Category: Locks, Blocks, and Deadlocks

John McCormack explains locks and deadlocks:

Blocking is the real world impact of locks being taken on resources and other lock types being requested which are incompatible with the existing lock. You need to have locks in order to have blocking. In the scenario where a row is being updated, the lock type of IX or X means that a simultaneous read operation will be blocked until the data modification lock has been released. Similarly, data being read blocks data from being modified. Again, there are exceptions to these based on the isolation level used.

Blocking then is a perfectly natural occurrence within SQL Server. In fact, it is vital to maintain ACID transactions. On a well optimised system, it can be hard to notice and doesn’t cause problems.

All things in moderation, even blocking.

Erik Darling talks about key lookups:

I go back and forth when it comes to Lookups.

On the one hand, I don’t think the optimizer uses them enough. There are times when hinting a nonclustered index, or re-writing a query to get it to use a nonclustered index can really help performance.

On the other hand, they can really exacerbate parameter sniffing problems, and can even lead to read queries blocking write queries. And quite often, they lead to people creating very wide indexes to make sure particular queries are covered.

Read on for one scenario around deadlocking due to key lookups.

Erik Darling hits on an interesting issue:

You’ll see the exchange event, and you’ll also see the same query deadlocking itself.

This is an admittedly odd situation, but one I’ve had to troubleshoot a bunch of times.

In other words, parallel threads on the same query causing the query to deadlock on itself. Click through to learn what you can do about it.

Dan Jackson provides us a primer on how SQL Server implements locking:

We have all heard of locks and can probably even name a few; shared or exclusive locks, for example. We notice them more when something goes wrong and we run into blocking or other performance problems, but what are locks and how do they work in SQL Server?

In this blog, I aim to give a basic answer to that question and provide you with an overview of the different lock modes in SQL Server and how it all works…

Let us make it clear from the off; locks are an essential part of SQL Server. In a multi-user system, there will be many users who wish to access the same resources at the same time. This means that SQL Server must have measures in place to handle concurrency and prevent adverse side effects. Locking is one of those measures.

Read the whole thing.

Erik Darling notes a locking scenario specifically with LOB data:

To backtrack a little bit, part of what I wanted to show was that using order by can sometimes result in “more” blocking. I don’t mean more locks; what I mean is that when we need to order by Score, but we don’t have Score indexed in a useful way, the query will get hung up without showing any rows whatsoever.

Read the whole thing.