SQL Server Execution Plans - Red Gate Software

Chapter 5: Controlling Execution Plans with Hints
though, a certain value, or subset of values, supplied to the parameters results in an
execution plan that performs extremely poorly. This is the parameter sniffing problem.
Parameter sniffing is a process that occurs with all stored procedures; it is normally at the
very least benign, and often very beneficial to the performance of the system. As values
are passed to a stored procedure or parameterized query, the optimizer uses those values
to evaluate how well an index will satisfy the query requirements, based on the available
statistics. In most cases, this produces more accurate execution plans.
However, situations can arise whereby the data distribution of a particular table or index
is such that most parameters will result in a good plan, but some parameters can result in
a bad one. Problems occur when the first execution of the parameterized query or stored
procedure happens to use a very non-representative parameter value. The resulting plan
will be stored in cache and reused, and often turns out to be highly inefficient for the
parameter values that are more representative. Alternatively, a "good" plan may be aged
out of the cache, or recompiled due to changes in the statistics or the code, and replaced
with a "bad" plan. As such, it becomes, to a degree, a gamble as to where and when the
problematic execution plan is the one that is created and cached.
In SQL Server 2000, only two options are available:
1. Recompile the plan every time, using the RECOMPILE hint.
2. Get a good plan and keep it, using the KEEPFIXED PLAN hint.
Both of these solutions (covered later in this chapter) could create as many problems as
they solve since, depending on its complexity and size, the RECOMPILE of the query could
be longer than the execution itself. The KEEPFIXED PLAN hint could be applied to the
problematic values as well as the useful ones.
In SQL Server 2005 and above, when you're hitting a bad parameter sniffing situation,
you can use the OPTIMIZE FOR hint. This hint allows you to instruct the optimizer to
optimize the query for the value that you supply, rather than the value passed to the
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