SQL Server Execution Plans - Red Gate Software

Chapter 2: Graphical Execution Plans for Basic Queries
Quite often, aggregations within queries can be expensive operations. About the only
way to "speed" the performance of an aggregation via code is to ensure that you have a
restrictive WHERE clause to limit the number of rows that need to be aggregated, thus
reducing the amount of aggregation that needs to be done. You can also pre-aggregate
data by using an indexed view.
Filter
If we add a simple HAVING clause to our previous query, our execution plan gets
more complex.
SELECT [City],
COUNT([City]) AS CityCount
FROM [Person].[Address]
GROUP BY [City]
HAVING COUNT([City]) > 1
Listing 2.13
The execution plan now looks as shown in Figure 2.31.
Figure 2.31
By adding the HAVING clause, the Filter operator appears in the execution plan. The
Filter operator is applied to limit the output to those values of the column, City, that are
greater than 1, in order to satisfy the HAVING clause. One useful bit of knowledge to take
away from this plan is that the optimizer does not apply the HAVING clause until all the
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