code: func (hg *Histogram) DecodeTo(tp *types.FieldType, timeZone *time.Location) error {
  	oldIter := chunk.NewIterator4Chunk(hg.Bounds)
  	hg.Bounds = chunk.NewChunkWithCapacity([]*types.FieldType{tp}, oldIter.Len())
  	hg.Tp = tp
  	for row := oldIter.Begin(); row != oldIter.End(); row = oldIter.Next() {
  		datum, err := tablecodec.DecodeColumnValue(row.GetBytes(0), tp, timeZone)
  		if err != nil {
  			return errors.Trace(err)
  		}
  		hg.Bounds.AppendDatum(0, &datum)
  	}
  	return nil
  }
docs: // DecodeTo decodes the histogram bucket values into `Tp`.