SQL Server Execution Plans - Red Gate Software

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Chapter 4: Understanding More Complex Query <strong>Plans</strong> Queries that contain aggregates are good candidates for indexed views because the creation of the aggregates only has to occur once when the index is created, and the aggregated results can be returned with a simple SELECT query, rather than having the added overhead of running the aggregates through a GROUP BY each time the query runs. For example, one of the indexed views supplied with AdventureWorks2008R2 is vStateProvinceCountryRegion. This combines the StateProvince table and the CountryRegion table into a single entity for querying, such as the query in Listing 4.12. SELECT * FROM Person.vStateProvinceCountryRegion; Listing 4.12 Figure 4.17 shows the execution plan for this query. Figure 4.17 From our previous experience with execution plans containing views, you might have expected to see two tables and the join in the execution plan. Instead, we see a single Clustered Index Scan operation. Rather than execute each step of the view, the optimizer went straight to the clustered index that makes this an indexed view. Since the indexes that define an indexed view are available to the optimizer, they are also available to queries that don't even refer to the view. For example, the query in Listing 4.13 gives the exact same execution plan as the one shown in Figure 4.17, because the optimizer recognizes the index as the best way to access the data. 162
Chapter 4: Understanding More Complex Query <strong>Plans</strong> SELECT sp.Name AS StateProvinceName, cr.Name AS CountryRegionName FROM Person.StateProvince sp INNER JOIN Person.CountryRegion cr ON sp.CountryRegionCode = cr.CountryRegionCode ; Listing 4.13 However, as the execution plan grows in complexity, this behavior is neither automatic nor guaranteed. For example, consider the query in Listing 4.14. SELECT a.City, v.StateProvinceName, v.CountryRegionName FROM Person.Address a JOIN Person.vStateProvinceCountryRegion v ON a.StateProvinceID = v.StateProvinceID WHERE a.AddressID = 22701 ; Listing 4.14 If you expected to see a join between the indexed view and the Person.Address table, you would be disappointed. Figure 4.18 163
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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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