SQL SERVER – Subquery or Join – Various Options – SQL Server Engine knows the Best

Posted by pinaldave on SQL Authority See other posts from SQL Authority or by pinaldave
Published on Sun, 06 Jun 2010 01:30:41 +0000 Indexed on 2010/06/06 1:33 UTC
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This is followup post of my earlier article SQL SERVER – Convert IN to EXISTS – Performance Talk, after reading all the comments I have received I felt that I could write more on the same subject to clear few things out.

First let us run following four queries, all of them are giving exactly same resultset.

USE AdventureWorks
GO
-- use of =
SELECT *
FROM HumanResources.Employee E
WHERE E.EmployeeID = ( SELECT EA.EmployeeID
FROM HumanResources.EmployeeAddress EA
WHERE EA.EmployeeID = E.EmployeeID)
GO
-- use of in
SELECT *
FROM HumanResources.Employee E
WHERE E.EmployeeID IN ( SELECT EA.EmployeeID
FROM HumanResources.EmployeeAddress EA
WHERE EA.EmployeeID = E.EmployeeID)
GO
-- use of exists
SELECT *
FROM HumanResources.Employee E
WHERE EXISTS ( SELECT EA.EmployeeID
FROM HumanResources.EmployeeAddress EA
WHERE EA.EmployeeID = E.EmployeeID)
GO
-- Use of Join
SELECT *
FROM HumanResources.Employee E
INNER JOIN HumanResources.EmployeeAddress EA ON E.EmployeeID = EA.EmployeeID
GO

Let us compare the execution plan of the queries listed above. Click on image to see larger image.

It is quite clear from the execution plan that in case of IN, EXISTS and JOIN SQL Server Engines is smart enough to figure out what is the best optimal plan of Merge Join for the same query and execute the same. However, in the case of use of Equal (=) Operator, SQL Server is forced to use Nested Loop and test each result of the inner query and compare to outer query, leading to cut the performance. Please note that here I no mean suggesting that Nested Loop is bad or Merge Join is better. This can very well vary on your machine and amount of resources available on your computer.

When I see Equal (=) operator used in query like above, I usually recommend to see if user can use IN or EXISTS or JOIN. As I said, this can very much vary on different system. What is your take in above query? I believe SQL Server Engines is usually pretty smart to figure out what is ideal execution plan and use it.

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


Filed under: Pinal Dave, SQL, SQL Authority, SQL Joins, SQL Optimization, SQL Performance, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, T SQL, Technology

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