Question

I need to write a query pulling distinct values from columns defined by a user for any given data set. There could be millions of rows so the statements must be as efficient as possible. Below is the code I have.

What is the order of this LINQ query? Is there a more efficient way of doing this?

var MyValues = from r in MyDataTable.AsEnumerable()
               orderby r.Field<double>(_varName)
               select r.Field<double>(_varName); 

IEnumerable result= MyValues.Distinct();

Answer 1

I can’t speak much to the AsEnumerable() call or the field conversions, but for the LINQ side of things, the orderby is a stable quick sort and should be O(n log n). If I had to guess, everything but the orderby should be O(n), so overall you’re still just O(n log n).

Update: the LINQ Distinct() call should also be O(n).

So altogether, the Big-Oh for this thing is still O(Kn log n), where K is some constant.