Parameter Sniffing in SQL Server

Parameter sniffing in SQL Server happens when a query execution plan is generated using specific parameter values. Subsequent executions of the same query may perform poorly with different parameter values due to an unsuitable cached plan. Here’s how to address this issue:

1. Use Query Hints

SQL Server provides query hints to control the behavior of query execution.

OPTIMIZE FOR UNKNOWN

This hint instructs SQL Server to disregard the specific parameter values. It instead uses a generic plan when generating the execution plan.

SELECT *
FROM Employees
WHERE Department = @Department
OPTION (OPTIMIZE FOR UNKNOWN);

OPTIMIZE FOR Specific Value

You can specify a value to optimize the query for, which can be representative of typical use cases.

SELECT *
FROM Employees
WHERE Department = @Department
OPTION (OPTIMIZE FOR (@Department = 'IT'));

2. Use RECOMPILE Option

The OPTION (RECOMPILE) forces SQL Server to generate a new plan for every execution of the query. This approach avoids reliance on the cached plan.

SELECT *
FROM Employees
WHERE Department = @Department
OPTION (RECOMPILE);

Alternatively, apply WITH RECOMPILE at the stored procedure level:

CREATE PROCEDURE GetEmployees
    @Department NVARCHAR(50)
WITH RECOMPILE
AS
BEGIN
    SELECT *
    FROM Employees
    WHERE Department = @Department;
END;

• Pros: Ensures the best execution plan is always used.
• Cons: Additional CPU cost due to repeated compilation.

3. Use Local Variables

Using local variables prevents SQL Server from sniffing the parameter value since the query plan will assume unknown values.

CREATE PROCEDURE GetEmployees
    @Department NVARCHAR(50)
AS
BEGIN
    DECLARE @LocalDepartment NVARCHAR(50) = @Department;

    SELECT *
    FROM Employees
    WHERE Department = @LocalDepartment;
END;

• Pros: Forces SQL Server to create a more generic plan.
• Cons: Can lead to suboptimal plans in certain cases.

4. Leverage Plan Guides

Plan guides allow you to influence the execution plan for a query without modifying the query itself.

1. Identify the query using sp_create_plan_guide.
2. Apply hints like OPTIMIZE FOR or OPTION (RECOMPILE) using the plan guide.

5. Use Query Store

The Query Store feature (available in SQL Server 2016 and later) allows you to monitor query performance and enforce specific execution plans.

• Steps:
  1. Identify the query causing parameter sniffing issues.
  2. Use Query Store to force a good plan.
  3. Monitor performance to ensure the chosen plan works for most parameter values.

6. Partitioned Views or Dynamic SQL

• Partitioned Views: Break down the query into multiple queries with fixed parameter values, which SQL Server optimizes individually.
• Dynamic SQL: Dynamically construct the SQL statement to ensure a new plan is created for each execution.

CREATE PROCEDURE GetEmployees
    @Department NVARCHAR(50)
AS
BEGIN
    DECLARE @SQL NVARCHAR(MAX);

    SET @SQL = N'SELECT * FROM Employees WHERE Department = ''' + @Department + '''';
    EXEC sp_executesql @SQL;
END;

7. Statistics and Index Tuning

• Ensure up-to-date statistics on columns involved in filtering, as outdated statistics can mislead the optimizer.
• Consider creating filtered indexes or indexed views for frequently used parameter values.

8. Parameterized Queries in Applications

For application-level queries, ensure consistent parameter usage to reduce variability in execution plans.

Conclusion

The best solution for parameter sniffing depends on your specific workload and query patterns. OPTIMIZE FOR UNKNOWN or RECOMPILE are good starting points for quick fixes. Consider more advanced approaches like Query Store or plan guides for persistent issues. Regularly monitoring and updating your database (e.g., statistics, indexes) also helps minimize parameter sniffing effects.