Why Does My Stored Procedure Suffer Performance Issues Even with a NULL Parameter, and How Can I Fix It?

Parameter Sniffing and Stored Procedure Execution Plan Performance

A stored procedure is experiencing performance issues when executed with @MyDate as NULL, even though the execution plan generated is suboptimal. This issue persists regardless of whether @MyDate is actually NULL or set to the current timestamp by an IF statement.

The problematic execution plan is believed to be caused by parameter sniffing, where SQL Server compiles the stored procedure based on the parameter values provided at the time of compilation. However, in this case, the issue arises even when the parameter is NULL, which contradicts typical parameter sniffing scenarios.

Disabling parameter sniffing by spoofing the parameter (@MyDate_Copy) resolves the issue. This suggests that the server is using an incorrect execution plan based on an arbitrary NULL value.

Insight into the Phenomenon

According to the accepted answer, parameter sniffing in SQL Server 2005 is known to have issues that can lead to poor execution plans. It's speculated that the server generates an ineffective plan based on unrepresentative parameter values, even when the parameter is NULL. Additionally, problems with LEFT JOINs and NULLs have been observed, which were resolved by replacing them with NOT IN or NOT EXISTS.

Solution

For SQL Server 2008, the OPTIMIZE FOR UNKNOWN option can be utilized to alleviate the issue.

The above is the detailed content of Why Does My Stored Procedure Suffer Performance Issues Even with a NULL Parameter, and How Can I Fix It?. For more information, please follow other related articles on the PHP Chinese website!