code: func (hg *Histogram) notNullCount() float64 {
  	if hg.Len() == 0 {
  		return 0
  	}
  	return float64(hg.Buckets[hg.Len()-1].Count)
  }
docs: // notNullCount indicates the count of non-null values in column histogram and single-column index histogram,
  // for multi-column index histogram, since we cannot define null for the row, we treat all rows as non-null, that means,
  // notNullCount would return same value as TotalRowCount for multi-column index histograms.