NOLOCK, some say it’s a fast option for queries and will never cause blocking, I say it’s quite dirty and MAY cause blocking.

I used to think that a NOLOCK (which is the same as read uncommitted) hint actually meant that a “no lock” was used, I was very wrong and the naming doesn’t really help. Let’s have a look.

I will run a random SELECT statement with the hint, such as

SELECT * FROM Sales.BigSalesOrderDetailEnlarged WITH (NOLOCK)

Then I will check the “go to” DMV for locks.

SELECT resource_type, resource_associated_entity_id,
    request_status, request_mode,request_session_id,
    resource_description
    FROM sys.dm_tran_locks
    WHERE resource_database_id = 13

A Sch-S (schema stability) lock is taken.  This is a lightweight lock; the only lock that can conflict with this is a Sch-m (schema modification) lock. (C = Conflict). This means that a NOLOCK can actually block for example against an ALTER TABLE command.

Below shows the example of when my NOLOCK query is being blocked by ALTER INDEX commands:

 SELECT resource_type, resource_associated_entity_id,
    request_status, request_mode,request_session_id,
    resource_description
    FROM sys.dm_tran_locks 

So there you go, it can be involved in blocking.

Dirty reads:

This is my example data.

In Window 1 using SQL Server management Studio I begin an open UPDATE statement – notice that it has NOT been committed.

BEGIN TRAN
UPDATE [HumanResources].[Department]
SET Name = 'Global'
WHERE DepartmentID = 16

In window 2 I do a NOLOCK select statement.

SELECT * FROM HumanResources.Department (NOLOCK)

Now that’s dirty.