Join order is an important aspect of SQL Server query optimization. It determines the sequence in which tables are joined together to retrieve the desired result set. However, there are cases where you may want to control the join order to meet specific requirements.
In a recent conversation, a user expressed their frustration with SQL Server’s optimizer rearranging the join order for their queries. They wanted to join tables in a certain way, regardless of performance implications. Initially, they used the FORCE ORDER hint to enforce their desired join order. However, they later discovered the Common Table Expression (CTE) and wondered if it could provide a better solution.
CTE is a powerful feature in SQL Server that allows you to define a temporary named result set within a query. It enhances the readability and usability of complex queries. However, there is a misconception that CTE can control the join order in a query. To clarify this, we conducted an experiment.
We created two examples, one without CTE and one with CTE, both using INNER JOINs. After executing the queries and comparing the results, we found that the join order was identical in both cases. The execution plans also showed no difference in the order of table joins.
This experiment confirmed that CTE does not impact the join order when all joins in the query are INNER JOINs. The reason for this is simple – CTE is not a temporary table or a feature that pre-executes the SELECT statement. It is merely an expression that represents the SELECT statement in a more readable format. The CTE is executed along with the regular SELECT statement, without any influence on the join order.
It’s important to note that this conclusion applies only to INNER JOINs. The behavior may differ when using OUTER JOINs, which we will cover in future blog posts.
Understanding the behavior of CTE and its impact on join order can help you write more efficient and readable queries. By leveraging CTE, you can improve the organization and structure of your SQL code without worrying about unintended changes to the join order.
If you’re interested in learning more about this topic, I recommend watching one of my favorite videos on the subject. It provides further insights into SQL Server join order optimization.
Thank you for reading!