SQL Server 2017 - Performance Issues with CTE and Recursive Queries on Large Datasets
I've looked through the documentation and I'm still confused about I'm experiencing significant performance issues when executing a recursive Common Table Expression (CTE) in SQL Server 2017. The CTE is designed to traverse a hierarchy of categories stored in my database, but on large datasets, the query takes an unexpectedly long time to execute, often timing out. Here’s a simplified version of my query: ```sql WITH CategoryCTE AS ( SELECT CategoryID, ParentCategoryID, CategoryName FROM Categories WHERE ParentCategoryID IS NULL UNION ALL SELECT c.CategoryID, c.ParentCategoryID, c.CategoryName FROM Categories c INNER JOIN CategoryCTE cc ON c.ParentCategoryID = cc.CategoryID ) SELECT * FROM CategoryCTE; ``` I have indexed both `CategoryID` and `ParentCategoryID`, but it seems that the CTE performs poorly as the number of records increases (currently around 500,000 rows). I also tried running the query with the `OPTION (MAXRECURSION 0)` to ensure it wasn't being limited by recursion depth, but it still takes a long time. Interestingly, when I run this query on a smaller dataset (about 50,000 rows), it executes quickly. Additionally, I monitored the SQL Server execution plans, and it appears that the recursive part of the CTE is causing the majority of the execution time, as it generates a large intermediate result set. I've also considered breaking the query into multiple parts or using a temporary table to store intermediate results, but I'm unsure if that would actually yield better performance. Has anyone else faced similar issues with recursive CTEs in SQL Server, and what approaches can I take to optimize it for larger datasets? Any insights or recommendations would be greatly appreciated! This is part of a larger CLI tool I'm building. Thanks in advance!