SQL Stored Procedure Execution Plan Optimization: Parameter Sniffing and Its Impact

The performance of SQL stored procedures can be significantly affected by a phenomenon known as parameter sniffing. This occurs when the database engine compiles a stored procedure's execution plan based on the values of the input parameters provided at the time of compilation.

Consider the following stored procedure:

CREATE PROCEDURE MyProc
    @MyDate DATETIME = NULL
AS
    IF @MyDate IS NULL SET @MyDate = CURRENT_TIMESTAMP
    -- Do Something using @MyDate

In this scenario, if @MyDate is passed in as NULL during initial compilation, the database optimizes the execution plan for this value. However, subsequent calls to the procedure with other input values may result in poor performance, even if those values are also NULL.

Impact of Parameter Sniffing

Parameter sniffing can have several negative consequences:

• Unpredictable Execution Times: The performance of the stored procedure can vary drastically depending on the input values passed in, leading to unpredictable execution times.
• Poor Execution Plans: The engine may generate inefficient execution plans that are not optimal for the actual data being processed, resulting in slow query execution.

Case Study: Parameter Sniffing Gone Wrong

In the case mentioned, the execution plan generated for @MyDate was poor even when the value used was NULL. This behavior is unusual and suggests an issue with parameter sniffing.

Solution: Disabling Parameter Sniffing

One solution is to disable parameter sniffing by using a parameter variable as follows:

CREATE PROCEDURE MyProc
    @MyDate DATETIME = NULL
AS
    DECLARE @MyDate_Copy DATETIME
    SET @MyDate_Copy = @MyDate
    IF @MyDate_Copy IS NULL SET @MyDate_Copy = CURRENT_TIMESTAMP
    -- Do Something using @MyDate_Copy

Insight into the Issue

The underlying issue in SQL Server 2005 is a known problem with parameter sniffing in certain patch levels. In SQL Server 2008, the OPTIMIZE FOR UNKNOWN clause can be used to resolve such issues.

Additional Considerations

To further improve performance when using stored procedures with input parameters, it is recommended to:

• Use parameterized queries to ensure the database optimizes the execution plan efficiently.
• Avoid using NULL values as input parameters, as they can trigger unintended optimization issues.
• Monitor the performance of stored procedures regularly and adjust the execution plans as needed.