Juan Carlos Oropeza Juan Carlos Oropeza - 3 months ago 7
SQL Question

Better way to calculate the SUM of one, two and three consecutive items

class

public class myItem
{
public int ID;
public long Value;
public long Sum1;
public long Sum2;
public long Sum3;
}


data

ID Value
1 25
2 45
3 56
4 21


result:

Sum1 Sum2 Sum3
1 25 25 + 45 25 + 45 + 56
2 45 45 + 56 45 + 56 + 21
3 56 56 + 21 56 + 21 + 0
4 21 21 + 0 21 + 0 + 0


Current procedure: Works but very slow (~10 minutes) with 100k rows.

List<myItem> list;
for (int i = 0; i < list.Count(); i++)
{
myItem m = list[i];
m.Sum1 = list.Where(x => x.ID == i).Sum(x => x.Value);
m.Sum2 = list.Where(x => x.ID >= i && x.ID <= i + 2).Sum(x => x.Value);
m.Sum3 = list.Where(x => x.ID >= i && x.ID <= i + 3).Sum(x => x.Value);
}


So I guess there should be a way to do it without the
for
to speed things up.

Answer

The root cause of the slowness is that your loop is O(n^2), since for each element you search the entire list for its successors. Below reduces that to O(n log n) (slowest part is the sort).

List<myItem> list;
list.Sort(<.. sort by id ..>);
for (int i = 0; i < list.Count(); i++)
{
   myItem m = list[i];
   m.Sum1 = m.Value;
   if (i < list.Count() - 1)
   {
       m.Sum2 = m.Value+ list[i + 1].Value;
   }
   if (i < list.Count() - 1)
   {
       m.Sum3 = m.Value + list[i + 1].Value + list[i + 2].Value;
   }
}