SQL Server Corruption: NOLOCK and Corruption

Over the years I have documented many of the concepts around corruption repair. Want to find out more, check out the corruption category at by blog.

NOLOCK is a query hint in SQL Server that allows a SELECT statement to read data from a table without acquiring a shared lock on it. This means that the query can read uncommitted data or data that is in the process of being modified by other transactions. Using NOLOCK can increase query performance, especially in high-concurrency environments, but it also carries a risk of returning inconsistent or incorrect data. It is also worth noting that NOLOCK only affects the table being queried, and not any associated indexes or views. Other similar hints in SQL Server include READUNCOMMITTED and SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED, which also allow reading uncommitted data.

Using the NOLOCK hint in queries can increase the risk of reading inconsistent data, but it does not directly cause corruption in data files. Corruption in data files can be caused by hardware failure, software bugs, or other factors. However, using NOLOCK can result in queries reading uncommitted or partially committed data, which can lead to incorrect results or data inconsistencies. Therefore, it is generally recommended to use NOLOCK with caution and only in cases where the potential benefits outweigh the risks.

In my experience I have never seen a situation where a corrupt data file was in any way related to the use of the NOLOCK option on queries. If you are seeing corruption, it is likely due to something else besides the NOLOCK option.

Want to learn more about how to prepare for corruption on SQL Server? Take a look at my class that will teach how to to be prepared if corruption strikes.

Corruption Class by Steve Stedman.