code: func (s *StatsInfo) ScaleByExpectCnt(expectCnt float64) *StatsInfo {
  	if expectCnt > s.RowCount {
  		return s
  	}
  	if s.RowCount > 1.0 { // if s.RowCount is too small, it will cause overflow
  		return s.Scale(expectCnt / s.RowCount)
  	}
  	return s
  }
docs: // ScaleByExpectCnt tries to Scale StatsInfo to an expectCnt which must be
  // smaller than the derived cnt.
  // TODO: try to use a better way to do this.