Category: SQL Server

Hi Folks,

Well this post is not related to Power Platform, but I want to bring it on here to specify the significance of using NOLOCK in Power Platform Implementations using SQL Server.

Recently during our Deployment activity, we had a SSIS job which is writing a lot of data into SQL Server, at the same time, we were trying to read the data from the same table. I received never ending Executing query … message. It is when I had arguments on this, hence I would like to share the significance of NOLOCK.

The default behaviour in SQL Server is for every query to acquire its own shared lock prior to reading data from a given table. This behaviour ensures that you are only reading committed data. However, the NOLOCK table hint allows you to instruct the query optimizer to read a given table without obtaining an exclusive or shared lock. The benefits of querying data using the NOLOCK table hint is that it requires less memory and prevents deadlocks from occurring with any other queries that may be reading similar data. 

In SQL Server, the NOLOCK hint, also known as the READUNCOMMITTED isolation level, allows a SELECT statement to read data from a table without acquiring shared locks on the data. This means it can potentially read uncommitted changes made by other transactions, which can lead to what’s called dirty reads.

Here’s an example:

Let’s say you have a table named Employee with columns EmployeeID and EmployeeName.

CREATE TABLE Employee (
    EmployeeID INT,
    EmployeeName VARCHAR(100)
);

INSERT INTO Employee (EmployeeID, EmployeeName)
VALUES (1, 'Alice'), (2, 'Bob'), (3, 'Charlie');

Now, if two transactions are happening concurrently:

Transaction 1:

BEGIN TRANSACTION
UPDATE Employee
SET EmployeeName = 'David'
WHERE EmployeeID = 1;

Transaction 2:

SELECT EmployeeName
FROM Employee WITH (NOLOCK)
WHERE EmployeeID = 1;

If Transaction 2 uses WITH (NOLOCK) when reading the Employee table, it might read the uncommitted change made by Transaction 1 and retrieve 'David' as the EmployeeName for EmployeeID 1. However, if Transaction 1 rolled back the update, Transaction 2 would have obtained inaccurate or non-existent data, resulting in a dirty read.

Key takeaways about NOLOCK:

• ✅ Pros: Reduces memory use, avoids blocking, speeds up reads.
• ❌ Cons: May read uncommitted or inconsistent data.

Using NOLOCK can be helpful in scenarios where you prioritize reading data speed over strict consistency. So, in my case as I want to just view the data, using NOLOCK is good without locking the query. However, it’s essential to be cautious since it can lead to inconsistent or inaccurate results, especially in critical transactional systems.

Other considerations like potential data inconsistencies, increased chance of reading uncommitted data, and potential performance implications should be weighed before using NOLOCK.

Conclusion:

There are benefits and drawbacks to specifying NOLOCK table hint as a result they should not just be included in every T-SQL script without a clear understanding of what they do. Nevertheless, should a decision be made to use NOLOCK table hint, it is recommended that you include the WITH keyword. Using NOLOCK without WITH Statement is deprecated. Always use a COMMIT keyword at the end of the transaction.

Hope this helps…

Cheers,

PMDY