@SergeiPavlov, I see that ColumnGatherer.GetColumns() changed return type to IEnumerable which forced a lot of places to make .ToList() or ToArray() but in other places. Did it worth it? What scenario overweight these enumerations + possible number of Array.Copy? Please, point me.

@SergeiPavlov, I see that ColumnGatherer.GetColumns() changed return type to IEnumerable which forced a lot of places to make .ToList() or ToArray() but in other places. Did it worth it? What scenario overweight these enumerations + possible number of Array.Copy? Please, point me.

There are a few places whe we need not to call neither .ToList() nor .ToArray(), but just to enumerate:

          var columns = orderByProjector
            .GetColumns(ColumnExtractionModes.TreatEntityAsKey | ColumnExtractionModes.Distinct);
          foreach (var c in columns) {
            if (!sortColumns.ContainsKey(c)) {
              sortColumns.Add(c, direction);
            }
          }

      var keySegment = visitedSource.ItemProjector.GetColumns(ColumnExtractionModes.TreatEntityAsKey);
      var keyPairs = keySegment
        .Select((leftIndex, rightIndex) => new Pair<int>(leftIndex, rightIndex))
        .ToArray();

I removed .ToList() from .GetColumns() implementation. So total number of list allocations has been decreased

@SergeiPavlov If we had both results would it be better?