Perform a ``StreamingRead`` API request for rows in a table.

        :type table: str
        :param table: name of the table from which to fetch data

        :type columns: list of str
        :param columns: names of columns to be retrieved

        :type keyset: :class:`~google.cloud.spanner_v1.keyset.KeySet`
        :param keyset: keys / ranges identifying rows to be retrieved

        :type index: str
        :param index: (Optional) name of index to use, rather than the
                      table's primary key

        :type limit: int
        :param limit: (Optional) maximum number of rows to return.
                      Incompatible with ``partition``.

        :type partition: bytes
        :param partition: (Optional) one of the partition tokens returned
                          from :meth:`partition_read`.  Incompatible with
                          ``limit``.

        :rtype: :class:`~google.cloud.spanner_v1.streamed.StreamedResultSet`
        :returns: a result set instance which can be used to consume rows.

        :raises ValueError:
            for reuse of single-use snapshots, or if a transaction ID is
            already pending for multiple-use snapshots.

def read(self, table, columns, keyset, index="", limit=0, partition=None):
        """Perform a ``StreamingRead`` API request for rows in a table.

        :type table: str
        :param table: name of the table from which to fetch data

        :type columns: list of str
        :param columns: names of columns to be retrieved

        :type keyset: :class:`~google.cloud.spanner_v1.keyset.KeySet`
        :param keyset: keys / ranges identifying rows to be retrieved

        :type index: str
        :param index: (Optional) name of index to use, rather than the
                      table's primary key

        :type limit: int
        :param limit: (Optional) maximum number of rows to return.
                      Incompatible with ``partition``.

        :type partition: bytes
        :param partition: (Optional) one of the partition tokens returned
                          from :meth:`partition_read`.  Incompatible with
                          ``limit``.

        :rtype: :class:`~google.cloud.spanner_v1.streamed.StreamedResultSet`
        :returns: a result set instance which can be used to consume rows.

        :raises ValueError:
            for reuse of single-use snapshots, or if a transaction ID is
            already pending for multiple-use snapshots.
        """
        if self._read_request_count > 0:
            if not self._multi_use:
                raise ValueError("Cannot re-use single-use snapshot.")
            if self._transaction_id is None:
                raise ValueError("Transaction ID pending.")

        database = self._session._database
        api = database.spanner_api
        metadata = _metadata_with_prefix(database.name)
        transaction = self._make_txn_selector()

        restart = functools.partial(
            api.streaming_read,
            self._session.name,
            table,
            columns,
            keyset._to_pb(),
            transaction=transaction,
            index=index,
            limit=limit,
            partition_token=partition,
            metadata=metadata,
        )

        iterator = _restart_on_unavailable(restart)

        self._read_request_count += 1

        if self._multi_use:
            return StreamedResultSet(iterator, source=self)
        else:
            return StreamedResultSet(iterator)