http://www.sqlservercentral.com/blogs/scarydba/2010/05/11/recompiles-and-constant-learning/

Printed 2014/10/22 06:15PM

Recompiles and Constant Learning

By Grant Fritchey, 2010/05/11

When faced with a procedure that looks like this:

CREATE PROCEDURE dbo.TestProc (@TestValue INT)
AS
BEGIN
IF @TestValue = 1
BEGIN
SELECT *
FROM Sales.SalesOrderHeader AS soh
JOIN Sales.SalesOrderDetail AS sod
ON soh.SalesOrderID = sod.SalesOrderID
WHERE soh.SalesOrderID = @TestValue
END
ELSE
BEGIN
SELECT *
FROM Production.Product AS p
JOIN Production.ProductDocument AS pd
ON p.ProductID = pd.ProductID
WHERE p.ProductID = @TestValue
END
END

I used to suggest creating a wrapper procedure in order to avoid the recompiles that occur when the different paths through the IF statement are taken by the optimizer. I mentioned that recently on a post over at SQL Server Central. Gail Shaw (blog | twitter) asked me why I thought there would be a recompile. She said that the optimizer took the query as a whole and created plans for it. I never seem to learn my lesson, so I suggested that she might be wrong about that. Gail being who she is, immediately went and made up a quick little test with simple queries. Sure enough, no recompiles. Ah, but I figured she was benefiting from trivial plans or something, so I created the procedure above to test the theory out. Each query, while relatively simple, goes through a full optimization process, so no trivial plans involved…

Short answer, Gail’s right and I was wrong. I don’t know where I got the idea that this type of query caused recompiles. I have now tested it on 2000, 2005 and 2008, no recompiles anywhere. I also cleared the cache, ran the procedure once, and then checked the cache using this query:

DBCC freeproccache ;

EXEC dbo.TestProc

@TestValue = 1 ;

SELECT deqp.query_plan

FROM sys.dm_exec_query_stats AS deqs

CROSS APPLY sys.dm_exec_query_plan(deqs.plan_handle) AS deqp

This is what I saw:

 

I swear, I’d never seen a plan like this before. I guess partly because I tend to only look at the actual execution plan rather than the estimated plan. It’s pretty clear that the optimizer just walked through and determined that there was more than one query involved and built a plan for them, including the conditional IF statement. If I’d just bothered to look at the estimated plan one time, I could have avoided my error of understanding.

To all those I’ve suggested wrapper procs in order to avoid recompiles… oops, sorry.



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