code: func (hg *Histogram) ToString(idxCols int) string {
  	strs := make([]string, 0, hg.Len()+1)
  	if idxCols > 0 {
  		strs = append(strs, fmt.Sprintf("index:%d ndv:%d", hg.ID, hg.NDV))
  	} else {
  		strs = append(strs, fmt.Sprintf("column:%d ndv:%d totColSize:%d", hg.ID, hg.NDV, hg.TotColSize))
  	}
  	for i := 0; i < hg.Len(); i++ {
  		strs = append(strs, hg.BucketToString(i, idxCols))
  	}
  	return strings.Join(strs, "\n")
  }
docs: // ToString gets the string representation for the histogram.