code: func (s *rowSegment) Columns() []uint64 {
  	a := make([]uint64, 0, s.Count())
  	itr := s.data.Iterator()
  	for v, eof := itr.Next(); !eof; v, eof = itr.Next() {
  		a = append(a, v)
  	}
  	return a
  }
docs: // Columns returns a list of all columns set in the segment.