How Does Parameter Sniffing Impact Stored Procedure Performance, and How Can It Be Mitigated?

Impact of Parameter Sniffing on Stored Procedure Performance

A stored procedure that leverages a date parameter (@MyDate) exhibits significant performance discrepancies based on the initial value of the parameter during compilation. When @MyDate is passed in as NULL initially, subsequent executions of the procedure suffer from poor performance, regardless of the actual input value. However, initializing @MyDate with a date or the current date during compilation ensures optimal performance for all subsequent invocations.

This behavior can be attributed to the SQL Server's parameter sniffing mechanism, which evaluates the values of query parameters during stored procedure compilation. In this case, when @MyDate is initially NULL, the optimizer generates an inefficient execution plan that persists even when @MyDate contains non-NULL values later.

To resolve this issue, disable parameter sniffing by using a copy of the @MyDate parameter (@MyDate_Copy). This ensures that the SQL Server optimizes the query plan based on the current value of @MyDate at runtime, eliminating the impact of the initial parameter value.

In SQL Server 2005, parameter sniffing is known to exhibit severe bugs, leading to performance bottlenecks and even non-completion of queries. Therefore, it is recommended to mask stored procedure parameters to avoid these issues. SQL Server 2008 provides the OPTIMIZE FOR UNKNOWN clause, which can be used to address parameter sniffing-related performance concerns.

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