Why is My SQL Query 3 Minutes Slower in C# Than in SSMS?

Query Performance Discrepancy Between Code and SSMS

A developer encountered a significant performance difference when executing the same query in C# code and SQL Server Management Studio (SSMS). While the query completed in less than 3 seconds in SSMS, it took over 3 minutes when run in the application.

Root Cause of Performance Issue

Upon further examination, the issue was identified to be a subtle difference between the query parameters used in the C# code and SSMS. In the C# code, the parameter for the client ID was passed as an NVARCHAR type, while in the SSMS query it was declared as a VARCHAR type.

Data Type Precedence Impact

Due to data type precedence rules, the NVARCHAR parameter in the C# query prevented the query from leveraging an index seek. This resulted in a table scan, significantly increasing the execution time. In contrast, the VARCHAR parameter in the SSMS query allowed the query optimizer to use an index seek, resulting in faster execution.

Solution to Performance Issue

To resolve the performance discrepancy, the developer had two options:

• Option 1: Use Parameter with Specified Type

  By using the constructor that accepts a type, the developer explicitly defined the parameter as a VARCHAR type:

  This ensures that the parameter is passed as the correct type and does not interfere with the SARG-ability of the query.

• Option 2: Cast Parameter in SQL Query

  Alternatively, the developer could cast the parameter within the SQL query as follows:

  This method also resolves the type issue and allows the query to leverage the index seek.

Additional Recommendations

• Use Explicit Parameter Length: To prevent cache pollution, the developer is advised to specify the length of the VARCHAR parameter explicitly when using Option 1.
• Reference Recommended Article: The developer is encouraged to refer to the article "Slow in the Application, Fast in SSMS? Understanding Performance Mysteries" for further insights into performance discrepancies between code and SSMS.

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