code: func (hg *Histogram) AvgCountPerNotNullValue(totalCount int64) float64 {
  	factor := hg.GetIncreaseFactor(totalCount)
  	totalNotNull := hg.notNullCount() * factor
  	curNDV := float64(hg.NDV) * factor
  	if curNDV == 0 {
  		curNDV = 1
  	}
  	return totalNotNull / curNDV
  }
docs: // AvgCountPerNotNullValue gets the average row count per value by the data of histogram.