Performance Comparison EXCEPT vs NOT IN – Interview Question of the Week #095

During SQLPass summit, I was asked following question by one of the attendees. The question was about Performance Comparison of Except vs NOT IN. Let us quickly discuss this question.

Question: Which of the operator is better for query performance – EXCEPT or NOT IN?

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Answer: The EXCEPT operator returns all of the distinct rows from the query to the left of the EXCEPT operator when there are no matching rows in the right query.  EXCEPT operator works the same way NOT IN.

In simple words, there is absolutely no difference between how these operators work and there is absolutely no difference in the performance. They are just the same operators with the different name. Let us prove this with a simple example. You can run following two different queries with different operators and see how they perform exactly the same.

-- EXCEPT 
USE AdventureWorks2014;
GO
SELECT ProductID
FROM Production.Product
EXCEPT
SELECT ProductID
FROM Production.WorkOrder; 
GO
-- NOT IN
USE AdventureWorks2014;
GO
SELECT ProductID
FROM Production.Product
WHERE ProductID
NOT IN (
SELECT ProductID
FROM Production.WorkOrder);
GO

When you run above script, it gives following execution plan.

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Upon carefully observing that you can see that execution plan is absolutely same for both the queries. This essentially proves that both the operators give the same results and the same performance.

Reference: Pinal Dave (http://blog.sqlauthority.com)

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