table.py

# Copyright 2015 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Define API Tables."""

from __future__ import absolute_import

import copy
import datetime
import functools
import operator
import pytz
import typing
from typing import Any, Dict, Iterable, Iterator, Optional, Tuple
import warnings

try:
    import pandas
except ImportError:  # pragma: NO COVER
    pandas = None

try:
    import pyarrow
except ImportError:  # pragma: NO COVER
    pyarrow = None

import google.api_core.exceptions
from google.api_core.page_iterator import HTTPIterator

import google.cloud._helpers
from google.cloud.bigquery import _helpers
from google.cloud.bigquery import _pandas_helpers
from google.cloud.bigquery.exceptions import LegacyBigQueryStorageError
from google.cloud.bigquery.schema import _build_schema_resource
from google.cloud.bigquery.schema import _parse_schema_resource
from google.cloud.bigquery.schema import _to_schema_fields
from google.cloud.bigquery._tqdm_helpers import get_progress_bar
from google.cloud.bigquery.external_config import ExternalConfig
from google.cloud.bigquery.encryption_configuration import EncryptionConfiguration

if typing.TYPE_CHECKING:  # pragma: NO COVER
    # Unconditionally import optional dependencies again to tell pytype that
    # they are not None, avoiding false "no attribute" errors.
    import pandas
    import pyarrow
    from google.cloud import bigquery_storage


_NO_PANDAS_ERROR = (
    "The pandas library is not installed, please install "
    "pandas to use the to_dataframe() function."
)
_NO_PYARROW_ERROR = (
    "The pyarrow library is not installed, please install "
    "pyarrow to use the to_arrow() function."
)

_TABLE_HAS_NO_SCHEMA = 'Table has no schema:  call "client.get_table()"'


def _reference_getter(table):
    """A :class:`~google.cloud.bigquery.table.TableReference` pointing to
    this table.

    Returns:
        google.cloud.bigquery.table.TableReference: pointer to this table.
    """
    from google.cloud.bigquery import dataset

    dataset_ref = dataset.DatasetReference(table.project, table.dataset_id)
    return TableReference(dataset_ref, table.table_id)


def _view_use_legacy_sql_getter(table):
    """bool: Specifies whether to execute the view with Legacy or Standard SQL.

    This boolean specifies whether to execute the view with Legacy SQL
    (:data:`True`) or Standard SQL (:data:`False`). The client side default is
    :data:`False`. The server-side default is :data:`True`. If this table is
    not a view, :data:`None` is returned.

    Raises:
        ValueError: For invalid value types.
    """
    view = table._properties.get("view")
    if view is not None:
        # The server-side default for useLegacySql is True.
        return view.get("useLegacySql", True)
    # In some cases, such as in a table list no view object is present, but the
    # resource still represents a view. Use the type as a fallback.
    if table.table_type == "VIEW":
        # The server-side default for useLegacySql is True.
        return True


class TableReference(object):
    """TableReferences are pointers to tables.

    See
    https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#tablereference

    Args:
        dataset_ref (google.cloud.bigquery.dataset.DatasetReference):
            A pointer to the dataset
        table_id (str): The ID of the table
    """

    def __init__(self, dataset_ref, table_id):
        self._project = dataset_ref.project
        self._dataset_id = dataset_ref.dataset_id
        self._table_id = table_id

    @property
    def project(self):
        """str: Project bound to the table"""
        return self._project

    @property
    def dataset_id(self):
        """str: ID of dataset containing the table."""
        return self._dataset_id

    @property
    def table_id(self):
        """str: The table ID."""
        return self._table_id

    @property
    def path(self):
        """str: URL path for the table's APIs."""
        return "/projects/%s/datasets/%s/tables/%s" % (
            self._project,
            self._dataset_id,
            self._table_id,
        )

    @classmethod
    def from_string(
        cls, table_id: str, default_project: str = None
    ) -> "TableReference":
        """Construct a table reference from table ID string.

        Args:
            table_id (str):
                A table ID in standard SQL format. If ``default_project``
                is not specified, this must included a project ID, dataset
                ID, and table ID, each separated by ``.``.
            default_project (Optional[str]):
                The project ID to use when ``table_id`` does not
                include a project ID.

        Returns:
            TableReference: Table reference parsed from ``table_id``.

        Examples:
            >>> TableReference.from_string('my-project.mydataset.mytable')
            TableRef...(DatasetRef...('my-project', 'mydataset'), 'mytable')

        Raises:
            ValueError:
                If ``table_id`` is not a fully-qualified table ID in
                standard SQL format.
        """
        from google.cloud.bigquery.dataset import DatasetReference

        (
            output_project_id,
            output_dataset_id,
            output_table_id,
        ) = _helpers._parse_3_part_id(
            table_id, default_project=default_project, property_name="table_id"
        )

        return cls(
            DatasetReference(output_project_id, output_dataset_id), output_table_id
        )

    @classmethod
    def from_api_repr(cls, resource: dict) -> "TableReference":
        """Factory:  construct a table reference given its API representation

        Args:
            resource (Dict[str, object]):
                Table reference representation returned from the API

        Returns:
            google.cloud.bigquery.table.TableReference:
                Table reference parsed from ``resource``.
        """
        from google.cloud.bigquery.dataset import DatasetReference

        project = resource["projectId"]
        dataset_id = resource["datasetId"]
        table_id = resource["tableId"]
        return cls(DatasetReference(project, dataset_id), table_id)

    def to_api_repr(self) -> dict:
        """Construct the API resource representation of this table reference.

        Returns:
            Dict[str, object]: Table reference represented as an API resource
        """
        return {
            "projectId": self._project,
            "datasetId": self._dataset_id,
            "tableId": self._table_id,
        }

    def to_bqstorage(self) -> str:
        """Construct a BigQuery Storage API representation of this table.

        Install the ``google-cloud-bigquery-storage`` package to use this
        feature.

        If the ``table_id`` contains a partition identifier (e.g.
        ``my_table$201812``) or a snapshot identifier (e.g.
        ``mytable@1234567890``), it is ignored. Use
        :class:`google.cloud.bigquery_storage.types.ReadSession.TableReadOptions`
        to filter rows by partition. Use
        :class:`google.cloud.bigquery_storage.types.ReadSession.TableModifiers`
        to select a specific snapshot to read from.

        Returns:
            str: A reference to this table in the BigQuery Storage API.
        """

        table_id, _, _ = self._table_id.partition("@")
        table_id, _, _ = table_id.partition("$")

        table_ref = "projects/{}/datasets/{}/tables/{}".format(
            self._project, self._dataset_id, table_id,
        )

        return table_ref

    def _key(self):
        """A tuple key that uniquely describes this field.

        Used to compute this instance's hashcode and evaluate equality.

        Returns:
            Tuple[str]: The contents of this :class:`DatasetReference`.
        """
        return (self._project, self._dataset_id, self._table_id)

    def __eq__(self, other):
        if not isinstance(other, TableReference):
            return NotImplemented
        return self._key() == other._key()

    def __ne__(self, other):
        return not self == other

    def __hash__(self):
        return hash(self._key())

    def __str__(self):
        return f"{self.project}.{self.dataset_id}.{self.table_id}"

    def __repr__(self):
        from google.cloud.bigquery.dataset import DatasetReference

        dataset_ref = DatasetReference(self._project, self._dataset_id)
        return "TableReference({}, '{}')".format(repr(dataset_ref), self._table_id)


class Table(object):
    """Tables represent a set of rows whose values correspond to a schema.

    See
    https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#resource-table

    Args:
        table_ref (Union[google.cloud.bigquery.table.TableReference, str]):
            A pointer to a table. If ``table_ref`` is a string, it must
            included a project ID, dataset ID, and table ID, each separated
            by ``.``.
        schema (Optional[Sequence[Union[ \
                :class:`~google.cloud.bigquery.schema.SchemaField`, \
                Mapping[str, Any] \
        ]]]):
            The table's schema. If any item is a mapping, its content must be
            compatible with
            :meth:`~google.cloud.bigquery.schema.SchemaField.from_api_repr`.
    """

    _PROPERTY_TO_API_FIELD = {
        "clustering_fields": "clustering",
        "created": "creationTime",
        "dataset_id": ["tableReference", "datasetId"],
        "description": "description",
        "encryption_configuration": "encryptionConfiguration",
        "etag": "etag",
        "expires": "expirationTime",
        "external_data_configuration": "externalDataConfiguration",
        "friendly_name": "friendlyName",
        "full_table_id": "id",
        "labels": "labels",
        "location": "location",
        "modified": "lastModifiedTime",
        "mview_enable_refresh": "materializedView",
        "mview_last_refresh_time": ["materializedView", "lastRefreshTime"],
        "mview_query": "materializedView",
        "mview_refresh_interval": "materializedView",
        "num_bytes": "numBytes",
        "num_rows": "numRows",
        "partition_expiration": "timePartitioning",
        "partitioning_type": "timePartitioning",
        "project": ["tableReference", "projectId"],
        "range_partitioning": "rangePartitioning",
        "time_partitioning": "timePartitioning",
        "schema": "schema",
        "snapshot_definition": "snapshotDefinition",
        "streaming_buffer": "streamingBuffer",
        "self_link": "selfLink",
        "table_id": ["tableReference", "tableId"],
        "time_partitioning": "timePartitioning",
        "type": "type",
        "view_use_legacy_sql": "view",
        "view_query": "view",
        "require_partition_filter": "requirePartitionFilter",
    }

    def __init__(self, table_ref, schema=None):
        table_ref = _table_arg_to_table_ref(table_ref)
        self._properties = {"tableReference": table_ref.to_api_repr(), "labels": {}}
        # Let the @property do validation.
        if schema is not None:
            self.schema = schema

    @property
    def project(self):
        """str: Project bound to the table."""
        return _helpers._get_sub_prop(
            self._properties, self._PROPERTY_TO_API_FIELD["project"]
        )

    @property
    def dataset_id(self):
        """str: ID of dataset containing the table."""
        return _helpers._get_sub_prop(
            self._properties, self._PROPERTY_TO_API_FIELD["dataset_id"]
        )

    @property
    def table_id(self):
        """str: ID of the table."""
        return _helpers._get_sub_prop(
            self._properties, self._PROPERTY_TO_API_FIELD["table_id"]
        )

    reference = property(_reference_getter)

    @property
    def path(self):
        """str: URL path for the table's APIs."""
        return "/projects/%s/datasets/%s/tables/%s" % (
            self.project,
            self.dataset_id,
            self.table_id,
        )

    @property
    def require_partition_filter(self):
        """bool: If set to true, queries over the partitioned table require a
        partition filter that can be used for partition elimination to be
        specified.
        """
        return self._properties.get(
            self._PROPERTY_TO_API_FIELD["require_partition_filter"]
        )

    @require_partition_filter.setter
    def require_partition_filter(self, value):
        self._properties[
            self._PROPERTY_TO_API_FIELD["require_partition_filter"]
        ] = value

    @property
    def schema(self):
        """Sequence[Union[ \
                :class:`~google.cloud.bigquery.schema.SchemaField`, \
                Mapping[str, Any] \
        ]]:
            Table's schema.

        Raises:
            Exception:
                If ``schema`` is not a sequence, or if any item in the sequence
                is not a :class:`~google.cloud.bigquery.schema.SchemaField`
                instance or a compatible mapping representation of the field.
        """
        prop = self._properties.get(self._PROPERTY_TO_API_FIELD["schema"])
        if not prop:
            return []
        else:
            return _parse_schema_resource(prop)

    @schema.setter
    def schema(self, value):
        api_field = self._PROPERTY_TO_API_FIELD["schema"]

        if value is None:
            self._properties[api_field] = None
        else:
            value = _to_schema_fields(value)
            self._properties[api_field] = {"fields": _build_schema_resource(value)}

    @property
    def labels(self):
        """Dict[str, str]: Labels for the table.

        This method always returns a dict. To change a table's labels,
        modify the dict, then call ``Client.update_table``. To delete a
        label, set its value to :data:`None` before updating.

        Raises:
            ValueError: If ``value`` type is invalid.
        """
        return self._properties.setdefault(self._PROPERTY_TO_API_FIELD["labels"], {})

    @labels.setter
    def labels(self, value):
        if not isinstance(value, dict):
            raise ValueError("Pass a dict")
        self._properties[self._PROPERTY_TO_API_FIELD["labels"]] = value

    @property
    def encryption_configuration(self):
        """google.cloud.bigquery.encryption_configuration.EncryptionConfiguration: Custom
        encryption configuration for the table.

        Custom encryption configuration (e.g., Cloud KMS keys) or :data:`None`
        if using default encryption.

        See `protecting data with Cloud KMS keys
        <https://cloud.google.com/bigquery/docs/customer-managed-encryption>`_
        in the BigQuery documentation.
        """
        prop = self._properties.get(
            self._PROPERTY_TO_API_FIELD["encryption_configuration"]
        )
        if prop is not None:
            prop = EncryptionConfiguration.from_api_repr(prop)
        return prop

    @encryption_configuration.setter
    def encryption_configuration(self, value):
        api_repr = value
        if value is not None:
            api_repr = value.to_api_repr()
        self._properties[
            self._PROPERTY_TO_API_FIELD["encryption_configuration"]
        ] = api_repr

    @property
    def created(self):
        """Union[datetime.datetime, None]: Datetime at which the table was
        created (:data:`None` until set from the server).
        """
        creation_time = self._properties.get(self._PROPERTY_TO_API_FIELD["created"])
        if creation_time is not None:
            # creation_time will be in milliseconds.
            return google.cloud._helpers._datetime_from_microseconds(
                1000.0 * float(creation_time)
            )

    @property
    def etag(self):
        """Union[str, None]: ETag for the table resource (:data:`None` until
        set from the server).
        """
        return self._properties.get(self._PROPERTY_TO_API_FIELD["etag"])

    @property
    def modified(self):
        """Union[datetime.datetime, None]: Datetime at which the table was last
        modified (:data:`None` until set from the server).
        """
        modified_time = self._properties.get(self._PROPERTY_TO_API_FIELD["modified"])
        if modified_time is not None:
            # modified_time will be in milliseconds.
            return google.cloud._helpers._datetime_from_microseconds(
                1000.0 * float(modified_time)
            )

    @property
    def num_bytes(self):
        """Union[int, None]: The size of the table in bytes (:data:`None` until
        set from the server).
        """
        return _helpers._int_or_none(
            self._properties.get(self._PROPERTY_TO_API_FIELD["num_bytes"])
        )

    @property
    def num_rows(self):
        """Union[int, None]: The number of rows in the table (:data:`None`
        until set from the server).
        """
        return _helpers._int_or_none(
            self._properties.get(self._PROPERTY_TO_API_FIELD["num_rows"])
        )

    @property
    def self_link(self):
        """Union[str, None]: URL for the table resource (:data:`None` until set
        from the server).
        """
        return self._properties.get(self._PROPERTY_TO_API_FIELD["self_link"])

    @property
    def full_table_id(self):
        """Union[str, None]: ID for the table (:data:`None` until set from the
        server).

        In the format ``project-id:dataset_id.table_id``.
        """
        return self._properties.get(self._PROPERTY_TO_API_FIELD["full_table_id"])

    @property
    def table_type(self):
        """Union[str, None]: The type of the table (:data:`None` until set from
        the server).

        Possible values are ``'TABLE'``, ``'VIEW'``, ``'MATERIALIZED_VIEW'`` or
        ``'EXTERNAL'``.
        """
        return self._properties.get(self._PROPERTY_TO_API_FIELD["type"])

    @property
    def range_partitioning(self):
        """Optional[google.cloud.bigquery.table.RangePartitioning]:
        Configures range-based partitioning for a table.

        .. note::
            **Beta**. The integer range partitioning feature is in a
            pre-release state and might change or have limited support.

        Only specify at most one of
        :attr:`~google.cloud.bigquery.table.Table.time_partitioning` or
        :attr:`~google.cloud.bigquery.table.Table.range_partitioning`.

        Raises:
            ValueError:
                If the value is not
                :class:`~google.cloud.bigquery.table.RangePartitioning` or
                :data:`None`.
        """
        resource = self._properties.get(
            self._PROPERTY_TO_API_FIELD["range_partitioning"]
        )
        if resource is not None:
            return RangePartitioning(_properties=resource)

    @range_partitioning.setter
    def range_partitioning(self, value):
        resource = value
        if isinstance(value, RangePartitioning):
            resource = value._properties
        elif value is not None:
            raise ValueError(
                "Expected value to be RangePartitioning or None, got {}.".format(value)
            )
        self._properties[self._PROPERTY_TO_API_FIELD["range_partitioning"]] = resource

    @property
    def time_partitioning(self):
        """Optional[google.cloud.bigquery.table.TimePartitioning]: Configures time-based
        partitioning for a table.

        Only specify at most one of
        :attr:`~google.cloud.bigquery.table.Table.time_partitioning` or
        :attr:`~google.cloud.bigquery.table.Table.range_partitioning`.

        Raises:
            ValueError:
                If the value is not
                :class:`~google.cloud.bigquery.table.TimePartitioning` or
                :data:`None`.
        """
        prop = self._properties.get(self._PROPERTY_TO_API_FIELD["time_partitioning"])
        if prop is not None:
            return TimePartitioning.from_api_repr(prop)

    @time_partitioning.setter
    def time_partitioning(self, value):
        api_repr = value
        if isinstance(value, TimePartitioning):
            api_repr = value.to_api_repr()
        elif value is not None:
            raise ValueError(
                "value must be google.cloud.bigquery.table.TimePartitioning " "or None"
            )
        self._properties[self._PROPERTY_TO_API_FIELD["time_partitioning"]] = api_repr

    @property
    def partitioning_type(self):
        """Union[str, None]: Time partitioning of the table if it is
        partitioned (Defaults to :data:`None`).

        """
        warnings.warn(
            "This method will be deprecated in future versions. Please use "
            "Table.time_partitioning.type_ instead.",
            PendingDeprecationWarning,
            stacklevel=2,
        )
        if self.time_partitioning is not None:
            return self.time_partitioning.type_

    @partitioning_type.setter
    def partitioning_type(self, value):
        warnings.warn(
            "This method will be deprecated in future versions. Please use "
            "Table.time_partitioning.type_ instead.",
            PendingDeprecationWarning,
            stacklevel=2,
        )
        api_field = self._PROPERTY_TO_API_FIELD["partitioning_type"]
        if self.time_partitioning is None:
            self._properties[api_field] = {}
        self._properties[api_field]["type"] = value

    @property
    def partition_expiration(self):
        """Union[int, None]: Expiration time in milliseconds for a partition.

        If :attr:`partition_expiration` is set and :attr:`type_` is
        not set, :attr:`type_` will default to
        :attr:`~google.cloud.bigquery.table.TimePartitioningType.DAY`.
        """
        warnings.warn(
            "This method will be deprecated in future versions. Please use "
            "Table.time_partitioning.expiration_ms instead.",
            PendingDeprecationWarning,
            stacklevel=2,
        )
        if self.time_partitioning is not None:
            return self.time_partitioning.expiration_ms

    @partition_expiration.setter
    def partition_expiration(self, value):
        warnings.warn(
            "This method will be deprecated in future versions. Please use "
            "Table.time_partitioning.expiration_ms instead.",
            PendingDeprecationWarning,
            stacklevel=2,
        )
        api_field = self._PROPERTY_TO_API_FIELD["partition_expiration"]

        if self.time_partitioning is None:
            self._properties[api_field] = {"type": TimePartitioningType.DAY}
        self._properties[api_field]["expirationMs"] = str(value)

    @property
    def clustering_fields(self):
        """Union[List[str], None]: Fields defining clustering for the table

        (Defaults to :data:`None`).

        Clustering fields are immutable after table creation.

        .. note::

           BigQuery supports clustering for both partitioned and
           non-partitioned tables.
        """
        prop = self._properties.get(self._PROPERTY_TO_API_FIELD["clustering_fields"])
        if prop is not None:
            return list(prop.get("fields", ()))

    @clustering_fields.setter
    def clustering_fields(self, value):
        """Union[List[str], None]: Fields defining clustering for the table

        (Defaults to :data:`None`).
        """
        api_field = self._PROPERTY_TO_API_FIELD["clustering_fields"]

        if value is not None:
            prop = self._properties.setdefault(api_field, {})
            prop["fields"] = value
        else:
            # In order to allow unsetting clustering fields completely, we explicitly
            # set this property to None (as oposed to merely removing the key).
            self._properties[api_field] = None

    @property
    def description(self):
        """Union[str, None]: Description of the table (defaults to
        :data:`None`).

        Raises:
            ValueError: For invalid value types.
        """
        return self._properties.get(self._PROPERTY_TO_API_FIELD["description"])

    @description.setter
    def description(self, value):
        if not isinstance(value, str) and value is not None:
            raise ValueError("Pass a string, or None")
        self._properties[self._PROPERTY_TO_API_FIELD["description"]] = value

    @property
    def expires(self):
        """Union[datetime.datetime, None]: Datetime at which the table will be
        deleted.

        Raises:
            ValueError: For invalid value types.
        """
        expiration_time = self._properties.get(self._PROPERTY_TO_API_FIELD["expires"])
        if expiration_time is not None:
            # expiration_time will be in milliseconds.
            return google.cloud._helpers._datetime_from_microseconds(
                1000.0 * float(expiration_time)
            )

    @expires.setter
    def expires(self, value):
        if not isinstance(value, datetime.datetime) and value is not None:
            raise ValueError("Pass a datetime, or None")
        value_ms = google.cloud._helpers._millis_from_datetime(value)
        self._properties[
            self._PROPERTY_TO_API_FIELD["expires"]
        ] = _helpers._str_or_none(value_ms)

    @property
    def friendly_name(self):
        """Union[str, None]: Title of the table (defaults to :data:`None`).

        Raises:
            ValueError: For invalid value types.
        """
        return self._properties.get(self._PROPERTY_TO_API_FIELD["friendly_name"])

    @friendly_name.setter
    def friendly_name(self, value):
        if not isinstance(value, str) and value is not None:
            raise ValueError("Pass a string, or None")
        self._properties[self._PROPERTY_TO_API_FIELD["friendly_name"]] = value

    @property
    def location(self):
        """Union[str, None]: Location in which the table is hosted

        Defaults to :data:`None`.
        """
        return self._properties.get(self._PROPERTY_TO_API_FIELD["location"])

    @property
    def view_query(self):
        """Union[str, None]: SQL query defining the table as a view (defaults
        to :data:`None`).

        By default, the query is treated as Standard SQL. To use Legacy
        SQL, set :attr:`view_use_legacy_sql` to :data:`True`.

        Raises:
            ValueError: For invalid value types.
        """
        api_field = self._PROPERTY_TO_API_FIELD["view_query"]
        return _helpers._get_sub_prop(self._properties, [api_field, "query"])

    @view_query.setter
    def view_query(self, value):
        if not isinstance(value, str):
            raise ValueError("Pass a string")

        api_field = self._PROPERTY_TO_API_FIELD["view_query"]
        _helpers._set_sub_prop(self._properties, [api_field, "query"], value)
        view = self._properties[api_field]
        # The service defaults useLegacySql to True, but this
        # client uses Standard SQL by default.
        if view.get("useLegacySql") is None:
            view["useLegacySql"] = False

    @view_query.deleter
    def view_query(self):
        """Delete SQL query defining the table as a view."""
        self._properties.pop(self._PROPERTY_TO_API_FIELD["view_query"], None)

    view_use_legacy_sql = property(_view_use_legacy_sql_getter)

    @view_use_legacy_sql.setter
    def view_use_legacy_sql(self, value):
        if not isinstance(value, bool):
            raise ValueError("Pass a boolean")

        api_field = self._PROPERTY_TO_API_FIELD["view_query"]
        if self._properties.get(api_field) is None:
            self._properties[api_field] = {}
        self._properties[api_field]["useLegacySql"] = value

    @property
    def mview_query(self):
        """Optional[str]: SQL query defining the table as a materialized
        view (defaults to :data:`None`).
        """
        api_field = self._PROPERTY_TO_API_FIELD["mview_query"]
        return _helpers._get_sub_prop(self._properties, [api_field, "query"])

    @mview_query.setter
    def mview_query(self, value):
        api_field = self._PROPERTY_TO_API_FIELD["mview_query"]
        _helpers._set_sub_prop(self._properties, [api_field, "query"], str(value))

    @mview_query.deleter
    def mview_query(self):
        """Delete SQL query defining the table as a materialized view."""
        self._properties.pop(self._PROPERTY_TO_API_FIELD["mview_query"], None)

    @property
    def mview_last_refresh_time(self):
        """Optional[datetime.datetime]: Datetime at which the materialized view was last
        refreshed (:data:`None` until set from the server).
        """
        refresh_time = _helpers._get_sub_prop(
            self._properties, self._PROPERTY_TO_API_FIELD["mview_last_refresh_time"]
        )
        if refresh_time is not None:
            # refresh_time will be in milliseconds.
            return google.cloud._helpers._datetime_from_microseconds(
                1000 * int(refresh_time)
            )

    @property
    def mview_enable_refresh(self):
        """Optional[bool]: Enable automatic refresh of the materialized view
        when the base table is updated. The default value is :data:`True`.
        """
        api_field = self._PROPERTY_TO_API_FIELD["mview_enable_refresh"]
        return _helpers._get_sub_prop(self._properties, [api_field, "enableRefresh"])

    @mview_enable_refresh.setter
    def mview_enable_refresh(self, value):
        api_field = self._PROPERTY_TO_API_FIELD["mview_enable_refresh"]
        return _helpers._set_sub_prop(
            self._properties, [api_field, "enableRefresh"], value
        )

    @property
    def mview_refresh_interval(self):
        """Optional[datetime.timedelta]: The maximum frequency at which this
        materialized view will be refreshed. The default value is 1800000
        milliseconds (30 minutes).
        """
        api_field = self._PROPERTY_TO_API_FIELD["mview_refresh_interval"]
        refresh_interval = _helpers._get_sub_prop(
            self._properties, [api_field, "refreshIntervalMs"]
        )
        if refresh_interval is not None:
            return datetime.timedelta(milliseconds=int(refresh_interval))

    @mview_refresh_interval.setter
    def mview_refresh_interval(self, value):
        if value is None:
            refresh_interval_ms = None
        else:
            refresh_interval_ms = str(value // datetime.timedelta(milliseconds=1))

        api_field = self._PROPERTY_TO_API_FIELD["mview_refresh_interval"]
        _helpers._set_sub_prop(
            self._properties, [api_field, "refreshIntervalMs"], refresh_interval_ms,
        )

    @property
    def streaming_buffer(self):
        """google.cloud.bigquery.StreamingBuffer: Information about a table's
        streaming buffer.
        """
        sb = self._properties.get(self._PROPERTY_TO_API_FIELD["streaming_buffer"])
        if sb is not None:
            return StreamingBuffer(sb)

    @property
    def external_data_configuration(self):
        """Union[google.cloud.bigquery.ExternalConfig, None]: Configuration for
        an external data source (defaults to :data:`None`).

        Raises:
            ValueError: For invalid value types.
        """
        prop = self._properties.get(
            self._PROPERTY_TO_API_FIELD["external_data_configuration"]
        )
        if prop is not None:
            prop = ExternalConfig.from_api_repr(prop)
        return prop

    @external_data_configuration.setter
    def external_data_configuration(self, value):
        if not (value is None or isinstance(value, ExternalConfig)):
            raise ValueError("Pass an ExternalConfig or None")
        api_repr = value
        if value is not None:
            api_repr = value.to_api_repr()
        self._properties[
            self._PROPERTY_TO_API_FIELD["external_data_configuration"]
        ] = api_repr

    @property
    def snapshot_definition(self) -> Optional["SnapshotDefinition"]:
        """Information about the snapshot. This value is set via snapshot creation.

        See: https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#Table.FIELDS.snapshot_definition
        """
        snapshot_info = self._properties.get(
            self._PROPERTY_TO_API_FIELD["snapshot_definition"]
        )
        if snapshot_info is not None:
            snapshot_info = SnapshotDefinition(snapshot_info)
        return snapshot_info

    @classmethod
    def from_string(cls, full_table_id: str) -> "Table":
        """Construct a table from fully-qualified table ID.

        Args:
            full_table_id (str):
                A fully-qualified table ID in standard SQL format. Must
                included a project ID, dataset ID, and table ID, each
                separated by ``.``.

        Returns:
            Table: Table parsed from ``full_table_id``.

        Examples:
            >>> Table.from_string('my-project.mydataset.mytable')
            Table(TableRef...(D...('my-project', 'mydataset'), 'mytable'))

        Raises:
            ValueError:
                If ``full_table_id`` is not a fully-qualified table ID in
                standard SQL format.
        """
        return cls(TableReference.from_string(full_table_id))

    @classmethod
    def from_api_repr(cls, resource: dict) -> "Table":
        """Factory: construct a table given its API representation

        Args:
            resource (Dict[str, object]):
                Table resource representation from the API

        Returns:
            google.cloud.bigquery.table.Table: Table parsed from ``resource``.

        Raises:
            KeyError:
                If the ``resource`` lacks the key ``'tableReference'``, or if
                the ``dict`` stored within the key ``'tableReference'`` lacks
                the keys ``'tableId'``, ``'projectId'``, or ``'datasetId'``.
        """
        from google.cloud.bigquery import dataset

        if (
            "tableReference" not in resource
            or "tableId" not in resource["tableReference"]
        ):
            raise KeyError(
                "Resource lacks required identity information:"
                '["tableReference"]["tableId"]'
            )
        project_id = _helpers._get_sub_prop(
            resource, cls._PROPERTY_TO_API_FIELD["project"]
        )
        table_id = _helpers._get_sub_prop(
            resource, cls._PROPERTY_TO_API_FIELD["table_id"]
        )
        dataset_id = _helpers._get_sub_prop(
            resource, cls._PROPERTY_TO_API_FIELD["dataset_id"]
        )
        dataset_ref = dataset.DatasetReference(project_id, dataset_id)

        table = cls(dataset_ref.table(table_id))
        table._properties = resource

        return table

    def to_api_repr(self) -> dict:
        """Constructs the API resource of this table

        Returns:
            Dict[str, object]: Table represented as an API resource
        """
        return copy.deepcopy(self._properties)

    def to_bqstorage(self) -> str:
        """Construct a BigQuery Storage API representation of this table.

        Returns:
            str: A reference to this table in the BigQuery Storage API.
        """
        return self.reference.to_bqstorage()

    def _build_resource(self, filter_fields):
        """Generate a resource for ``update``."""
        return _helpers._build_resource_from_properties(self, filter_fields)

    def __repr__(self):
        return "Table({})".format(repr(self.reference))


class TableListItem(object):
    """A read-only table resource from a list operation.

    For performance reasons, the BigQuery API only includes some of the table
    properties when listing tables. Notably,
    :attr:`~google.cloud.bigquery.table.Table.schema` and
    :attr:`~google.cloud.bigquery.table.Table.num_rows` are missing.

    For a full list of the properties that the BigQuery API returns, see the
    `REST documentation for tables.list
    <https://cloud.google.com/bigquery/docs/reference/rest/v2/tables/list>`_.


    Args:
        resource (Dict[str, object]):
            A table-like resource object from a table list response. A
            ``tableReference`` property is required.

    Raises:
        ValueError:
            If ``tableReference`` or one of its required members is missing
            from ``resource``.
    """

    def __init__(self, resource):
        if "tableReference" not in resource:
            raise ValueError("resource must contain a tableReference value")
        if "projectId" not in resource["tableReference"]:
            raise ValueError(
                "resource['tableReference'] must contain a projectId value"
            )
        if "datasetId" not in resource["tableReference"]:
            raise ValueError(
                "resource['tableReference'] must contain a datasetId value"
            )
        if "tableId" not in resource["tableReference"]:
            raise ValueError("resource['tableReference'] must contain a tableId value")

        self._properties = resource

    @property
    def created(self):
        """Union[datetime.datetime, None]: Datetime at which the table was
        created (:data:`None` until set from the server).
        """
        creation_time = self._properties.get("creationTime")
        if creation_time is not None:
            # creation_time will be in milliseconds.
            return google.cloud._helpers._datetime_from_microseconds(
                1000.0 * float(creation_time)
            )

    @property
    def expires(self):
        """Union[datetime.datetime, None]: Datetime at which the table will be
        deleted.
        """
        expiration_time = self._properties.get("expirationTime")
        if expiration_time is not None:
            # expiration_time will be in milliseconds.
            return google.cloud._helpers._datetime_from_microseconds(
                1000.0 * float(expiration_time)
            )

    @property
    def project(self):
        """str: Project bound to the table."""
        return self._properties["tableReference"]["projectId"]

    @property
    def dataset_id(self):
        """str: ID of dataset containing the table."""
        return self._properties["tableReference"]["datasetId"]

    @property
    def table_id(self):
        """str: ID of the table."""
        return self._properties["tableReference"]["tableId"]

    reference = property(_reference_getter)

    @property
    def labels(self):
        """Dict[str, str]: Labels for the table.

        This method always returns a dict. To change a table's labels,
        modify the dict, then call ``Client.update_table``. To delete a
        label, set its value to :data:`None` before updating.
        """
        return self._properties.setdefault("labels", {})

    @property
    def full_table_id(self):
        """Union[str, None]: ID for the table (:data:`None` until set from the
        server).

        In the format ``project_id:dataset_id.table_id``.
        """
        return self._properties.get("id")

    @property
    def table_type(self):
        """Union[str, None]: The type of the table (:data:`None` until set from
        the server).

        Possible values are ``'TABLE'``, ``'VIEW'``, or ``'EXTERNAL'``.
        """
        return self._properties.get("type")

    @property
    def time_partitioning(self):
        """google.cloud.bigquery.table.TimePartitioning: Configures time-based
        partitioning for a table.
        """
        prop = self._properties.get("timePartitioning")
        if prop is not None:
            return TimePartitioning.from_api_repr(prop)

    @property
    def partitioning_type(self):
        """Union[str, None]: Time partitioning of the table if it is
        partitioned (Defaults to :data:`None`).
        """
        warnings.warn(
            "This method will be deprecated in future versions. Please use "
            "TableListItem.time_partitioning.type_ instead.",
            PendingDeprecationWarning,
            stacklevel=2,
        )
        if self.time_partitioning is not None:
            return self.time_partitioning.type_

    @property
    def partition_expiration(self):
        """Union[int, None]: Expiration time in milliseconds for a partition.

        If this property is set and :attr:`type_` is not set, :attr:`type_`
        will default to :attr:`TimePartitioningType.DAY`.
        """
        warnings.warn(
            "This method will be deprecated in future versions. Please use "
            "TableListItem.time_partitioning.expiration_ms instead.",
            PendingDeprecationWarning,
            stacklevel=2,
        )
        if self.time_partitioning is not None:
            return self.time_partitioning.expiration_ms

    @property
    def friendly_name(self):
        """Union[str, None]: Title of the table (defaults to :data:`None`)."""
        return self._properties.get("friendlyName")

    view_use_legacy_sql = property(_view_use_legacy_sql_getter)

    @property
    def clustering_fields(self):
        """Union[List[str], None]: Fields defining clustering for the table

        (Defaults to :data:`None`).

        Clustering fields are immutable after table creation.

        .. note::

           BigQuery supports clustering for both partitioned and
           non-partitioned tables.
        """
        prop = self._properties.get("clustering")
        if prop is not None:
            return list(prop.get("fields", ()))

    @classmethod
    def from_string(cls, full_table_id: str) -> "TableListItem":
        """Construct a table from fully-qualified table ID.

        Args:
            full_table_id (str):
                A fully-qualified table ID in standard SQL format. Must
                included a project ID, dataset ID, and table ID, each
                separated by ``.``.

        Returns:
            Table: Table parsed from ``full_table_id``.

        Examples:
            >>> Table.from_string('my-project.mydataset.mytable')
            Table(TableRef...(D...('my-project', 'mydataset'), 'mytable'))

        Raises:
            ValueError:
                If ``full_table_id`` is not a fully-qualified table ID in
                standard SQL format.
        """
        return cls(
            {"tableReference": TableReference.from_string(full_table_id).to_api_repr()}
        )

    def to_bqstorage(self) -> str:
        """Construct a BigQuery Storage API representation of this table.

        Returns:
            str: A reference to this table in the BigQuery Storage API.
        """
        return self.reference.to_bqstorage()

    def to_api_repr(self) -> dict:
        """Constructs the API resource of this table

        Returns:
            Dict[str, object]: Table represented as an API resource
        """
        return copy.deepcopy(self._properties)


def _row_from_mapping(mapping, schema):
    """Convert a mapping to a row tuple using the schema.

    Args:
        mapping (Dict[str, object])
            Mapping of row data: must contain keys for all required fields in
            the schema. Keys which do not correspond to a field in the schema
            are ignored.
        schema (List[google.cloud.bigquery.schema.SchemaField]):
            The schema of the table destination for the rows

    Returns:
        Tuple[object]:
            Tuple whose elements are ordered according to the schema.

    Raises:
        ValueError: If schema is empty.
    """
    if len(schema) == 0:
        raise ValueError(_TABLE_HAS_NO_SCHEMA)

    row = []
    for field in schema:
        if field.mode == "REQUIRED":
            row.append(mapping[field.name])
        elif field.mode == "REPEATED":
            row.append(mapping.get(field.name, ()))
        elif field.mode == "NULLABLE":
            row.append(mapping.get(field.name))
        else:
            raise ValueError("Unknown field mode: {}".format(field.mode))
    return tuple(row)


class StreamingBuffer(object):
    """Information about a table's streaming buffer.

    See https://cloud.google.com/bigquery/streaming-data-into-bigquery.

    Args:
        resource (Dict[str, object]):
            streaming buffer representation returned from the API
    """

    def __init__(self, resource):
        self.estimated_bytes = None
        if "estimatedBytes" in resource:
            self.estimated_bytes = int(resource["estimatedBytes"])
        self.estimated_rows = None
        if "estimatedRows" in resource:
            self.estimated_rows = int(resource["estimatedRows"])
        self.oldest_entry_time = None
        if "oldestEntryTime" in resource:
            self.oldest_entry_time = google.cloud._helpers._datetime_from_microseconds(
                1000.0 * int(resource["oldestEntryTime"])
            )


class SnapshotDefinition:
    """Information about base table and snapshot time of the snapshot.

    See https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#snapshotdefinition

    Args:
        resource: Snapshot definition representation returned from the API.
    """

    def __init__(self, resource: Dict[str, Any]):
        self.base_table_reference = None
        if "baseTableReference" in resource:
            self.base_table_reference = TableReference.from_api_repr(
                resource["baseTableReference"]
            )

        self.snapshot_time = None
        if "snapshotTime" in resource:
            self.snapshot_time = google.cloud._helpers._rfc3339_to_datetime(
                resource["snapshotTime"]
            )


class Row(object):
    """A BigQuery row.

    Values can be accessed by position (index), by key like a dict,
    or as properties.

    Args:
        values (Sequence[object]): The row values
        field_to_index (Dict[str, int]):
            A mapping from schema field names to indexes
    """

    # Choose unusual field names to try to avoid conflict with schema fields.
    __slots__ = ("_xxx_values", "_xxx_field_to_index")

    def __init__(self, values, field_to_index):
        self._xxx_values = values
        self._xxx_field_to_index = field_to_index

    def values(self):
        """Return the values included in this row.

        Returns:
            Sequence[object]: A sequence of length ``len(row)``.
        """
        return copy.deepcopy(self._xxx_values)

    def keys(self) -> Iterable[str]:
        """Return the keys for using a row as a dict.

        Returns:
            Iterable[str]: The keys corresponding to the columns of a row

        Examples:

            >>> list(Row(('a', 'b'), {'x': 0, 'y': 1}).keys())
            ['x', 'y']
        """
        return self._xxx_field_to_index.keys()

    def items(self) -> Iterable[Tuple[str, Any]]:
        """Return items as ``(key, value)`` pairs.

        Returns:
            Iterable[Tuple[str, object]]:
                The ``(key, value)`` pairs representing this row.

        Examples:

            >>> list(Row(('a', 'b'), {'x': 0, 'y': 1}).items())
            [('x', 'a'), ('y', 'b')]
        """
        for key, index in self._xxx_field_to_index.items():
            yield (key, copy.deepcopy(self._xxx_values[index]))

    def get(self, key: str, default: Any = None) -> Any:
        """Return a value for key, with a default value if it does not exist.

        Args:
            key (str): The key of the column to access
            default (object):
                The default value to use if the key does not exist. (Defaults
                to :data:`None`.)

        Returns:
            object:
                The value associated with the provided key, or a default value.

        Examples:
            When the key exists, the value associated with it is returned.

            >>> Row(('a', 'b'), {'x': 0, 'y': 1}).get('x')
            'a'

            The default value is :data:`None` when the key does not exist.

            >>> Row(('a', 'b'), {'x': 0, 'y': 1}).get('z')
            None

            The default value can be overrided with the ``default`` parameter.

            >>> Row(('a', 'b'), {'x': 0, 'y': 1}).get('z', '')
            ''

            >>> Row(('a', 'b'), {'x': 0, 'y': 1}).get('z', default = '')
            ''
        """
        index = self._xxx_field_to_index.get(key)
        if index is None:
            return default
        return self._xxx_values[index]

    def __getattr__(self, name):
        value = self._xxx_field_to_index.get(name)
        if value is None:
            raise AttributeError("no row field {!r}".format(name))
        return self._xxx_values[value]

    def __len__(self):
        return len(self._xxx_values)

    def __getitem__(self, key):
        if isinstance(key, str):
            value = self._xxx_field_to_index.get(key)
            if value is None:
                raise KeyError("no row field {!r}".format(key))
            key = value
        return self._xxx_values[key]

    def __eq__(self, other):
        if not isinstance(other, Row):
            return NotImplemented
        return (
            self._xxx_values == other._xxx_values
            and self._xxx_field_to_index == other._xxx_field_to_index
        )

    def __ne__(self, other):
        return not self == other

    def __repr__(self):
        # sort field dict by value, for determinism
        items = sorted(self._xxx_field_to_index.items(), key=operator.itemgetter(1))
        f2i = "{" + ", ".join("%r: %d" % item for item in items) + "}"
        return "Row({}, {})".format(self._xxx_values, f2i)


class _NoopProgressBarQueue(object):
    """A fake Queue class that does nothing.

    This is used when there is no progress bar to send updates to.
    """

    def put_nowait(self, item):
        """Don't actually do anything with the item."""


class RowIterator(HTTPIterator):
    """A class for iterating through HTTP/JSON API row list responses.

    Args:
        client (Optional[google.cloud.bigquery.Client]):
            The API client instance. This should always be non-`None`, except for
            subclasses that do not use it, namely the ``_EmptyRowIterator``.
        api_request (Callable[google.cloud._http.JSONConnection.api_request]):
            The function to use to make API requests.
        path (str): The method path to query for the list of items.
        schema (Sequence[Union[ \
                :class:`~google.cloud.bigquery.schema.SchemaField`, \
                Mapping[str, Any] \
        ]]):
            The table's schema. If any item is a mapping, its content must be
            compatible with
            :meth:`~google.cloud.bigquery.schema.SchemaField.from_api_repr`.
        page_token (str): A token identifying a page in a result set to start
            fetching results from.
        max_results (Optional[int]): The maximum number of results to fetch.
        page_size (Optional[int]): The maximum number of rows in each page
            of results from this request. Non-positive values are ignored.
            Defaults to a sensible value set by the API.
        extra_params (Optional[Dict[str, object]]):
            Extra query string parameters for the API call.
        table (Optional[Union[ \
            google.cloud.bigquery.table.Table, \
            google.cloud.bigquery.table.TableReference, \
        ]]):
            The table which these rows belong to, or a reference to it. Used to
            call the BigQuery Storage API to fetch rows.
        selected_fields (Optional[Sequence[google.cloud.bigquery.schema.SchemaField]]):
            A subset of columns to select from this table.
        total_rows (Optional[int]):
            Total number of rows in the table.
        first_page_response (Optional[dict]):
            API response for the first page of results. These are returned when
            the first page is requested.
    """

    def __init__(
        self,
        client,
        api_request,
        path,
        schema,
        page_token=None,
        max_results=None,
        page_size=None,
        extra_params=None,
        table=None,
        selected_fields=None,
        total_rows=None,
        first_page_response=None,
    ):
        super(RowIterator, self).__init__(
            client,
            api_request,
            path,
            item_to_value=_item_to_row,
            items_key="rows",
            page_token=page_token,
            max_results=max_results,
            extra_params=extra_params,
            page_start=_rows_page_start,
            next_token="pageToken",
        )
        schema = _to_schema_fields(schema)
        self._field_to_index = _helpers._field_to_index_mapping(schema)
        self._page_size = page_size
        self._preserve_order = False
        self._project = client.project if client is not None else None
        self._schema = schema
        self._selected_fields = selected_fields
        self._table = table
        self._total_rows = total_rows
        self._first_page_response = first_page_response

    def _is_completely_cached(self):
        """Check if all results are completely cached.

        This is useful to know, because we can avoid alternative download
        mechanisms.
        """
        if self._first_page_response is None or self.next_page_token:
            return False

        return self._first_page_response.get(self._next_token) is None

    def _validate_bqstorage(self, bqstorage_client, create_bqstorage_client):
        """Returns if the BigQuery Storage API can be used.

        Returns:
            bool
                True if the BigQuery Storage client can be used or created.
        """
        using_bqstorage_api = bqstorage_client or create_bqstorage_client
        if not using_bqstorage_api:
            return False

        if self._is_completely_cached():
            return False

        if self.max_results is not None:
            warnings.warn(
                "Cannot use bqstorage_client if max_results is set, "
                "reverting to fetching data with the REST endpoint.",
                stacklevel=2,
            )
            return False

        try:
            from google.cloud import bigquery_storage  # noqa: F401
        except ImportError:
            return False

        try:
            _helpers.BQ_STORAGE_VERSIONS.verify_version()
        except LegacyBigQueryStorageError as exc:
            warnings.warn(str(exc))
            return False

        return True

    def _get_next_page_response(self):
        """Requests the next page from the path provided.

        Returns:
            Dict[str, object]:
                The parsed JSON response of the next page's contents.
        """
        if self._first_page_response:
            response = self._first_page_response
            self._first_page_response = None
            return response

        params = self._get_query_params()
        if self._page_size is not None:
            if self.page_number and "startIndex" in params:
                del params["startIndex"]
            params["maxResults"] = self._page_size
        return self.api_request(
            method=self._HTTP_METHOD, path=self.path, query_params=params
        )

    @property
    def schema(self):
        """List[google.cloud.bigquery.schema.SchemaField]: The subset of
        columns to be read from the table."""
        return list(self._schema)

    @property
    def total_rows(self):
        """int: The total number of rows in the table."""
        return self._total_rows

    def _to_page_iterable(
        self, bqstorage_download, tabledata_list_download, bqstorage_client=None
    ):
        if not self._validate_bqstorage(bqstorage_client, False):
            bqstorage_client = None

        result_pages = (
            bqstorage_download()
            if bqstorage_client is not None
            else tabledata_list_download()
        )
        yield from result_pages

    def _to_arrow_iterable(self, bqstorage_client=None):
        """Create an iterable of arrow RecordBatches, to process the table as a stream."""
        bqstorage_download = functools.partial(
            _pandas_helpers.download_arrow_bqstorage,
            self._project,
            self._table,
            bqstorage_client,
            preserve_order=self._preserve_order,
            selected_fields=self._selected_fields,
        )
        tabledata_list_download = functools.partial(
            _pandas_helpers.download_arrow_row_iterator, iter(self.pages), self.schema
        )
        return self._to_page_iterable(
            bqstorage_download,
            tabledata_list_download,
            bqstorage_client=bqstorage_client,
        )

    # If changing the signature of this method, make sure to apply the same
    # changes to job.QueryJob.to_arrow()
    def to_arrow(
        self,
        progress_bar_type: str = None,
        bqstorage_client: "bigquery_storage.BigQueryReadClient" = None,
        create_bqstorage_client: bool = True,
    ) -> "pyarrow.Table":
        """[Beta] Create a class:`pyarrow.Table` by loading all pages of a
        table or query.

        Args:
            progress_bar_type (Optional[str]):
                If set, use the `tqdm <https://tqdm.github.io/>`_ library to
                display a progress bar while the data downloads. Install the
                ``tqdm`` package to use this feature.

                Possible values of ``progress_bar_type`` include:

                ``None``
                  No progress bar.
                ``'tqdm'``
                  Use the :func:`tqdm.tqdm` function to print a progress bar
                  to :data:`sys.stderr`.
                ``'tqdm_notebook'``
                  Use the :func:`tqdm.tqdm_notebook` function to display a
                  progress bar as a Jupyter notebook widget.
                ``'tqdm_gui'``
                  Use the :func:`tqdm.tqdm_gui` function to display a
                  progress bar as a graphical dialog box.
            bqstorage_client (Optional[google.cloud.bigquery_storage_v1.BigQueryReadClient]):
                A BigQuery Storage API client. If supplied, use the faster BigQuery
                Storage API to fetch rows from BigQuery. This API is a billable API.

                This method requires the ``pyarrow`` and
                ``google-cloud-bigquery-storage`` libraries.

                This method only  exposes a subset of the capabilities of the
                BigQuery Storage API.  For full access to all features
                (projections, filters, snapshots) use the Storage API directly.
            create_bqstorage_client (Optional[bool]):
                If ``True`` (default), create a BigQuery Storage API client using
                the default API settings. The BigQuery Storage API is a faster way
                to fetch rows from BigQuery. See the ``bqstorage_client`` parameter
                for more information.

                This argument does nothing if ``bqstorage_client`` is supplied.

                .. versionadded:: 1.24.0

        Returns:
            pyarrow.Table
                A :class:`pyarrow.Table` populated with row data and column
                headers from the query results. The column headers are derived
                from the destination table's schema.

        Raises:
            ValueError: If the :mod:`pyarrow` library cannot be imported.

        .. versionadded:: 1.17.0
        """
        if pyarrow is None:
            raise ValueError(_NO_PYARROW_ERROR)

        if not self._validate_bqstorage(bqstorage_client, create_bqstorage_client):
            create_bqstorage_client = False
            bqstorage_client = None

        owns_bqstorage_client = False
        if not bqstorage_client and create_bqstorage_client:
            bqstorage_client = self.client._ensure_bqstorage_client()
            owns_bqstorage_client = bqstorage_client is not None

        try:
            progress_bar = get_progress_bar(
                progress_bar_type, "Downloading", self.total_rows, "rows"
            )

            record_batches = []
            for record_batch in self._to_arrow_iterable(
                bqstorage_client=bqstorage_client
            ):
                record_batches.append(record_batch)

                if progress_bar is not None:
                    # In some cases, the number of total rows is not populated
                    # until the first page of rows is fetched. Update the
                    # progress bar's total to keep an accurate count.
                    progress_bar.total = progress_bar.total or self.total_rows
                    progress_bar.update(record_batch.num_rows)

            if progress_bar is not None:
                # Indicate that the download has finished.
                progress_bar.close()
        finally:
            if owns_bqstorage_client:
                bqstorage_client._transport.grpc_channel.close()

        if record_batches:
            return pyarrow.Table.from_batches(record_batches)
        else:
            # No records, use schema based on BigQuery schema.
            arrow_schema = _pandas_helpers.bq_to_arrow_schema(self._schema)
            return pyarrow.Table.from_batches(record_batches, schema=arrow_schema)

    def to_dataframe_iterable(
        self,
        bqstorage_client: "bigquery_storage.BigQueryReadClient" = None,
        dtypes: Dict[str, Any] = None,
        max_queue_size: int = _pandas_helpers._MAX_QUEUE_SIZE_DEFAULT,
    ) -> "pandas.DataFrame":
        """Create an iterable of pandas DataFrames, to process the table as a stream.

        Args:
            bqstorage_client (Optional[google.cloud.bigquery_storage_v1.BigQueryReadClient]):
                A BigQuery Storage API client. If supplied, use the faster
                BigQuery Storage API to fetch rows from BigQuery.

                This method requires the ``pyarrow`` and
                ``google-cloud-bigquery-storage`` libraries.

                This method only exposes a subset of the capabilities of the
                BigQuery Storage API. For full access to all features
                (projections, filters, snapshots) use the Storage API directly.

            dtypes (Optional[Map[str, Union[str, pandas.Series.dtype]]]):
                A dictionary of column names pandas ``dtype``s. The provided
                ``dtype`` is used when constructing the series for the column
                specified. Otherwise, the default pandas behavior is used.

            max_queue_size (Optional[int]):
                The maximum number of result pages to hold in the internal queue when
                streaming query results over the BigQuery Storage API. Ignored if
                Storage API is not used.

                By default, the max queue size is set to the number of BQ Storage streams
                created by the server. If ``max_queue_size`` is :data:`None`, the queue
                size is infinite.

                .. versionadded:: 2.14.0

        Returns:
            pandas.DataFrame:
                A generator of :class:`~pandas.DataFrame`.

        Raises:
            ValueError:
                If the :mod:`pandas` library cannot be imported.
        """
        if pandas is None:
            raise ValueError(_NO_PANDAS_ERROR)
        if dtypes is None:
            dtypes = {}

        column_names = [field.name for field in self._schema]
        bqstorage_download = functools.partial(
            _pandas_helpers.download_dataframe_bqstorage,
            self._project,
            self._table,
            bqstorage_client,
            column_names,
            dtypes,
            preserve_order=self._preserve_order,
            selected_fields=self._selected_fields,
            max_queue_size=max_queue_size,
        )
        tabledata_list_download = functools.partial(
            _pandas_helpers.download_dataframe_row_iterator,
            iter(self.pages),
            self.schema,
            dtypes,
        )
        return self._to_page_iterable(
            bqstorage_download,
            tabledata_list_download,
            bqstorage_client=bqstorage_client,
        )

    # If changing the signature of this method, make sure to apply the same
    # changes to job.QueryJob.to_dataframe()
    def to_dataframe(
        self,
        bqstorage_client: "bigquery_storage.BigQueryReadClient" = None,
        dtypes: Dict[str, Any] = None,
        progress_bar_type: str = None,
        create_bqstorage_client: bool = True,
        date_as_object: bool = True,
    ) -> "pandas.DataFrame":
        """Create a pandas DataFrame by loading all pages of a query.

        Args:
            bqstorage_client (Optional[google.cloud.bigquery_storage_v1.BigQueryReadClient]):
                A BigQuery Storage API client. If supplied, use the faster
                BigQuery Storage API to fetch rows from BigQuery.

                This method requires the ``pyarrow`` and
                ``google-cloud-bigquery-storage`` libraries.

                This method only exposes a subset of the capabilities of the
                BigQuery Storage API. For full access to all features
                (projections, filters, snapshots) use the Storage API directly.

            dtypes (Optional[Map[str, Union[str, pandas.Series.dtype]]]):
                A dictionary of column names pandas ``dtype``s. The provided
                ``dtype`` is used when constructing the series for the column
                specified. Otherwise, the default pandas behavior is used.
            progress_bar_type (Optional[str]):
                If set, use the `tqdm <https://tqdm.github.io/>`_ library to
                display a progress bar while the data downloads. Install the
                ``tqdm`` package to use this feature.

                Possible values of ``progress_bar_type`` include:

                ``None``
                  No progress bar.
                ``'tqdm'``
                  Use the :func:`tqdm.tqdm` function to print a progress bar
                  to :data:`sys.stderr`.
                ``'tqdm_notebook'``
                  Use the :func:`tqdm.tqdm_notebook` function to display a
                  progress bar as a Jupyter notebook widget.
                ``'tqdm_gui'``
                  Use the :func:`tqdm.tqdm_gui` function to display a
                  progress bar as a graphical dialog box.

                .. versionadded:: 1.11.0
            create_bqstorage_client (Optional[bool]):
                If ``True`` (default), create a BigQuery Storage API client
                using the default API settings. The BigQuery Storage API
                is a faster way to fetch rows from BigQuery. See the
                ``bqstorage_client`` parameter for more information.

                This argument does nothing if ``bqstorage_client`` is supplied.

                .. versionadded:: 1.24.0

            date_as_object (Optional[bool]):
                If ``True`` (default), cast dates to objects. If ``False``, convert
                to datetime64[ns] dtype.

                .. versionadded:: 1.26.0

        Returns:
            pandas.DataFrame:
                A :class:`~pandas.DataFrame` populated with row data and column
                headers from the query results. The column headers are derived
                from the destination table's schema.

        Raises:
            ValueError:
                If the :mod:`pandas` library cannot be imported, or the
                :mod:`google.cloud.bigquery_storage_v1` module is
                required but cannot be imported.

        """
        if pandas is None:
            raise ValueError(_NO_PANDAS_ERROR)
        if dtypes is None:
            dtypes = {}

        if not self._validate_bqstorage(bqstorage_client, create_bqstorage_client):
            create_bqstorage_client = False
            bqstorage_client = None

        record_batch = self.to_arrow(
            progress_bar_type=progress_bar_type,
            bqstorage_client=bqstorage_client,
            create_bqstorage_client=create_bqstorage_client,
        )

        # When converting timestamp values to nanosecond precision, the result
        # can be out of pyarrow bounds. To avoid the error when converting to
        # Pandas, we set the timestamp_as_object parameter to True, if necessary.
        types_to_check = {
            pyarrow.timestamp("us"),
            pyarrow.timestamp("us", tz=pytz.UTC),
        }

        for column in record_batch:
            if column.type in types_to_check:
                try:
                    column.cast("timestamp[ns]")
                except pyarrow.lib.ArrowInvalid:
                    timestamp_as_object = True
                    break
        else:
            timestamp_as_object = False

        extra_kwargs = {"timestamp_as_object": timestamp_as_object}

        df = record_batch.to_pandas(date_as_object=date_as_object, **extra_kwargs)

        for column in dtypes:
            df[column] = pandas.Series(df[column], dtype=dtypes[column])

        return df


class _EmptyRowIterator(RowIterator):
    """An empty row iterator.

    This class prevents API requests when there are no rows to fetch or rows
    are impossible to fetch, such as with query results for DDL CREATE VIEW
    statements.
    """

    schema = ()
    pages = ()
    total_rows = 0

    def __init__(
        self, client=None, api_request=None, path=None, schema=(), *args, **kwargs
    ):
        super().__init__(
            client=client,
            api_request=api_request,
            path=path,
            schema=schema,
            *args,
            **kwargs,
        )

    def to_arrow(
        self,
        progress_bar_type=None,
        bqstorage_client=None,
        create_bqstorage_client=True,
    ) -> "pyarrow.Table":
        """[Beta] Create an empty class:`pyarrow.Table`.

        Args:
            progress_bar_type (str): Ignored. Added for compatibility with RowIterator.
            bqstorage_client (Any): Ignored. Added for compatibility with RowIterator.
            create_bqstorage_client (bool): Ignored. Added for compatibility with RowIterator.

        Returns:
            pyarrow.Table: An empty :class:`pyarrow.Table`.
        """
        if pyarrow is None:
            raise ValueError(_NO_PYARROW_ERROR)
        return pyarrow.Table.from_arrays(())

    def to_dataframe(
        self,
        bqstorage_client=None,
        dtypes=None,
        progress_bar_type=None,
        create_bqstorage_client=True,
        date_as_object=True,
    ) -> "pandas.DataFrame":
        """Create an empty dataframe.

        Args:
            bqstorage_client (Any): Ignored. Added for compatibility with RowIterator.
            dtypes (Any): Ignored. Added for compatibility with RowIterator.
            progress_bar_type (Any): Ignored. Added for compatibility with RowIterator.
            create_bqstorage_client (bool): Ignored. Added for compatibility with RowIterator.
            date_as_object (bool): Ignored. Added for compatibility with RowIterator.

        Returns:
            pandas.DataFrame: An empty :class:`~pandas.DataFrame`.
        """
        if pandas is None:
            raise ValueError(_NO_PANDAS_ERROR)
        return pandas.DataFrame()

    def to_dataframe_iterable(
        self,
        bqstorage_client: Optional["bigquery_storage.BigQueryReadClient"] = None,
        dtypes: Optional[Dict[str, Any]] = None,
        max_queue_size: Optional[int] = None,
    ) -> Iterator["pandas.DataFrame"]:
        """Create an iterable of pandas DataFrames, to process the table as a stream.

        .. versionadded:: 2.21.0

        Args:
            bqstorage_client:
                Ignored. Added for compatibility with RowIterator.

            dtypes (Optional[Map[str, Union[str, pandas.Series.dtype]]]):
                Ignored. Added for compatibility with RowIterator.

            max_queue_size:
                Ignored. Added for compatibility with RowIterator.

        Returns:
            An iterator yielding a single empty :class:`~pandas.DataFrame`.

        Raises:
            ValueError:
                If the :mod:`pandas` library cannot be imported.
        """
        if pandas is None:
            raise ValueError(_NO_PANDAS_ERROR)
        return iter((pandas.DataFrame(),))

    def __iter__(self):
        return iter(())


class PartitionRange(object):
    """Definition of the ranges for range partitioning.

    .. note::
        **Beta**. The integer range partitioning feature is in a pre-release
        state and might change or have limited support.

    Args:
        start (Optional[int]):
            Sets the
            :attr:`~google.cloud.bigquery.table.PartitionRange.start`
            property.
        end (Optional[int]):
            Sets the
            :attr:`~google.cloud.bigquery.table.PartitionRange.end`
            property.
        interval (Optional[int]):
            Sets the
            :attr:`~google.cloud.bigquery.table.PartitionRange.interval`
            property.
        _properties (Optional[dict]):
            Private. Used to construct object from API resource.
    """

    def __init__(self, start=None, end=None, interval=None, _properties=None):
        if _properties is None:
            _properties = {}
        self._properties = _properties

        if start is not None:
            self.start = start
        if end is not None:
            self.end = end
        if interval is not None:
            self.interval = interval

    @property
    def start(self):
        """int: The start of range partitioning, inclusive."""
        return _helpers._int_or_none(self._properties.get("start"))

    @start.setter
    def start(self, value):
        self._properties["start"] = _helpers._str_or_none(value)

    @property
    def end(self):
        """int: The end of range partitioning, exclusive."""
        return _helpers._int_or_none(self._properties.get("end"))

    @end.setter
    def end(self, value):
        self._properties["end"] = _helpers._str_or_none(value)

    @property
    def interval(self):
        """int: The width of each interval."""
        return _helpers._int_or_none(self._properties.get("interval"))

    @interval.setter
    def interval(self, value):
        self._properties["interval"] = _helpers._str_or_none(value)

    def _key(self):
        return tuple(sorted(self._properties.items()))

    def __eq__(self, other):
        if not isinstance(other, PartitionRange):
            return NotImplemented
        return self._key() == other._key()

    def __ne__(self, other):
        return not self == other

    def __repr__(self):
        key_vals = ["{}={}".format(key, val) for key, val in self._key()]
        return "PartitionRange({})".format(", ".join(key_vals))

    __hash__ = None


class RangePartitioning(object):
    """Range-based partitioning configuration for a table.

    .. note::
        **Beta**. The integer range partitioning feature is in a pre-release
        state and might change or have limited support.

    Args:
        range_ (Optional[google.cloud.bigquery.table.PartitionRange]):
            Sets the
            :attr:`google.cloud.bigquery.table.RangePartitioning.range_`
            property.
        field (Optional[str]):
            Sets the
            :attr:`google.cloud.bigquery.table.RangePartitioning.field`
            property.
        _properties (Optional[dict]):
            Private. Used to construct object from API resource.
    """

    def __init__(self, range_=None, field=None, _properties=None):
        if _properties is None:
            _properties = {}
        self._properties = _properties

        if range_ is not None:
            self.range_ = range_
        if field is not None:
            self.field = field

    # Trailing underscore to prevent conflict with built-in range() function.
    @property
    def range_(self):
        """google.cloud.bigquery.table.PartitionRange: Defines the
        ranges for range partitioning.

        Raises:
            ValueError:
                If the value is not a :class:`PartitionRange`.
        """
        range_properties = self._properties.setdefault("range", {})
        return PartitionRange(_properties=range_properties)

    @range_.setter
    def range_(self, value):
        if not isinstance(value, PartitionRange):
            raise ValueError("Expected a PartitionRange, but got {}.".format(value))
        self._properties["range"] = value._properties

    @property
    def field(self):
        """str: The table is partitioned by this field.

        The field must be a top-level ``NULLABLE`` / ``REQUIRED`` field. The
        only supported type is ``INTEGER`` / ``INT64``.
        """
        return self._properties.get("field")

    @field.setter
    def field(self, value):
        self._properties["field"] = value

    def _key(self):
        return (("field", self.field), ("range_", self.range_))

    def __eq__(self, other):
        if not isinstance(other, RangePartitioning):
            return NotImplemented
        return self._key() == other._key()

    def __ne__(self, other):
        return not self == other

    def __repr__(self):
        key_vals = ["{}={}".format(key, repr(val)) for key, val in self._key()]
        return "RangePartitioning({})".format(", ".join(key_vals))

    __hash__ = None


class TimePartitioningType(object):
    """Specifies the type of time partitioning to perform."""

    DAY = "DAY"
    """str: Generates one partition per day."""

    HOUR = "HOUR"
    """str: Generates one partition per hour."""

    MONTH = "MONTH"
    """str: Generates one partition per month."""

    YEAR = "YEAR"
    """str: Generates one partition per year."""


class TimePartitioning(object):
    """Configures time-based partitioning for a table.

    Args:
        type_ (Optional[google.cloud.bigquery.table.TimePartitioningType]):
            Specifies the type of time partitioning to perform. Defaults to
            :attr:`~google.cloud.bigquery.table.TimePartitioningType.DAY`.

            Supported values are:

            * :attr:`~google.cloud.bigquery.table.TimePartitioningType.HOUR`
            * :attr:`~google.cloud.bigquery.table.TimePartitioningType.DAY`
            * :attr:`~google.cloud.bigquery.table.TimePartitioningType.MONTH`
            * :attr:`~google.cloud.bigquery.table.TimePartitioningType.YEAR`

        field (Optional[str]):
            If set, the table is partitioned by this field. If not set, the
            table is partitioned by pseudo column ``_PARTITIONTIME``. The field
            must be a top-level ``TIMESTAMP``, ``DATETIME``, or ``DATE``
            field. Its mode must be ``NULLABLE`` or ``REQUIRED``.

            See the `time-unit column-partitioned tables guide
            <https://cloud.google.com/bigquery/docs/creating-column-partitions>`_
            in the BigQuery documentation.
        expiration_ms(Optional[int]):
            Number of milliseconds for which to keep the storage for a
            partition.
        require_partition_filter (Optional[bool]):
            DEPRECATED: Use
            :attr:`~google.cloud.bigquery.table.Table.require_partition_filter`,
            instead.
    """

    def __init__(
        self, type_=None, field=None, expiration_ms=None, require_partition_filter=None
    ):
        self._properties = {}
        if type_ is None:
            self.type_ = TimePartitioningType.DAY
        else:
            self.type_ = type_
        if field is not None:
            self.field = field
        if expiration_ms is not None:
            self.expiration_ms = expiration_ms
        if require_partition_filter is not None:
            self.require_partition_filter = require_partition_filter

    @property
    def type_(self):
        """google.cloud.bigquery.table.TimePartitioningType: The type of time
        partitioning to use.
        """
        return self._properties.get("type")

    @type_.setter
    def type_(self, value):
        self._properties["type"] = value

    @property
    def field(self):
        """str: Field in the table to use for partitioning"""
        return self._properties.get("field")

    @field.setter
    def field(self, value):
        self._properties["field"] = value

    @property
    def expiration_ms(self):
        """int: Number of milliseconds to keep the storage for a partition."""
        return _helpers._int_or_none(self._properties.get("expirationMs"))

    @expiration_ms.setter
    def expiration_ms(self, value):
        if value is not None:
            # Allow explicitly setting the expiration to None.
            value = str(value)
        self._properties["expirationMs"] = value

    @property
    def require_partition_filter(self):
        """bool: Specifies whether partition filters are required for queries

        DEPRECATED: Use
        :attr:`~google.cloud.bigquery.table.Table.require_partition_filter`,
        instead.
        """
        warnings.warn(
            (
                "TimePartitioning.require_partition_filter will be removed in "
                "future versions. Please use Table.require_partition_filter "
                "instead."
            ),
            PendingDeprecationWarning,
            stacklevel=2,
        )
        return self._properties.get("requirePartitionFilter")

    @require_partition_filter.setter
    def require_partition_filter(self, value):
        warnings.warn(
            (
                "TimePartitioning.require_partition_filter will be removed in "
                "future versions. Please use Table.require_partition_filter "
                "instead."
            ),
            PendingDeprecationWarning,
            stacklevel=2,
        )
        self._properties["requirePartitionFilter"] = value

    @classmethod
    def from_api_repr(cls, api_repr: dict) -> "TimePartitioning":
        """Return a :class:`TimePartitioning` object deserialized from a dict.

        This method creates a new ``TimePartitioning`` instance that points to
        the ``api_repr`` parameter as its internal properties dict. This means
        that when a ``TimePartitioning`` instance is stored as a property of
        another object, any changes made at the higher level will also appear
        here::

            >>> time_partitioning = TimePartitioning()
            >>> table.time_partitioning = time_partitioning
            >>> table.time_partitioning.field = 'timecolumn'
            >>> time_partitioning.field
            'timecolumn'

        Args:
            api_repr (Mapping[str, str]):
                The serialized representation of the TimePartitioning, such as
                what is output by :meth:`to_api_repr`.

        Returns:
            google.cloud.bigquery.table.TimePartitioning:
                The ``TimePartitioning`` object.
        """
        instance = cls()
        instance._properties = api_repr
        return instance

    def to_api_repr(self) -> dict:
        """Return a dictionary representing this object.

        This method returns the properties dict of the ``TimePartitioning``
        instance rather than making a copy. This means that when a
        ``TimePartitioning`` instance is stored as a property of another
        object, any changes made at the higher level will also appear here.

        Returns:
            dict:
                A dictionary representing the TimePartitioning object in
                serialized form.
        """
        return self._properties

    def _key(self):
        # because we are only "renaming" top level keys shallow copy is sufficient here.
        properties = self._properties.copy()
        # calling repr for non built-in type objects.
        properties["type_"] = repr(properties.pop("type"))
        if "field" in properties:
            # calling repr for non built-in type objects.
            properties["field"] = repr(properties["field"])
        if "requirePartitionFilter" in properties:
            properties["require_partition_filter"] = properties.pop(
                "requirePartitionFilter"
            )
        if "expirationMs" in properties:
            properties["expiration_ms"] = properties.pop("expirationMs")
        return tuple(sorted(properties.items()))

    def __eq__(self, other):
        if not isinstance(other, TimePartitioning):
            return NotImplemented
        return self._key() == other._key()

    def __ne__(self, other):
        return not self == other

    def __hash__(self):
        return hash(self._key())

    def __repr__(self):
        key_vals = ["{}={}".format(key, val) for key, val in self._key()]
        return "TimePartitioning({})".format(",".join(key_vals))


def _item_to_row(iterator, resource):
    """Convert a JSON row to the native object.

    .. note::

        This assumes that the ``schema`` attribute has been
        added to the iterator after being created, which
        should be done by the caller.

    Args:
        iterator (google.api_core.page_iterator.Iterator): The iterator that is currently in use.
        resource (Dict): An item to be converted to a row.

    Returns:
        google.cloud.bigquery.table.Row: The next row in the page.
    """
    return Row(
        _helpers._row_tuple_from_json(resource, iterator.schema),
        iterator._field_to_index,
    )


def _row_iterator_page_columns(schema, response):
    """Make a generator of all the columns in a page from tabledata.list.

    This enables creating a :class:`pandas.DataFrame` and other
    column-oriented data structures such as :class:`pyarrow.RecordBatch`
    """
    columns = []
    rows = response.get("rows", [])

    def get_column_data(field_index, field):
        for row in rows:
            yield _helpers._field_from_json(row["f"][field_index]["v"], field)

    for field_index, field in enumerate(schema):
        columns.append(get_column_data(field_index, field))

    return columns


# pylint: disable=unused-argument
def _rows_page_start(iterator, page, response):
    """Grab total rows when :class:`~google.cloud.iterator.Page` starts.

    Args:
        iterator (google.api_core.page_iterator.Iterator): The iterator that is currently in use.
        page (google.api_core.page_iterator.Page): The page that was just created.
        response (Dict): The JSON API response for a page of rows in a table.
    """
    # Make a (lazy) copy of the page in column-oriented format for use in data
    # science packages.
    page._columns = _row_iterator_page_columns(iterator._schema, response)

    total_rows = response.get("totalRows")
    if total_rows is not None:
        total_rows = int(total_rows)
    iterator._total_rows = total_rows


# pylint: enable=unused-argument


def _table_arg_to_table_ref(value, default_project=None):
    """Helper to convert a string or Table to TableReference.

    This function keeps TableReference and other kinds of objects unchanged.
    """
    if isinstance(value, str):
        value = TableReference.from_string(value, default_project=default_project)
    if isinstance(value, (Table, TableListItem)):
        value = value.reference
    return value


def _table_arg_to_table(value, default_project=None):
    """Helper to convert a string or TableReference to a Table.

    This function keeps Table and other kinds of objects unchanged.
    """
    if isinstance(value, str):
        value = TableReference.from_string(value, default_project=default_project)
    if isinstance(value, TableReference):
        value = Table(value)
    if isinstance(value, TableListItem):
        newvalue = Table(value.reference)
        newvalue._properties = value._properties
        value = newvalue

    return value