code: func (hg *Histogram) BucketToString(bktID, idxCols int) string {
  	upperVal, err := ValueToString(hg.GetUpper(bktID), idxCols)
  	terror.Log(errors.Trace(err))
  	lowerVal, err := ValueToString(hg.GetLower(bktID), idxCols)
  	terror.Log(errors.Trace(err))
  	return fmt.Sprintf("num: %d lower_bound: %s upper_bound: %s repeats: %d", hg.bucketCount(bktID), lowerVal, upperVal, hg.Buckets[bktID].Repeat)
  }
docs: // BucketToString change the given bucket to string format.