Adaptive Joins and Join Hints
At a recent all-day seminar on query performance tuning I was asked a question that I didn't know the answer to: "How do join hints affect adaptive joins?" I don't know. Let's find out together. Adaptive Joins Here's a query that we can run against AdventureWorks: SELECT p.Name, COUNT(th.ProductID) AS CountProductID, SUM(th.Quantity) AS SumQuantity, AVG(th.ActualCost) AS AvgActualCost FROM Production.TransactionHistory AS th JOIN Production.Product AS p ON p.ProductID = th.ProductID GROUP BY th.ProductID, p.Name; Without a columnstore index in SQL Server 2017, the execution plan looks like this: Let's introduce a columnstore index: CREATE NONCLUSTERED COLUMNSTORE INDEX ix_csTest ON Production.TransactionHistory ( ProductID, Quantity, ActualCost ); Now, if we run the same query, the execution plan changes to use an adaptive join like this: You can read more on adaptive joins here…