Parameter Sniffing

What is "parameter sniffing"? When a stored procedure is first executed SQL Server looks at the input parameters and uses this as guidance to build the query plan. This is known as "parameter sniffing".

This is good as long as the input parameters for the first invocation are typical for future invocations. But if that is not the case this will cause performance problems.

For example, a procedure is supposed to retrieve all rows for customer orders with non-clustered index on the customer column. If the first invocation returns a small set of orders it may be most efficient to use index seek. Further invocations may be for large set of orders, but the first cached plan with index seek will be used resulting in poor performance (instead of using a scan).

Here is one example stored procedure and different methods to handle parameter sniffing.

CREATE PROCEDURE GetCustomerOrders
 @customerid NCHAR(5)
AS
BEGIN
 
    SELECT orderid, customerid, orderdate, shippeddate
    FROM Orders
    WHERE customerid = @customerid;
 
END


Replace parameters with local variables

This solution is based on assigning the stored procedure parameters to local variables and then using the local variables in the query. This works because SQL Server is not sniffing local variables and using the local variables in place of parameters forces plan generated based on statistics (in effect this disables parameter sniffing). 

CREATE PROCEDURE GetCustomerOrders
 @customerid NCHAR(5)
AS
BEGIN
 
    DECLARE @local_customerid NCHAR(5);
 
    SET @local_customerid = @customerid;
 
    SELECT orderid, customerid, orderdate, shippeddate
    FROM Orders
    WHERE customerid = @local_customerid;
 
END


Execute using WITH RECOMPILE

This solution forces recompile of the stored procedure on each run, that way forcing a fresh query plan for the current parameters. Note that this will recompile all statements inside the stored procedure.

EXEC GetCustomerOrders @customerid = N'CACYK' WITH RECOMPILE;


Query hint RECOMPILE

SQL Server 2005 offers the new query hint RECOMPILE which will force recompilation of the individual query. This method is better than the prior method because recompilation will affect only one statement and all other queries in the stored procedure will not be recompiled.

CREATE PROCEDURE GetCustomerOrders
 @customerid NCHAR(5)
AS
BEGIN
 
    SELECT orderid, customerid, orderdate, shippeddate
    FROM Orders
    WHERE customerid = @customerid
    OPTION (RECOMPILE);
 
END


Query hint OPTIMIZE FOR

Another new query hint in SQL Server 2005 is OPTIMIZE FOR. It allows specifying a constant that will be used to optimize the query plan instead of the variable. This could be useful if it is known that particular selective value is frequently used to invoke the stored procedure. However, any other parameter value will suffer the same performance problems.

CREATE PROCEDURE GetCustomerOrders
 @customerid NCHAR(5)
AS
BEGIN
 
    SELECT orderid, customerid, orderdate, shippeddate
    FROM Orders
    WHERE customerid = @customerid
    OPTION (OPTIMIZE FOR (@customerid = N'CACYK'));
 
END

Note: SQL Server 2008 adds a new option to specify "OPTION (OPTIMIZE FOR UNKNOWN)". This specifies that the query optimizer will use statistical data instead of the initial value to determine the value for query optimization.

Plan Guides

Plan guides in SQL Server 2005 provide the opportunity to optimize a query without changing the actual code of the query. This is especially useful when dealing with third party vendor applications where access to code may not be available. A plan guide allows associating query hints with a query without changing the query.

EXEC sp_create_plan_guide 
     @name = N'SolveParameterSniffing',
     @stmt = N'SELECT orderid, customerid, orderdate, shippeddate
               FROM Orders
               WHERE customerid = @customerid',
     @type = N'OBJECT',
     @module_or_batch = N'GetCustomerOrders',
     @params = NULL,
     @hints = N'OPTION (RECOMPILE)';


USE PLAN query hint
Another plan stability feature in SQL Server 2005 is the USE PLAN "xml_plan" query hint, which allows forcing the use of a specific plan every time the query is run.

Additional Resources:

Batch Compilation, Recompilation, and Plan Caching Issues in SQL Server 2005
http://www.microsoft.com/technet/prodtechnol/sql/2005/recomp.mspx

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