code: func UpdateHistogram(h *Histogram, feedback *QueryFeedback) *Histogram {
  	buckets, isNewBuckets, totalCount := splitBuckets(h, feedback)
  	buckets = mergeBuckets(buckets, isNewBuckets, float64(totalCount))
  	hist := buildNewHistogram(h, buckets)
  	// Update the NDV of primary key column.
  	if feedback.Tp == PkType {
  		hist.NDV = int64(hist.TotalRowCount())
  	}
  	return hist
  }
docs: // UpdateHistogram updates the histogram according buckets.