def update_database_ddl(
        self,
        database,
        statements,
        operation_id=None,
        retry=google.api_core.gapic_v1.method.DEFAULT,
        timeout=google.api_core.gapic_v1.method.DEFAULT,
        metadata=None,
    ):
        """
        Updates the schema of a Cloud Spanner database by
        creating/altering/dropping tables, columns, indexes, etc. The returned
        ``long-running operation`` will have a name of the format
        ``<database_name>/operations/<operation_id>`` and can be used to track
        execution of the schema change(s). The ``metadata`` field type is
        ``UpdateDatabaseDdlMetadata``. The operation has no response.

        Example:
            >>> from google.cloud import spanner_admin_database_v1
            >>>
            >>> client = spanner_admin_database_v1.DatabaseAdminClient()
            >>>
            >>> database = client.database_path('[PROJECT]', '[INSTANCE]', '[DATABASE]')
            >>>
            >>> # TODO: Initialize `statements`:
            >>> statements = []
            >>>
            >>> response = client.update_database_ddl(database, statements)
            >>>
            >>> def callback(operation_future):
            ...     # Handle result.
            ...     result = operation_future.result()
            >>>
            >>> response.add_done_callback(callback)
            >>>
            >>> # Handle metadata.
            >>> metadata = response.metadata()

        Args:
            database (str): Required. The database to update.
            statements (list[str]): DDL statements to be applied to the database.
            operation_id (str): If empty, the new update request is assigned an automatically-generated
                operation ID. Otherwise, ``operation_id`` is used to construct the name
                of the resulting ``Operation``.

                Specifying an explicit operation ID simplifies determining whether the
                statements were executed in the event that the ``UpdateDatabaseDdl``
                call is replayed, or the return value is otherwise lost: the
                ``database`` and ``operation_id`` fields can be combined to form the
                ``name`` of the resulting ``longrunning.Operation``:
                ``<database>/operations/<operation_id>``.

                ``operation_id`` should be unique within the database, and must be a
                valid identifier: ``[a-z][a-z0-9_]*``. Note that automatically-generated
                operation IDs always begin with an underscore. If the named operation
                already exists, ``UpdateDatabaseDdl`` returns ``ALREADY_EXISTS``.
            retry (Optional[google.api_core.retry.Retry]):  A retry object used
                to retry requests. If ``None`` is specified, requests will not
                be retried.
            timeout (Optional[float]): The amount of time, in seconds, to wait
                for the request to complete. Note that if ``retry`` is
                specified, the timeout applies to each individual attempt.
            metadata (Optional[Sequence[Tuple[str, str]]]): Additional metadata
                that is provided to the method.

        Returns:
            A :class:`~google.cloud.spanner_admin_database_v1.types._OperationFuture` instance.

        Raises:
            google.api_core.exceptions.GoogleAPICallError: If the request
                    failed for any reason.
            google.api_core.exceptions.RetryError: If the request failed due
                    to a retryable error and retry attempts failed.
            ValueError: If the parameters are invalid.
        """
        # Wrap the transport method to add retry and timeout logic.
        if "update_database_ddl" not in self._inner_api_calls:
            self._inner_api_calls[
                "update_database_ddl"
            ] = google.api_core.gapic_v1.method.wrap_method(
                self.transport.update_database_ddl,
                default_retry=self._method_configs["UpdateDatabaseDdl"].retry,
                default_timeout=self._method_configs["UpdateDatabaseDdl"].timeout,
                client_info=self._client_info,
            )

        request = spanner_database_admin_pb2.UpdateDatabaseDdlRequest(
            database=database, statements=statements, operation_id=operation_id
        )
        if metadata is None:
            metadata = []
        metadata = list(metadata)
        try:
            routing_header = [("database", database)]
        except AttributeError:
            pass
        else:
            routing_metadata = google.api_core.gapic_v1.routing_header.to_grpc_metadata(
                routing_header
            )
            metadata.append(routing_metadata)

        operation = self._inner_api_calls["update_database_ddl"](
            request, retry=retry, timeout=timeout, metadata=metadata
        )
        return google.api_core.operation.from_gapic(
            operation,
            self.transport._operations_client,
            empty_pb2.Empty,
            metadata_type=spanner_database_admin_pb2.UpdateDatabaseDdlMetadata,
        )