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LISTING 8-17 Parameter Snif ng Example
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CREATE TABLE param1(col1 INT, col2 INT); go SET NOCOUNT ON; BEGIN TRANSACTION; DECLARE @a INT=0; WHILE @a < 10000 BEGIN INSERT INTO param1(col1, col2) VALUES (@a, @a); SET @a+=1; END; COMMIT TRANSACTION; go CREATE INDEX i1 ON param1(col1); go CREATE INDEX i2 ON param1(col2); go DECLARE @b INT; DECLARE @c INT; SELECT * FROM param1 WHERE col1=@b AND col2=@c;
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Microsoft SQL Server 2008 Internals
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FIGURE 8-78 Non-sniffed parameters use index i1
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SELECT * FROM param1 WHERE col1=23 AND col2=5;
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FIGURE 8-79 Sniffed parameters use i2
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DECLARE @b INT; DECLARE @c INT; SELECT * FROM param1 WHERE col1=@b AND col2=@c OPTION (OPTIMIZE FOR (@b=22));
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Using the OPTIMIZE FOR hint instructs the Query Optimizer to use a known common value when generating the plan so that it works for a wide range of parameter values.
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PARAMETERIZATION {SIMPLE | FORCED}
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SIMPLE parameterization is the model that has existed in SQL Server for many releases. This corresponds to the concept of the trivial plan explained in this chapter. FORCED parameterization always replaces most literals in the query with parameters. As the plan quality can suffer, using FORCED should be done with care and an understanding of the global behavior of your application. Usually, FORCED mode should be used only in an OLTP system with many almost equivalent queries that (almost) always yield the same query plan. Essentially, you are betting that the plans will not change between possible parameter values. If all the queries are very small, the risk of this bet is smaller. The reasoning for this hint is that some OLTP systems with ad-hoc queries spent a large fraction of their time compiling the same (or similar) queries repeatedly. When possible, this is a good case to consider adding parameters into your application s queries.
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8
The Query Optimizer
NOEXPAND
By default, the query processor expands view de nitions when parsing and binding the query tree. While the Query Optimizer usually matches the indexed views during optimization (as well as portions of any query even when the indexed view was not speci ed), there are some cases where the internal queries are rewritten such that it is not possible to match indexed views anymore. The NOEXPAND hint forces the query processor to force the use of the indexed view in the nal query plan. In many cases, this can speed up the execution of the query plan because the indexed view often pre-computes an expensive portion of a query. However, this is not always true the Query Optimizer may be able to nd a better plan using the information from the fully expanded query tree.
USE PLAN
The USE PLAN N xml plan hint directs the Query Optimizer to try to generate a plan that looks like the plan in the supplied XML string. The Query Optimizer has been instrumented to use the shape of this plan as a series of hints to guide the optimization process to get the desired plan shape. Note that this does not guarantee that the _exact_ same plan is selected, but it will usually be identical or very close. The common use of this hint is a DBA or database developer who wishes to x a plan regression in the Query Optimizer. If a baseline of good/expected query plans is saved when the application is developed or rst deployed, these can be used later to force a query plan to change back to what was expected if the Query Optimizer later determines to change to a different plan that is not performing well. This could be necessary to force a join order to avoid locking deadlocks or merely to get the right physical plan shape and algorithms to be chosen. In some scenarios, the Query Optimizer does not have enough information to make a good decision about a portion of the query plan (for example, the join order) and it can lead to a suboptimal plan choice. DBAs should use this option with care forcing the original query plan may actually degrade performance further because the plan was likely created for different data volumes and distributions. Try out a plan hint on a test database before deploying it, when possible. Although this feature was added in SQL Server 2005, the feature has been improved in SQL Server 2008 with the inclusion of scripting support through Management Studio and the ability to hint more types of queries. For example, INSERT/DELETE/UPDATE/MERGE queries are now supported in USE PLAN hints, which can be very useful in forcing speci c update plans that avoid deadlocks in stress scenarios. While SQL Server 2008 supports additional query types, some are not supported with this feature. These include:
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