SQL Server Execution Plans - Red Gate Software

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Figure 4.3 Chapter 4: Understanding More Complex Query <strong>Plans</strong> What appears to be a somewhat complicated query turns out to have a straightforward execution plan. Reading it in physical operator order, there are two Clustered Index Scans against Production.Product and Production.ProductListPriceHistory. These two data streams are combined using the Merge Join operator. The Merge Join requires that both data inputs are ordered on the join key, in this case, ProductId. The data resulting from a clustered index is always retrieved in an ordered fashion, so no additional sort operation is required here. A Merge Join takes a row each from the two ordered inputs and compares them. The inputs are sorted, so the operation will only scan each input one time (except in the case of a many-to-many join; more on this shortly). The operation will scan through the right side of the operation until it reaches a row that is different from the row on the left side. At that point, it will step the left side forward a row and begin scanning the right side until it reaches a changed data point. This operation continues until all the rows are processed. With the data already sorted, as in this case, it makes for a very fast operation. Although we don't have this situation in our example, many-to-many Merge Joins create a worktable to complete the operation. The creation of a worktable adds a great deal of cost to the operation because it will mean writing data out to tempdb. However, the 142
Chapter 4: Understanding More Complex Query <strong>Plans</strong> Merge Join operation will generally still be less costly than the use of a Hash Match join, which is the other choice the query optimizer can make to solve the same type of join. We can see that a worktable was not necessary, because the ToolTips property labeled Many to Many (see figure below) is set to False. Figure 4.4 143
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SQL Handbooks SQL Server Execution
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First Edition 2008 Copyright Grant
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Chapter 2: Graphical Execution Plan
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Table Hints _______________________
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About the Author Grant Fritchey is
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Foreword I have attended many SQL S
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Introduction Every day, out in the
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If you are specifically looking for
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Some of the code may not work withi
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Chapter 1: Execution Plan Basics Wh
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The query optimizer Chapter 1: Exec
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SQL Server Execution Plans - Red Gate Software

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