How MERGE on two different rows can still deadlock you

I recently ran into a curious deadlock issue. I have a process that performs a lot of updates in a “state” table using multiple, concurrent connections. The business logic in the application guarantees that two connections won’t try to update the same item, so we shouldn’t ever run into any locking issues. And yet, we keep getting deadlocks.

What’s going on here? Hint: it has to do with isolation levels and range locks.

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How to create, find and eliminate deadlocks

Deadlocks are typically encountered in high-load scenarios or in complex concurrent transactions. And the worst part is, you probably won’t see them until you’re live in production. Luckily, finding and solving deadlock issues isn’t as complicated as it may sound. All it takes is a bit of logging and patience, along with some basic know-how of how locking mechanisms work in SQL Server.

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