In SQL Server, Isolation Levels Don’t Control Locking

Different Engines

I seem to have gotten quite a few questions about this lately, so I’m going to write down some thoughts here.

It’s probably crappy form for a blog post, but what the heck.

Queries Decide The Isolation Level

They can choose anything from read uncommitted to serializable. Read uncommitted is the more honest term for what’s going on. When people see the “nolock” hint, they often assume that means their query won’t take any locks. What it really means is that it will ignore locks taken by other queries. The more accurate term would be “norespect”. This is probably what leads to confusion: reading uncommitted data sounds different than not locking data. But they’re both the same.

The Storage Engine Decides Which Locks Are Used

And if lock escalation is attempted. The storage engine will respect the query’s isolation level, and any table-level settings related to lock granularity, like not allowing row or page locks. It may not fully respect any query level hints regarding lock granularity.

One thing that helps reduce the chance of lock escalation is having a good index to help your modification query¬†find rows. Though if you need to find a million rows, don’t expect SQL Server to happily take a million row locks, just because of an index.

Batching modifications is one way to avoid lock escalation when you need to modify a lot of rows, though it isn’t always possible to do this. If for some reason you need to roll the entire change back, you’d have to keep track of all the committed batches somewhere, or wrap the entire thing in a transaction (which would defeat the purpose, largely).

One thing that increases the chance of lock escalation is having many indexes present on a table. For inserts and deletes, all of those indexes will get touched (unless they’re filtered around the rows to be inserted or deleted. For updates, any indexes containing the column(s) to be modified will need to be touched (again, barring filtering around the updated portion). Lock counts are cumulative across objects.

Not All “Blocking” Is “Locking”

In other words, when you see queries being blocked, there may not be an LCK wait involved. Some “blocking” can happen with resource contention, whether it’s physical (CPU, memory, disk), logical (like if there’s latch or spinlock contention), or even programmatic (if you’re lucky enough to see the source code).

This can happen in tempdb if you’re creating a lot of objects rapid-fire, even if you’re using table variables. Table variables can avoid some of the overhead that temp tables incur under high frequency execution, but not all of it.

Thanks for reading!

Going Further

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