SQL Server Locks

NOLOCK and ROWLOCK

In the last section we discussed what row-locking was, why it was done, and its impact on database performance. In this part we'll examine two query-level hints you can provide SQL Server with to specify how you want SQL to handle locking!

NOLOCK
Using NOLOCK politely asks SQL Server to ignore locks and read directly from the tables. This means you completely circumvent the lock system, which is a major performance and scalability improvement. However, you also completely circumvent the lock system, which means your code is living dangerously. You might read the not-necessarily-valid uncommitted modifications of a running transaction. This is a calculated risk.

For financial code and denormalized aggregates (those little counters of related data that you stash away and try desperately to keep accurate), you should play it safe and not use this technique. But I think you'll find that for better than 90% of your application, it would not be that big of a deal if a user (or even intermediate code) saw an uncommitted modification. In fact, you'll probably find that most of your data never or only very rarely changes, in which case the overhead of locking the data is almost always completely wasted.

For example, if I want to count all users that joined Streamload.com between June 1 and August 31 of Y2K, there's no reason for me to lock anything: that number was cast in stone the moment September 1, 2000 rolled around. Another example is the file listings you see on Streamload.com: it doesn't much matter if you don't see the exact perfect data, since either you don't own the data and it doesn't much matter what you see, or you do own the data and you know perfectly well whether you just modified the data or not and whether new files have finished uploading.

Just don't use this type of data as the basis for modifications to the database, and don't use it when it's really important that the user not see the wrong thing (an account statement or balance, for instance).

ROWLOCK
Using ROWLOCK politely asks SQL Server to only use row-level locks. You can use this in SELECT, UPDATE, and DELETE statements, but I only use it in UPDATE and DELETE statements. You'd think that an UPDATE in which you specify the primary key would always cause a row lock, but when SQL Server gets a batch with a bunch of these, and some of them happen to be in the same page (depending on this situation, this can be quite likely, e.g. updating all files in a folder, files which were created at pretty much the same time), you'll see page locks, and bad things will happen. And if you don't specify a primary key for an UPDATE or DELETE, there's no reason the database wouldn't assume that a lot won't be affected, so it probably goes right to page locks, and bad things happen.

By specifically requesting row-level locks, these problems are avoided. However, be aware that if you are wrong and lots of rows are affected, either the database will take the initiative and escalate to page locks, or you'll have a whole army of row locks filling your server's memory and bogging down processing. One thing to be particularly aware of is the "Management/Current Activity" folder with Enterprise Manager. It takes a long time to load information about a lot of locks. The information is valuable, and this technique is very helpful, but don't be surprised if you see hundreds of locks in the "Locks/Processes" folder after employing this technique. Just be glad you don't have lock timeouts or deadlocks.

Notes:
I get the sense that SQL Server honors NOLOCK requests religiously, but is more discretional with ROWLOCK requests. You can only use NOLOCK in SELECT statements. This includes inner queries, and the SELECT clause of the INSERT statement. You can and should use NOLOCK in joins:

SELECT COUNT(Users.UserID)
FROM Users WITH (NOLOCK)
   JOIN UsersInUserGroups WITH (NOLOCK) ON
      Users.UserID = UsersInUserGroups.UserID

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