Return generator through chunked result set.

def _query_iterator(self, result, chunksize, columns, coerce_float=True,
                        parse_dates=None):
        """Return generator through chunked result set."""

        while True:
            data = result.fetchmany(chunksize)
            if not data:
                break
            else:
                self.frame = DataFrame.from_records(
                    data, columns=columns, coerce_float=coerce_float)

                self._harmonize_columns(parse_dates=parse_dates)

                if self.index is not None:
                    self.frame.set_index(self.index, inplace=True)

                yield self.frame