Question

I have a dataset which contains more than 50k rows. I decided to take average of the first fifty values, then second fifty values and so on (so that I will get half of the Dataset with average of 50 value groups).

Here is my code:

var Rate = (from dr in ds.Tables[0].AsEnumerable()
                                    select new
                                    {
                                        rate = dr.Field<double>(columnName)
                                    }.rate).ToList();
                            if (Rate.Count > 50)
                            {
                                var avg = Rate.CheckRateValue();
                            }

And here is the code for my extension method:

public static IEnumerable<double> CheckRateValue(this IEnumerable<double> values)
{
    int i = 1;
    int j = 0;
    for (; i < values.Count(); )
    {
        yield return values.Skip(j * 2).Take(2).Average();
        i = i + 2;
        j++;
    }
}

Problem :
It works fine but it’s slow. Does anyone have any suggestions on how to speed it up?

Answer 1

By using row_number() i solved it.

declare @Skip int = 20
declare @Take int = 10

select SomeColumn
from (
   select SomeColumn,
          row_number() over(order by SomeColumnToOrderBy) as rn
   from YourTable
 ) T
where rn > @Skip and 
  rn <= @Skip + @Take