sql parameter sniffing

In this article explains SQL Server parameter sniffing, which is a process used by SQL Server to optimize the query execution plan for stored procedures. It discusses the benefits and drawbacks of parameter sniffing, how to identify when it is causing performance issues, and various ways to fix it. It also provides real-world SQL script examples and best practices for avoiding parameter sniffing in T-SQL stored procedures.

What is SQL Server Parameter Sniffing?

SQL Server parameter sniffing is a process used by SQL Server to optimize the query execution plan for stored procedures. When a stored procedure is executed for the first time, SQL Server sniffs the parameter values and generates an execution plan optimized for those values. This execution plan is then cached and reused for subsequent executions of the stored procedure with the same parameter values.

Benefits and Drawbacks of Parameter Sniffing

Parameter sniffing can greatly improve query performance by reusing the same execution plan for the same query with the same parameter values. However, it can also lead to suboptimal or even incorrect results, especially if the data distribution or query workload varies significantly. 

How to Identify When Parameter Sniffing is Causing Performance Issues

Parameter sniffing can cause performance issues when the data distribution or query workload changes significantly. Symptoms of parameter sniffing issues include:

• Stored procedure performance execution performance shows inconsistency.
• Sudden degradation of procedure performance.
• If there is fixing in the stored procedure, it starts working fine.

Ways to Fix Parameter Sniffing Issues

• There are several ways to fix parameter sniffing issues, including:
• Use local variables instead of parameters in the stored procedure.
• Use the OPTIMIZE FOR UNKNOWN hint to generate a generic execution plan.
• Use the WITH RECOMPILE option to recompile the stored procedure each time it is executed.
• Use the OPTION (RECOMPILE) hint to recompile the query each time it is executed.
• Use the OPTION (OPTIMIZE FOR) hint to optimize the query for specific parameter values.
• Disable parameter sniffing at the instance level using the Trace Flag 4136.

Real-World SQL Script Variable Sniffing Example

Here is an example of a stored procedure that uses parameter sniffing:

CREATE PROCEDURE dbo.GetOrdersByCustomer 

@CustomerID int 

AS 

BEGIN 

-- Declare local variable and assign parameter value 

DECLARE @CustID int = @CustomerID 

-- Use local variable in the query 

SELECT OrderID, OrderDate, TotalAmount 

FROM dbo.Orders 

WHERE CustomerID = @CustID 

END

Best Practices for Avoiding Parameter Sniffing in T-SQL Stored Procedures

• Here are some best practices for avoiding parameter sniffing in T-SQL stored procedures:
• Use local variables instead of parameters in the stored procedure.
• Use the OPTIMIZE FOR hint to optimize the query for specific parameter values.
• Use the WITH RECOMPILE option to recompile the stored procedure each time it is executed.
• Use the OPTION (RECOMPILE) hint to recompile the query each time it is executed.