Have you ever wondered what determines the order of the result set when you execute a SELECT statement on a clustered indexed table without using the ORDER BY clause? This question was recently posed to me by one of the tech team managers in my company, and it sparked an interesting discussion. In this article, I will share my insights on this topic.

There is a common misconception that if the ORDER BY clause is not explicitly specified and the table has a clustered index, the result set will be returned in the order of how the clustered index is built. While this theory holds true in most cases, SQL Server does not always follow this logic when returning the result set.

SQL Server’s primary objective is to return the result set as quickly as possible. In most scenarios, the fastest way to retrieve the result set is by utilizing the clustered index. However, there are certain situations where SQL Server may deviate from using the clustered index.

One such scenario is when parallelism is employed to retrieve query results. Due to variations in CPU speed and workload distribution, the result set may not necessarily align with the order of the clustered index. Additionally, SQL Server parametrization and the SQL Server cache can also impact the order of the result set, potentially overriding the order defined by the clustered index.

It’s worth noting that the observations mentioned in this article assume the presence of an ORDER BY clause in the SELECT statement. Personally, I am a strong advocate for using clustered indexes, particularly as primary keys. In fact, every table in my database server farm is equipped with a clustered index, although it may not always be the primary key.

Ultimately, SQL Server prioritizes efficiency and speed when returning the result set. If SQL Server can retrieve the result set faster using an alternative method, it will do so. Regardless of the approach taken, the end user benefits from faster query results.

Understanding the factors that influence the order of the result set in SQL Server can help you optimize your queries and improve overall performance. By leveraging the power of clustered indexes and considering the impact of parallelism and caching, you can ensure that your queries return results in the most efficient manner possible.

Thank you for reading this article. I hope it has provided you with valuable insights into the order of result sets in SQL Server. If you have any questions or would like to share your own experiences, please feel free to leave a comment below.