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The SET STATISTICS XML ON command, as covered in chapter 13, is ideal in diagnosing undesirable parameter-sniffing problems. For a given stored procedure execution, the <ParameterList> element includes both the compiled parameter(s) and the runtime parameter(s). If a given procedure appears to be executing slower than expected, you can compare the compiled and runtime parameters, which may reveal significant differences resulting in inappropriate index usage.
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In dealing with ad hoc/parameterization problems such as we ve just covered, the nature of the application usually determines the available options for improvement. For example, while an in-house-developed application could be modified to use stored procedures, an off-the-shelf vendor-supplied application cannot. In dealing with these cases, we have a number of options including Forced Parameterization and Optimize for Ad Hoc Workloads.
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FORCED PARAMETERIZATION
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In figure 17.7, shown earlier, we inspected the procedure cache after executing three SQL statements and found that three ad hoc plans with single usecounts were created in addition to a single prepared plan with three usecounts. This is an example of SQL Server s simple parameterization mechanism. This mechanism has detected that the three statements are essentially the same, with the only difference being the product number parameters, and therefore share the prepared plan. The three ad hoc plans are referred to as shell plans and point to the full prepared plan. The shells are saved for later executions of exactly the same statement, which reuses the same prepared plan. The key word here is exactly; a single character difference, for example, an extra space, is enough to cause a new plan to be created. Simple parameterization is exactly that. There are many conditions in which simple parameterization cannot be used. For example, consider the three queries in listing 17.4.
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Listing 17.4 Queries that cannot be parameterized with simple parameterization
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DBCC FREEPROCCACHE GO SELECT * FROM Production.Product WHERE left(ProductNumber, 3) = 'FW-' GO SELECT * FROM Production.Product WHERE left(ProductNumber, 3) = 'HN-' GO SELECT * FROM Production.Product WHERE left(ProductNumber, 3) = 'BE-' GO
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The use of the left function means these three commands cannot be parameterized using simple parameterization. Let s inspect the procedure cache to see how SQL Server has cached these queries. Figure 17.9 displays the results. In this case, there is no prepared plan, with each ad hoc compiled plan containing the full plan; notice that the size of each plan is larger than the shell plans from the earlier simple parameterization example in figure 17.7. If queries such as these were executed many times per day, we would have an even worse problem than the one described earlier. In SQL Server 2005, an option called Forced Parameterization was introduced, which is more aggressive in its parameterization. Each database contains a Parameterization
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Queries of moderate complexity cannot be parameterized using simple parameterization.
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Waits and queues: a performance-tuning methodology
Figure 17.10 After we enabled the Forced Parameterization option, the same three queries are parameterized.
property, which by default is set to Simple. Setting this value to Forced, through either Management Studio or using the ALTER DATABASE [dbname] SET PARAMETERIZATION FORCED command, will parameterize queries more frequently. For example, after enabling this property, rerunning the three queries from listing 17.4 will reveal a procedure cache, as shown in figure 17.10. In this case, the three ad hoc plans are created as shell plans with a link to the full prepared plan. Subsequent queries of the same form will also benefit from reusing the prepared plan, thereby reducing compilation and plan cache size, which in turn enables more efficient use of both RAM and CPU resources. Of course, modifying any configuration setting is not without a possible downside. SQL Server Books Online documents all of the considerations for using this option, the primary one being the possibility of reusing an inappropriate plan, with reduced performance a common outcome. However, for applications in which modifying code to use stored procedures is not an option, forced parameterization presents an opportunity for reducing compilation overhead and procedure cache size. Before enabling this option on a production system, a full load test with an appropriate workload should be observed in a testing environment to measure the positive (or negative) impact.
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