\n\n\n

Parallelism Deadlocks: A Self-Inflicted Stalemate<\/h2>\n\n\n\n

Imagine, if you will, that you’re in a Western-style showdown with\u2026<\/p>\n\n\n\n

\u2026 yourself.<\/p>\n\n\n\n

And because you’re facing off against yourself, you have the same reflexes, the same aim, and the same sense of timing, so both versions of you shoot\u2026 at the same time.<\/p>\n\n\n\n

And you both hit in the same general area at the same time.<\/p>\n\n\n\n

Needless to say, no one wins.<\/p>\n\n\n\n

This is essentially what happens in a type of deadlock known as a “parallelism deadlock” (also known as an “intra-query” deadlock or “exchange” deadlock), where a process splits into different threads to complete a task more efficiently (as is standard for parallelism) and ends up attempting to access the same resource at the same time, even though the resource is locked by\u2026 itself. And, naturally, as tends to happen with deadlocks, one process becomes the deadlock victim, but since both “processes” are the same <\/em>process, the other “process” also becomes a deadlock victim.<\/p>\n\n\n\n

Why is this unusual?<\/em><\/p>\n\n\n\n

Understanding Traditional SQL Server Deadlocks<\/h2>\n\n\n\n

In case you need a refresher, “traditional” deadlocks involve two processes (let’s call them “Process A” and “Process B”), each competing for a resource (let’s call them “Resource A” and “Resource B”) that the other has locked. For example:<\/p>\n\n\n\n

    \n
  1. Process A has a lock on Resource A.<\/li>\n\n\n\n
  2. At the same time, Process B has a lock on Resource B.<\/li>\n\n\n\n
  3. Process A needs to access Resource B, but Process B still has a lock on it.<\/li>\n\n\n\n
  4. Process B needs to access Resource A, but Process A still has a lock on it.<\/li>\n<\/ol>\n\n\n\n

    It’s a SQL Server stalemate (a digital Mexican standoff, if you will) that can only be resolved by choosing one process at random (although this can be manipulated by “deadlock priorities,” which is a topic for another day) as the “deadlock victim.” Let’s say, in this example, that Process B gets the short straw. Process B gets killed, freeing up Resource B so that Process A can resume. <\/p>\n\n\n\n

    Error Message 1205<\/h3>\n\n\n\n

    When this happens, you’ll see a version of Error Message 1205, which says something similar to:<\/p>\n\n\n\n

    Your transaction (process ID <\/em>[Process B SPID]<\/em><\/strong>) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun your transaction.<\/em><\/p>\n\n\n\n

    Now, remember that (say it with me, folks) it is expected to have deadlocks<\/em>. And not in the “it’s not a bug, it’s a feature” way, either. Sometimes, deadlocks happen; you can’t necessarily control which processes try to access which resources at which times, so it’s pointless to have the mindset of “I need to make sure NO deadlocks happen EVER!”. The critical thing to remember when tackling deadlocks is to have sufficient error handling in your code so that when a process does<\/em> get deadlocked, it just automatically reruns, giving it another chance to complete successfully.<\/p>\n\n\n\n

    Now that that’s out of the way, let’s get back to the topic.<\/p>\n\n\n\n

    How Do I Know if I Have a Parallelism Deadlock?<\/h2>\n\n\n\n

    In your deadlock report (which you would have if you enabled trace flags 1222 and 1204 on your instance or if you have deadlock monitoring through our VDBA monitoring software), this can be shown in a handful of different ways:<\/p>\n\n\n\n