Tag Archive for Execution Plan

T-SQL Tuesday #51- Don’t Crap Out While Betting On Table Functions

My good friend Jason Brimhall (b|t) is hosting this month’s T-SQL Tuesday blog party. The party was started by Adam Machanic (b|t) in December of 2009. As a compliment to the upcoming debut of the Las Vegas SQL Saturday, Jason has taken up a betting theme. He wants to know our stories of when we bet it all on a risky solution and won or lost.

Instead of telling you about the past, I want to help you win big at the table today. I really don’t want you to crap out while betting on the wrong table functions.

Snake Eyes

There are two types of table functions Multi-line Table Functions and In-Line Table Functions. There is a huge difference between the two of them.

Multi-line table functions sound great. You write as much code as you need in them and they will return all the data in a table variable. This is where the weighted dice rolls snake eyes every single time. You see, the statistics for a table variable always, always says there is only one row in the table being returned. It doesn’t matter if there are a hundred, a thousand, or a million rows. The statistics will say one. Which means the optimizer has a good chance of loosing when it picks the execution plan for that query.

Let’s Take a Look at the Bets

For my example, I have a simple query that returns 43 rows out of a Tally table. Notice that the index estimates 43 rows will be returned, which is great, because that is exactly on the money!



If we put that same query inside of a multi-line table function, we get an estimated number of rows of 1 (snake eyes!).






Double Down

An in-line table function will return the same result set, but there are some limitations on its construction. The entire query within the in-line table function needs to be done in only one statement.

Note: You can get very creative with Common Table Expressions (CTE) if need be.

There are two benefits to using an in-line Table Function. One, is that the Estimated Number of Rows will be accurate (or as accurate as the statistics on the table), and two, the “inside” of the in-line table function is not masked in the Execution Plan. It is plopped right into the middle of the calling query. (Yes, “plopped” is a technical term. )





Last Call

SoRemember to double down on in-line table functions and don’t crap out on the snake eyes of the multi-line table function.

Thanks for all the fish

Thanks go out to Jason Brimhall for hosting this month’s T-SQL Tuesday blog party. Please visit his website at http://jasonbrimhall.info/, or better yet come to Las Vegas for their SQL Saturday and thank him in person.

T-SQL Tuesday #43: Give Me a Key Lookup Operator for $1200, Please

My good friend Rob Farley (b|t) is hosting this month’s T-SQL Tuesday blog party. The party was started by Adam Machanic (b|t) in December of 2009.  The topic this month is on Plan Operators that are used in execution plans to tell us how the Optimizer is going to run our query statements.

What is a Key Lookup Operator?

The Optimizer uses indexes to retrieve the fields needed from a table. If there are missing fields in the index being used, then the Optimizer has to go back to the Clustered Index to get the other fields. This has to be done for every row in the table. This action is noted in the execution plan by the Key Lookup Operator. The RID Lookup Operator is used instead of the Key Lookup Operator when a Clustered Index is not used and a Heap is used instead.

Show Me the Money

For my example I used the AdventureWorks2008R2 database. I ran the following query and looked at the execution plan.

   ,s.BusinessEntityID AS StoreID
   ,s.Name AS StoreName
   ,st.Name AS TerritoryName
	Sales.Customer AS c
	JOIN Sales.Store AS s ON c.StoreID = s.BusinessEntityID
	JOIN Sales.SalesTerritory AS st ON c.TerritoryID = st.TerritoryID
	c.StoreID IN (1284, 994, 1356, 1282, 992, 1358, 1280)


TSQLTuesday43 - KeyLookup

Two indexes were used to retrieve all the fields needed from the Sales.Customer table. The ix_Customer_StoreID index was missing the TerritoryID field so the Optimizer had to go to the PK_Customer_CustomerID Clustered Index to retrieve it. If you add the cost of both operators, then 66% of the cost of the query was used to retrieve fields from the Sales.Customer table.

For reference I removed the Clustered Index to show you what the execution plan would look like when a Heap is involved.

TSQLTuesday43 - RIDLookup


Since there was already a good index for the Customer table, I added the TerritoryID to the INCLUDE part of the index script. This turned the index into a covering index. A covering index is an index that contains all the fields from a table that are needed by a query statement. The INCLUDE part of an index allow extra fields to be part of the index, without the overhead of the data being sorted. Any fields that are part of predicates or filters should be part of the index, all other fields from the table should be part of the INCLUDE. Be cautious though, don’t throw the whole kitchen sink there. Those fields still take up space.

TSQLTuesday43 - Index

When I ran the execution plan again, I saw that the Key Lookup Operator was removed and the total cost to retrieve the fields from the Customer table was now reduced to 12%. This would also be true if the table was using a Heap instead of a Clustered Index.

TSQLTuesday43 - CoveringIndex

The Bottom Line

When you see a Key Lookup Operator or a  RID Lookup Operator, look to see if it makes sense to modify the corresponding index to be a covering index.

Thanks for all the fish

Thanks go out to Rob Farley for hosting this month’s T-SQL Tuesday blog party. Please visit Rob’s blog at http://sqlblog.com/blogs/rob_farley/default.aspx.

Shameless Plug for Grant Fritchey’s (b|t) Book: SQL Server Execution Plans, Second EditionYup, this is a shameless plug for one of my FAVORITE books. Grant did an outstanding job on this book. He goes through the majority of the execution plan operators, discussing what they represent. He also goes into how best to read an execution plan and how changing your query can affect the operators that are shown in the execution plan. I highly recommend adding this book to your collection if you don’t already have it.This book is available from Red Gate. You can download a FREE eBook, buy a hard copy from Amazon, or if you are lucky you can pick up a copy from a Red Gate event.


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