code: func (ds *DataSource) getColumnNDV(colID int64) (ndv float64) {
  	hist, ok := ds.statisticTable.Columns[colID]
  	if ok && hist.Count > 0 {
  		factor := float64(ds.statisticTable.Count) / float64(hist.Count)
  		ndv = float64(hist.NDV) * factor
  	} else {
  		ndv = float64(ds.statisticTable.Count) * distinctFactor
  	}
  	return ndv
  }
docs: // getColumnNDV computes estimated NDV of specified column using the original
  // histogram of `DataSource` which is retrieved from storage(not the derived one).