SQL Server: Performance implementing complex subquery in a JOIN operation
I'm relatively new to this, so bear with me. I'm trying to figure out I'm experiencing important performance optimization when executing a query that includes a complex subquery within a JOIN operation on my SQL Server 2019 instance. The subquery is supposed to aggregate data from a related table, but the execution time is over 30 seconds, which is unacceptable for a production environment. Here’s a simplified version of my query: ```sql SELECT a.*, b.TotalSales FROM Orders a JOIN ( SELECT CustomerID, SUM(TotalAmount) AS TotalSales FROM Sales WHERE OrderDate >= '2023-01-01' GROUP BY CustomerID ) b ON a.CustomerID = b.CustomerID WHERE a.OrderStatus = 'Completed' ORDER BY a.OrderDate DESC; ``` I’ve tried adding indexes to the `CustomerID` and `OrderDate` columns in both the `Orders` and `Sales` tables, but it doesn't seem to make much difference. I also ran an execution plan and noticed that the subquery is being executed for each row in the `Orders` table, which seems inefficient. Could this be a case of improper indexing, or is there a better way to structure the query to improve performance? I've even considered using a Common Table Expression (CTE) instead of the subquery, but I want to ensure it won’t negatively impact performance further. Has anyone faced similar issues, and what optimizations would you recommend? This is part of a larger desktop app I'm building. Is there a simpler solution I'm overlooking? What's the correct way to implement this?