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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