Why is my SQL Query Fast in SSMS but Slow in my C# Application?
Code Runs Slow in Application but Fast in SSMS
Problem:
A query that returns data quickly in SQL Server Management Studio (SSMS) takes a significant amount of time when run from a C# application.
Code:
The C# code uses a SqlDataAdapter and its Fill method to execute the query.
using (var conn = new SqlConnection(...))
using (var ada = new SqlDataAdapter(...))
{
ada.Fill(Logs);
}
The SSMS query is identical to the one executed in the application.
Cause:
The discrepancy in execution time is caused by a difference in the way parameters are handled in the application and SSMS. In the application, the AddWithValue method is used to add a parameter of type NVARCHAR, while in SSMS, the parameter is declared as VARCHAR.
Solution:
To resolve this issue, either:
- Use the constructor that accepts a type when adding parameters in the application code:
ada.SelectCommand.Parameters.Add("@clientID", SqlDbType.Varchar, 200);
- Cast the parameter in the SQL text:
where client_id = cast(@clientID as varchar(200))
The first solution is preferred as it also addresses a potential cache pollution issue.
Additional Considerations:
- Ensure that both the application and SSMS are using the same computer and login.
- The tables referenced in the query have appropriate indexes and are performing well in SSMS.
- Consider the effect of parameter caching on query performance.
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