Why is my SQL query fast in SSMS but slow in ASP.NET?

Understanding SQL Performance Discrepancies Between SSMS and ASP.NET

You're experiencing a significant performance difference when executing a SQL query in SSMS compared to ASP.NET. The query, shown below, runs in a matter of seconds in SSMS, but it takes minutes on your website.

DECLARE @customerID INT
SET @customerID = @CustID
DECLARE @MyTable table(
Iden int NOT NULL IDENTITY(1,1),
ProductID int)

INSERT INTO @MyTable(ProductID)
SELECT P.ProductID FROM Product P WITH (NOLOCK)
left join Compunix_ProductMMY cpmmy with (nolock) on p.ProductID = cpmmy.ProductID
left join Compunix_CustomerMMY ccmmy with (nolock) on ccmmy.mmyid = cpmmy.mmyid
WHERE P.Deleted=0 AND P.Published=1 and (ccmmy.customerid = @customerID OR cpmmy.productid IS NULL)

SELECT c.Name, c.SeName, c.CategoryID
FROM Category c WITH (NOLOCK) 
JOIN ProductCategory PC With (NOLOCK) ON C.CategoryID = PC.CategoryID
JOIN @MyTable MT ON PC.ProductID=MT.ProductID
WHERE C.Published = 1
GROUP BY c.Name, c.SeName, c.CategoryID
ORDER BY c.Name

The Culprit: Parameter Sniffing

The most likely cause of this discrepancy is parameter sniffing. Parameter sniffing is a performance optimization technique where SQL Server analyzes the query parameters the first time it executes and creates an execution plan based on those parameters. If the query parameters change later, SQL Server might not recompile the execution plan and continue to use the original plan, which may not be optimal for the new parameters.

In this case, the query performance likely degrades because the execution plan optimized for the first time you ran the query with a specific value for @CustID is not ideal for subsequent runs with different @CustID values.

Resolving the Issue

To mitigate parameter sniffing, you can use the FORCESEEK hint in the query. This forces SQL Server to create a new execution plan for each execution of the query, ensuring that it uses the optimal plan for the given parameters.

To resolve the issue, modify your query as follows:

DECLARE @customerID INT
SET @customerID = @CustID
DECLARE @MyTable table(
Iden int NOT NULL IDENTITY(1,1),
ProductID int)

WITH (FORCESEEK)
INSERT INTO @MyTable(ProductID)
SELECT P.ProductID FROM Product P WITH (NOLOCK)
left join Compunix_ProductMMY cpmmy with (nolock) on p.ProductID = cpmmy.ProductID
left join Compunix_CustomerMMY ccmmy with (nolock) on ccmmy.mmyid = cpmmy.mmyid
WHERE P.Deleted=0 AND P.Published=1 and (ccmmy.customerid = @customerID OR cpmmy.productid IS NULL)

SELECT c.Name, c.SeName, c.CategoryID
FROM Category c WITH (NOLOCK) 
JOIN ProductCategory PC With (NOLOCK) ON C.CategoryID = PC.CategoryID
JOIN @MyTable MT ON PC.ProductID=MT.ProductID
WHERE C.Published = 1
GROUP BY c.Name, c.SeName, c.CategoryID
ORDER BY c.Name

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