Why is My ASP.NET Query Slower Than in SSMS?

Slow Query Performance in ASP.NET Compared to SSMS

The discrepancy in query execution time between SQL Server Management Studio (SSMS) and an ASP.NET application can be attributed to a phenomenon known as "parameter sniffing."

Parameter sniffing occurs when SQL Server compiles and executes a query based on the values of its parameters during the first execution. These values may be different from those used in subsequent executions, leading to suboptimal execution plans and performance degradation.

In your specific scenario, the query's performance varies on the ASP.NET website because the values of the @customerID parameter are changing. The first time the query is executed after a change in the parameter value, it creates an efficient execution plan based on those values. However, subsequent executions with different parameter values may result in slower performance due to the use of the cached, non-optimal execution plan.

To resolve this issue, consider the following techniques:

• Recompile the query with sp_recompile: This stored procedure forces SQL Server to recompile the query each time it is executed, ensuring that the execution plan is optimized for the current parameter values.
• Use query hints: The "OPTION (RECOMPILE)" hint instructs SQL Server to recompile the query each time it is executed, similar to sp_recompile.
• Parameterize the query in your ASP.NET code: ASP.NET's data access framework (e.g., ADO.NET) automatically parameterizes queries, preventing SQL Server from incorrectly assuming parameter types.
• Clear the procedure cache: Using the "DBCC FREEPROCCACHE" command clears the procedure cache, forcing SQL Server to recompile any stored procedures or queries that have been cached.

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