SQL Server Output Parameter is returning mixed results

Question

** EDIT ***************** I changed from GETDATE() to SYSDATETIME(). I also changed my measure from milliseconds to microseconds. The processing was happening so fast that milliseconds weren't able to measure it.

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My goal with this code is to calculate the processing time of the query string that's passed into the stored procedure. The query string is created by the user, a student taking a sql/database course. In a subsequent query, the string is compared against the key to determine if it produces the correct results for the assignment. As you'll see, it's VB.NET code (.NET 4.5 project) calling a C# method (.NET 3.5 project) that calls a stored procedure (SQL SERVER 2012).

My initial call from the main program project is here:

Dim executionTime As Integer = exercise.GetSqlTime(txtUserSQL, connectionString)

The method called is here:

public int GetSqlTime(string SqlString, string ConnectionString)//bookmark
    {
        int executionTime = 0;

        SqlConnection con = new SqlConnection(ConnectionString);
        using (SqlCommand cmd = new SqlCommand("QueryExecutionTime", con))
        {
            cmd.CommandType = CommandType.StoredProcedure;

            cmd.Parameters.Add("@UserSQLstring", SqlDbType.NVarChar).Value = SqlString;
            cmd.Parameters.Add("@Duration", SqlDbType.Int);
            cmd.Parameters["@Duration"].Direction = ParameterDirection.Output;

            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
            executionTime = Convert.ToInt32(cmd.Parameters["@Duration"].Value); 
        }
        return executionTime;
    }

My stored procedure is here:

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

ALTER PROCEDURE [dbo].[QueryExecutionTime] 
@UserSQLstring NvarChar(3000),
@Duration int OUTPUT
AS
BEGIN
SET NOCOUNT ON;

DECLARE @StartTime datetime = GETDATE() 
EXEC sp_executesql @UserSQLstring
DECLARE @EndTime datetime = GETDATE()   

SET @Duration = DATEDIFF(MILLISECOND,@StartTime,@EndTime)
END

The result is 0 when the query string is correct. When the string is incorrect, it returns a value between 1.4 and 2.5 seconds

This is the first time I've used a stored procedure with an output parameter. I had no idea it would prove to be so challenging.

Am I overlooking a simple detail that would make this work correctly? I have already tried at least five or six examples here on Stack Overflow alone - along with several other websites. What you're seeing here is my latest iteration. If you need additional information, please let me know.

If you know of a better way of doing this, I'm open to any and all suggestions.

Answer 1

It is possible that your results are correct. If your SQL SERVER version is 2008 or greater use SYSDATETIME() rather than GETDATE() because it has greater accuracy (or resolution). Otherwise you can try running the query string multiple times then dividing the result by the number of runs. The execution time from a badly formed query string should be discarded.