Wednesday, December 2, 2009

Parameter Sniffing and OPTIMZE FOR

Parameter Sniffing refers to a process whereby SQL Server’s execution environment “sniffs” the parameter values during first invocation, and passes it along to the query optimizer so that they can be used to generate optimal query execution plans.


While parameter sniffing is certainly a powerful feature, it can cause problems when a procedure’s plan happens to have been kicked out of the procedure cache (or was never in it) just prior to the procedure being called with atypical parameter values. This can result in a plan that is skewed toward atypical use, one that is not optimal when called with typical values. Since, once cached, a query plan can be reused for parameter values that vary widely, the ideal situation is to have a plan in the cache that covers the typical usage of the procedure as much as possible. If a plan makes it into the cache that is oriented toward atypical parameter values, it can have a devastating effect on performance when executed with typical values. 

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