code: func DecodeColumnValue(data []byte, ft *types.FieldType, loc *time.Location) (types.Datum, error) {
  	_, d, err := codec.DecodeOne(data)
  	if err != nil {
  		return types.Datum{}, errors.Trace(err)
  	}
  	colDatum, err := unflatten(d, ft, loc)
  	if err != nil {
  		return types.Datum{}, errors.Trace(err)
  	}
  	return colDatum, nil
  }
docs: // DecodeColumnValue decodes data to a Datum according to the column info.