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For SQL Server to determine that there is an appropriate plan guide to use, the statement text in the plan guide must match the query being compiled. This must be an exact character-for-character match, including case, whitespace, and comments, just as when SQL Server is determining whether it can reuse adhoc query plans, as we discussed earlier in the chapter. If your statement text is close, but not quite an exact match, this can lead to a situation that is very dif cult to troubleshoot. When matching a SQL template, whether the de nition also contains a batch that the statement must be part of, SQL Server does allow more leeway in the de nition of the batch. In particular, keyword case, whitespace, and comments are ignored. To make sure your plan guides use the exact text that is submitted by your applications, you can run a trace using SQL Server Pro ler and capture the SQL:BatchCompleted and RPC:Completed
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Microsoft SQL Server 2008 Internals
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events. After the relevant batch (the one you want to create a plan guide for) shows up in the top window of your Pro ler output, you can right-click the event and select Extract Event Data to save the SQL Text of the batch to a text le. It is not suf cient to copy and paste from the lower window in the Pro ler because the output there can introduce extra line breaks. To verify that your plan guide was used, you can look at the XML plan for the query. If you can run the query directly, you can use the option SET SHOWPLAN_XML ON, or you can capture the showplan XML through a trace. An XML plan has two speci c items, indicating that the query used a plan guide. These items are PlanGuideDB and PlanGuideName. If the plan guide was a template plan guide, the XML plan also has the items TemplatePlanGuideDB and TemplatePlanGuideName. When a query is submitted for processing, if there are any plan guides in the database at all, SQL Server rst checks to see if the statement matches a SQL plan guide or Object plan guide. The query string is hashed to make it faster to nd any matching strings in the database s existing plan guides. If no matching SQL or Object plan guides are found, SQL Server then checks for a TEMPLATE plan guide. If it nds a TEMPLATE guide, it then tries to match the resulting parameterized query to a SQL plan guide. This gives you the possibility of applying additional hints to your queries using forced parameterization. Figure 9-3, copied from SQL Server Books Online, shows the process that SQL Server uses to check for applicable plan guides.
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Start Request to compile Statement within Batch No Match Attempt to match the SQL plan guide to Statement and Batch Match Parameterization failed Attempt to parameterize Statement using forced parameterization
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Attempt to parameterize Statement by using simple parameterization
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Parameterization succeeded
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Match Modify Statement based on the plan guide
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Attempt to match the SQL plan guide to Statement and Parameterized Statement
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No Match Compile plan for Statement End
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FIGURE 9-3 Checking for applicable plan guides
9
Plan Caching and Recompilation
The key steps are the following, which follow the owchart from the top left, take the top branch to the right, the middle branch down, and then right at the center, to the point where the statement is modi ed based on the plan guide and its hints: 1. For a speci c statement within the batch, SQL Server tries to match the statement to a SQL-based plan guide, whose @module_or_batch argument matches that of the incoming batch text, including any constant literal values, and whose @stmt argument also matches the statement in the batch. If this kind of plan guide exists and the match succeeds, the statement text is modi ed to include the query hints speci ed in the plan guide. The statement is then compiled using the speci ed hints. 2. If a plan guide is not matched to the statement in step 1, SQL Server tries to parameterize the statement by using forced parameterization. In this step, parameterization can fail for any one of the following reasons:
The statement is already parameterized or contains local variables. The PARAMETERIZATION SIMPLE database SET option is applied (the default setting), and there is no plan guide of type TEMPLATE that applies to the statement and speci es the PARAMETERIZATION FORCED query hint. A plan guide of type TEMPLATE exists that applies to the statement and speci es the PARAMETERIZATION SIMPLE query hint.
Let s look at an example that involves the distribution of data in the SpecialOfferID column in the Sales.SalesOrderDetail table in the AdventureWorks2008 database. There are 12 different SpecialOfferID values, and most of them occur only a few hundred times (out of the 121317 rows in the Sales.SalesOrderDetail) at most, as the following script and output illustrates:
USE AdventureWorks2008 GO SELECT SpecialOfferID, COUNT(*) as Total FROM Sales.SalesOrderDetail GROUP BY SpecialOfferID; RESULTS: SpecialOfferID -------------1 2 3 4 5 7 8 9 11 13 14 16
Total ----------115884 3428 606 80 2 137 98 61 84 524 244 169
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