Getting Around Parameter Sniffing in SQL Server 2005

Question

I have seen people suggest copying the parameters to a local variable to avoid parameter sniffing in a stored proc. Say you have

CREATE PROCEDURE List_orders_3 @fromdate datetime AS
 DECLARE @fromdate_copy datetime
 SELECT @fromdate_copy = @fromdate
 SELECT * FROM Orders WHERE OrderDate > @fromdate_copy

(I got this from http://www.sommarskog.se/query-plan-mysteries.html but I need more details to understand it fully).

But what does this actually do to the query plan cache and query plan optimizer? If it is true that the optimizer makes no assumptions about @fromdate_copy, then why is it that it won't cache a plan that is most likely going to be a full table scan (since it makes no assumptions, how could it generate anything else)?

Is this technique basically like a "no inputs will run well, but no input will run terribly either" ?

Answer 1

Actually, you need to assign a default variable to the @fromdate_copy field that you declare, so that when the query engine looks at the query itself, it bases a plan on the value that is 'hard-coded' - but instead, when the query actually gets executed, it gets executed with the value being passed in and switched..

Ken Henderson (the Guru himself) explained this in great detail: http://blogs.msdn.com/b/khen1234/archive/2005/06/02/424228.aspx

If you can, read his books - they offer a plethora of information about sql server internals: http://www.amazon.com/Gurus-Guide-Server-Architecture-Internals/dp/0201700476/ref=pd_bxgy_b_text_c

I'm not sure if he has anything written for the newer versions, but some of the fundamentals haven't changed that much...