How Can I Resolve Poor Stored Procedure Performance Caused by Parameter Sniffing in SQL Server?

Poor Stored Procedure Performance Due to Parameter Sniffing

You're encountering performance issues with a stored procedure where an input parameter (@MyDate) can be NULL or a date. When the procedure is first compiled with @MyDate as NULL, subsequent executions exhibit poor performance regardless of the actual parameter value.

Parameter Sniffing Behavior

SQL Server performs "parameter sniffing" to optimize execution plans based on the parameter values during compilation. It captures these values in the procedure cache and uses them to estimate future executions.

In this case, parameter sniffing is causing the problem because it generates a suboptimal execution plan for all parameter values when @MyDate is NULL initially. Even when @MyDate is explicitly set to NULL at execution time, the cached plan remains.

Disabling Parameter Sniffing

You've encountered the classic symptom of "parameter sniffing gone bad," where the plan generated for the initial parameter values is used even when they are not representative of actual executions. To disable parameter sniffing, you spoofed the parameter with @MyDate_Copy.

Root Cause and Solution

The issue stems from a SQL Server 2005 bug where parameter sniffing can misbehave. In SQL Server 2008, you can mitigate this issue by using the OPTIMIZE FOR UNKNOWN option, which forces the optimizer to consider all possible values for the parameter during plan compilation.

Additional Insights

• Plan-affecting predicates are complex expressions that can cause plan changes.
• Masking parameters can help prevent parameter sniffing issues.
• In severe cases, using OPTIMIZE FOR UNKNOWN can force the optimizer to consider all parameter values.
• Consult Microsoft documentation for further details and updates on parameter sniffing behavior.

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