def process(self, piece_index, worker_predicate, shuffle_row_drop_partition):
        """Main worker function. Loads and returns all rows matching the predicate from a rowgroup

        Looks up the requested piece (a single row-group in a parquet file). If a predicate is specified,
        columns needed by the predicate are loaded first. If no rows in the rowgroup matches the predicate criteria
        the rest of the columns are not loaded.

        :param piece_index:
        :param shuffle_row_drop_partition: A tuple 2 of the current row drop partition and the total number
            of partitions.
        :return:
        """

        if not self._dataset:
            self._dataset = pq.ParquetDataset(
                self._dataset_path_or_paths,
                filesystem=self._filesystem,
                validate_schema=False, filters=self._arrow_filters)

        piece = self._split_pieces[piece_index]

        # Create pyarrow file system
        parquet_file = ParquetFile(self._dataset.fs.open(piece.path))

        if not isinstance(self._local_cache, NullCache):
            if worker_predicate:
                raise RuntimeError('Local cache is not supported together with predicates, '
                                   'unless the dataset is partitioned by the column the predicate operates on.')
            if shuffle_row_drop_partition[1] != 1:
                raise RuntimeError('Local cache is not supported together with shuffle_row_drop_partitions > 1')

        if worker_predicate:
            all_cols = self._load_rows_with_predicate(parquet_file, piece, worker_predicate, shuffle_row_drop_partition)
        else:
            # Using hash of the dataset path with the relative path in order to:
            #  1. Make sure if a common cache serves multiple processes (e.g. redis), we don't have conflicts
            #  2. Dataset path is hashed, to make sure we don't create too long keys, which maybe incompatible with
            #     some cache implementations
            #  3. Still leave relative path and the piece_index in plain text to make it easier to debug
            if isinstance(self._dataset_path_or_paths, list):
                path_str = ','.join(self._dataset_path_or_paths)
            else:
                path_str = self._dataset_path_or_paths
            cache_key = '{}:{}:{}'.format(hashlib.md5(path_str.encode('utf-8')).hexdigest(),
                                          piece.path, piece_index)
            all_cols = self._local_cache.get(cache_key,
                                             lambda: self._load_rows(parquet_file, piece, shuffle_row_drop_partition))

        if all_cols:
            self.publish_func(all_cols)
Main worker function. Loads and returns all rows matching the predicate from a rowgroup

        Looks up the requested piece (a single row-group in a parquet file). If a predicate is specified,
        columns needed by the predicate are loaded first. If no rows in the rowgroup matches the predicate criteria
        the rest of the columns are not loaded.

        :param piece_index:
        :param shuffle_row_drop_partition: A tuple 2 of the current row drop partition and the total number
            of partitions.
        :return:
        
petastorm/arrow_reader_worker.py