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Techniques to Improve Query Performance
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Scalar UDFs
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A scalar UDF is a function that returns a single value (not a result set). This type of function is frequently used in queries and can significantly degrade performance. The reason for this is that these functions are not expanded and optimized into the main query plan by the optimizer; rather, they are just called from the execution plan without any optimization based on the context into which it is inserted in the plan. This also means that the cost of whatever is done inside the function is not included in the cost estimates found in the graphical execution plan for the query. This same problem occurs for the output of the SET STATISTICS IO ON statement, which contains no references to what is done inside the UDF. As an example, this section compares the performance of a query using a UDF and that of another query using a correlated subquery. The UDF and the subquery perform identical SELECT statements. Because the cost of the UDF is not reflected in the query cost metric, the more appropriate performance metric for these queries is the execution time, which is returned using the SET STATISTICS TIME ON statement.
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When using query execution times as a performance metric, it is typically a good idea to execute each query a few times and use either the lowest execution time or the median as the metric. Also, note that metrics in a test environment might not accurately reflect performance in a production environment. This depends on a number of factors, including how users actually interact with real-life data.
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The following is a query using a UDF. The execution plan produced for the query is shown in Figure 6-3:
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USE AdventureWorks; GO
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CREATE FUNCTION dbo.fnGetCustomerAccountNumber(@CustomerID INT) RETURNS VARCHAR(10) AS BEGIN RETURN ISNULL( ( SELECT AccountNumber FROM Sales.Customer WHERE CustomerID = @CustomerID ), 'NOT FOUND'); END GO
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Lesson 1: Tuning Queries
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SET STATISTICS IO ON; SET STATISTICS TIME ON;
SELECT soh.SalesOrderID ,soh.OrderDate ,dbo.fnGetCustomerAccountNumber(soh.CustomerID) FROM Sales.SalesOrderHeader AS soh;
FiGURe 6-3 The actual execution plan from SSMS for the query using a UDF
The cost of this query is 0.56 and the number of page reads is 706 (neither metric is accurate and thus cannot be used to gauge performance), while the execution time on the test machine used in this example is 25 seconds. Examine the graphical execution plan and note that it contains no reference to the Sales.Customer table. The following is an example query that uses a correlated subquery in place of the UDF. The query s execution plan is shown in Figure 6-4:
USE AdventureWorks;
SET STATISTICS IO ON; SET STATISTICS TIME ON;
SELECT soh.SalesOrderID ,soh.OrderDate ,ISNULL( ( SELECT AccountNumber FROM Sales.Customer WHERE CustomerID = soh.CustomerID ), 'NOT FOUND') FROM Sales.SalesOrderHeader AS soh;
The cost of the query without the UDF goes up to 1.05 and the number of page reads to 742 (both of which are accurate values). At the same time, the execution time drops to about 1 second. As you can see, the first query using the UDF is about 25 times slower than the latter query because of the UDF use.
Techniques to Improve Query Performance
FiGURe 6-4 The actual execution plan from SSMS for the query using a correlated subquery
Why was the inline table-valued UDF so much faster The use of inline table-valued UDFs or views does not incur the same performance penalty as the use of scalar UDFs because both inline table-valued UDFs and views are optimized (expanded) into the query plan.
Table-Valued UDFs
There are three different types of table-valued UDFs. Two of them can be developed in T-SQL and the other one in a CLR language, such as C# or Microsoft Visual Basic .NET:
n n n
T-SQL inline table-valued UDF T-SQL multistatement table-valued UDF CLR table-valued UDF
t-SQL anD cLR UDFs
More info
5, Programming Microsoft SQL Server with T-SQL User-Defined Stored Procedures, Functions, Triggers, and Views, explains T-SQL UDFs in greater detail. 7, Extending Microsoft SQL Server Functionality with XML, SQLCLR, and Filestream, gives more information about CLR UDFs.
These different types of functions behave differently. A T-SQL inline table-valued UDF is actually just a view that can accept parameters. It is optimized in the same way as a view or any SELECT statement would be. An advantage of using inline table-valued UDFs instead of views is that you can require the user that uses the function to supply parameters. In this way you can make sure that a filter is always used for the query inside the function (based on the parameters provided).
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