base.py

# Copyright (c) 2017 The sqlalchemy-bigquery Authors
#
# Permission is hereby granted, free of charge, to any person obtaining a copy of
# this software and associated documentation files (the "Software"), to deal in
# the Software without restriction, including without limitation the rights to
# use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
# the Software, and to permit persons to whom the Software is furnished to do so,
# subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
# FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
# COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
# IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

"""Integration between SQLAlchemy and BigQuery."""

from __future__ import absolute_import
from __future__ import unicode_literals

from decimal import Decimal
import random
import operator
import uuid

from google import auth
import google.api_core.exceptions
from google.cloud.bigquery import dbapi
from google.cloud.bigquery.table import TableReference
from google.api_core.exceptions import NotFound

import sqlalchemy
import sqlalchemy.sql.expression
import sqlalchemy.sql.functions
import sqlalchemy.sql.sqltypes
import sqlalchemy.sql.type_api
from sqlalchemy.exc import NoSuchTableError
from sqlalchemy import util
from sqlalchemy.sql.compiler import (
    SQLCompiler,
    GenericTypeCompiler,
    DDLCompiler,
    IdentifierPreparer,
)
from sqlalchemy.sql.sqltypes import Integer, String, NullType, Numeric
from sqlalchemy.engine.default import DefaultDialect, DefaultExecutionContext
from sqlalchemy.engine.base import Engine
from sqlalchemy.sql.schema import Column
from sqlalchemy.sql.schema import Table
from sqlalchemy.sql import elements, selectable
import re

from .parse_url import parse_url
from . import _helpers, _struct, _types

FIELD_ILLEGAL_CHARACTERS = re.compile(r"[^\w]+")

TABLE_VALUED_ALIAS_ALIASES = "bigquery_table_valued_alias_aliases"


def assert_(cond, message="Assertion failed"):  # pragma: NO COVER
    if not cond:
        raise AssertionError(message)


class BigQueryIdentifierPreparer(IdentifierPreparer):
    """
    Set containing everything
    https://github.com/dropbox/PyHive/blob/master/pyhive/sqlalchemy_presto.py
    """

    def __init__(self, dialect):
        super(BigQueryIdentifierPreparer, self).__init__(
            dialect, initial_quote="`",
        )

    def quote_column(self, value):
        """
        Quote a column.
        Fields are quoted separately from the record name.
        """

        parts = value.split(".")
        return ".".join(self.quote_identifier(x) for x in parts)

    def quote(self, ident, force=None, column=False):
        """
        Conditionally quote an identifier.
        """

        force = getattr(ident, "quote", None)
        if force is None or force:
            return self.quote_column(ident) if column else self.quote_identifier(ident)
        else:
            return ident

    def format_label(self, label, name=None):
        name = name or label.name

        # Fields must start with a letter or underscore
        if not name[0].isalpha() and name[0] != "_":
            name = "_" + name

        # Fields must contain only letters, numbers, and underscores
        name = FIELD_ILLEGAL_CHARACTERS.sub("_", name)

        result = self.quote(name)
        return result


class BigQueryExecutionContext(DefaultExecutionContext):
    def create_cursor(self):
        # Set arraysize
        c = super(BigQueryExecutionContext, self).create_cursor()
        if self.dialect.arraysize:
            c.arraysize = self.dialect.arraysize
        return c

    def get_insert_default(self, column):  # pragma: NO COVER
        # Only used by compliance tests
        if isinstance(column.type, Integer):
            return random.randint(-9223372036854775808, 9223372036854775808)  # 1<<63
        elif isinstance(column.type, String):
            return str(uuid.uuid4())

    __remove_type_from_empty_in = _helpers.substitute_string_re_method(
        r"""
        \sIN\sUNNEST\(\[\s               # ' IN UNNEST([ '
        (
        (?:NULL|\(NULL(?:,\sNULL)+\))\)  # '(NULL)' or '((NULL, NULL, ...))'
        \s(?:AND|OR)\s\(1\s!?=\s1        # ' and 1 != 1' or ' or 1 = 1'
        )
        (?:[:][A-Z0-9]+)?                # Maybe ':TYPE' (e.g. ':INT64')
        \s\]\)                           # Close: ' ])'
        """,
        flags=re.IGNORECASE | re.VERBOSE,
        repl=r" IN(\1)",
    )

    @_helpers.substitute_re_method(
        r"""
        \sIN\sUNNEST\(\[\s       # ' IN UNNEST([ '
        (                        # Placeholders. See below.
        %\([^)]+_\d+\)s          # Placeholder '%(foo_1)s'
        (?:,\s                   # 0 or more placeholders
        %\([^)]+_\d+\)s
        )*
        )?
        :([A-Z0-9]+)             # Type ':TYPE' (e.g. ':INT64')
        \s\]\)                   # Close: ' ])'
        """,
        flags=re.IGNORECASE | re.VERBOSE,
    )
    def __distribute_types_to_expanded_placeholders(self, m):
        # If we have an in parameter, it sometimes gets expaned to 0 or more
        # parameters and we need to move the type marker to each
        # parameter.
        # (The way SQLAlchemy handles this is a bit awkward for our
        # purposes.)

        # In the placeholder part of the regex above, the `_\d+
        # suffixes refect that when an array parameter is expanded,
        # numeric suffixes are added.  For example, a placeholder like
        # `%(foo)s` gets expaneded to `%(foo_0)s, `%(foo_1)s, ...`.
        placeholders, type_ = m.groups()
        if placeholders:
            placeholders = placeholders.replace(")", f":{type_})")
        else:
            placeholders = ""
        return f" IN UNNEST([ {placeholders} ])"

    def pre_exec(self):
        self.statement = self.__distribute_types_to_expanded_placeholders(
            self.__remove_type_from_empty_in(self.statement)
        )


class BigQueryCompiler(_struct.SQLCompiler, SQLCompiler):

    compound_keywords = SQLCompiler.compound_keywords.copy()
    compound_keywords[selectable.CompoundSelect.UNION] = "UNION DISTINCT"
    compound_keywords[selectable.CompoundSelect.UNION_ALL] = "UNION ALL"

    def __init__(self, dialect, statement, *args, **kwargs):
        if isinstance(statement, Column):
            kwargs["compile_kwargs"] = util.immutabledict({"include_table": False})
        super(BigQueryCompiler, self).__init__(dialect, statement, *args, **kwargs)

    def visit_insert(self, insert_stmt, asfrom=False, **kw):
        # The (internal) documentation for `inline` is confusing, but
        # having `inline` be true prevents us from generating default
        # primary-key values when we're doing executemany, which seem broken.

        # We can probably do this in the constructor, but I want to
        # make sure this only affects insert, because I'm paranoid. :)

        self.inline = False

        return super(BigQueryCompiler, self).visit_insert(
            insert_stmt, asfrom=False, **kw
        )

    def visit_table_valued_alias(self, element, **kw):
        # When using table-valued functions, like UNNEST, BigQuery requires a
        # FROM for any table referenced in the function, including expressions
        # in function arguments.
        #
        # For example, given SQLAlchemy code:
        #
        #   print(
        #      select([func.unnest(foo.c.objects).alias('foo_objects').column])
        #      .compile(engine))
        #
        # Left to it's own devices, SQLAlchemy would outout:
        #
        #   SELECT `foo_objects`
        #   FROM unnest(`foo`.`objects`) AS `foo_objects`
        #
        # But BigQuery diesn't understand the `foo` reference unless
        # we add as reference to `foo` in the FROM:
        #
        #   SELECT foo_objects
        #   FROM `foo`, UNNEST(`foo`.`objects`) as foo_objects
        #
        # This is tricky because:
        # 1. We have to find the table references.
        # 2. We can't know practically if there's already a FROM for a table.
        #
        # We leverage visit_column to find a table reference.  Whenever we find
        # one, we create an alias for it, so as not to conflict with an existing
        # reference if one is present.
        #
        # This requires communicating between this function and visit_column.
        # We do this by sticking a dictionary in the keyword arguments.
        # This dictionary:
        # a. Tells visit_column that it's an a table-valued alias expresssion, and
        # b. Gives it a place to record the aliases it creates.
        #
        # This function creates aliases in the FROM list for any aliases recorded
        # by visit_column.

        kw[TABLE_VALUED_ALIAS_ALIASES] = {}
        ret = super().visit_table_valued_alias(element, **kw)
        aliases = kw.pop(TABLE_VALUED_ALIAS_ALIASES)
        if aliases:
            aliases = ", ".join(
                f"{self.preparer.quote(tablename)} {self.preparer.quote(alias)}"
                for tablename, alias in aliases.items()
            )
            ret = f"{aliases}, {ret}"
        return ret

    def visit_column(
        self,
        column,
        add_to_result_map=None,
        include_table=True,
        result_map_targets=(),
        **kwargs,
    ):
        name = orig_name = column.name
        if name is None:
            name = self._fallback_column_name(column)

        is_literal = column.is_literal
        if not is_literal and isinstance(name, elements._truncated_label):
            name = self._truncated_identifier("colident", name)

        if add_to_result_map is not None:
            targets = (column, name, column.key) + result_map_targets
            if getattr(column, "_tq_label", None):
                # _tq_label was added in SQLAlchemy 1.4
                targets += (column._tq_label,)

            add_to_result_map(name, orig_name, targets, column.type)

        if is_literal:
            name = self.escape_literal_column(name)
        else:
            name = self.preparer.quote(name, column=True)
        table = column.table
        if table is None or not include_table or not table.named_with_column:
            return name
        else:
            tablename = table.name
            if isinstance(tablename, elements._truncated_label):
                tablename = self._truncated_identifier("alias", tablename)
            elif TABLE_VALUED_ALIAS_ALIASES in kwargs:
                known_tables = set(
                    from_.name
                    for from_ in self.compile_state.froms
                    if isinstance(from_, Table)
                )
                if tablename not in known_tables:
                    aliases = kwargs[TABLE_VALUED_ALIAS_ALIASES]
                    if tablename not in aliases:
                        aliases[tablename] = self.anon_map[
                            f"{TABLE_VALUED_ALIAS_ALIASES} {tablename}"
                        ]
                    tablename = aliases[tablename]

            return self.preparer.quote(tablename) + "." + name

    def visit_label(self, *args, within_group_by=False, **kwargs):
        # Use labels in GROUP BY clause.
        #
        # Flag set in the group_by_clause method. Works around missing
        # equivalent to supports_simple_order_by_label for group by.
        if within_group_by:
            kwargs["render_label_as_label"] = args[0]
        return super(BigQueryCompiler, self).visit_label(*args, **kwargs)

    def group_by_clause(self, select, **kw):
        return super(BigQueryCompiler, self).group_by_clause(
            select, **kw, within_group_by=True
        )

    ############################################################################
    # Handle parameters in in

    # Due to details in the way sqlalchemy arranges the compilation we
    # expect the bind parameter as an array and unnest it.

    # As it happens, bigquery can handle arrays directly, but there's
    # no way to tell sqlalchemy that, so it works harder than
    # necessary and makes us do the same.

    __sqlalchemy_version_info = tuple(map(int, sqlalchemy.__version__.split(".")))

    __expandng_text = (
        "EXPANDING" if __sqlalchemy_version_info < (1, 4) else "POSTCOMPILE"
    )

    __in_expanding_bind = _helpers.substitute_string_re_method(
        fr"""
        \sIN\s\(                     # ' IN ('
        (
        \[                           # Expanding placeholder
        {__expandng_text}            #   e.g. [EXPANDING_foo_1]
        _[^\]]+                      #
        \]
        (:[A-Z0-9]+)?                # type marker (e.g. ':INT64'
        )
        \)$                          # close w ending )
        """,
        flags=re.IGNORECASE | re.VERBOSE,
        repl=r" IN UNNEST([ \1 ])",
    )

    def visit_in_op_binary(self, binary, operator_, **kw):
        return self.__in_expanding_bind(
            self._generate_generic_binary(binary, " IN ", **kw)
        )

    def visit_empty_set_expr(self, element_types):
        return ""

    def visit_not_in_op_binary(self, binary, operator, **kw):
        return (
            "("
            + self.__in_expanding_bind(
                self._generate_generic_binary(binary, " NOT IN ", **kw)
            )
            + ")"
        )

    visit_notin_op_binary = visit_not_in_op_binary  # before 1.4

    ############################################################################

    ############################################################################
    # Correct for differences in the way that SQLAlchemy escape % and _ (/)
    # and BigQuery does (\\).

    @staticmethod
    def _maybe_reescape(binary):
        binary = binary._clone()
        escape = binary.modifiers.pop("escape", None)
        if escape and escape != "\\":
            binary.right.value = escape.join(
                v.replace(escape, "\\")
                for v in binary.right.value.split(escape + escape)
            )
        return binary

    def visit_contains_op_binary(self, binary, operator, **kw):
        return super(BigQueryCompiler, self).visit_contains_op_binary(
            self._maybe_reescape(binary), operator, **kw
        )

    def visit_notcontains_op_binary(self, binary, operator, **kw):
        return super(BigQueryCompiler, self).visit_notcontains_op_binary(
            self._maybe_reescape(binary), operator, **kw
        )

    def visit_startswith_op_binary(self, binary, operator, **kw):
        return super(BigQueryCompiler, self).visit_startswith_op_binary(
            self._maybe_reescape(binary), operator, **kw
        )

    def visit_notstartswith_op_binary(self, binary, operator, **kw):
        return super(BigQueryCompiler, self).visit_notstartswith_op_binary(
            self._maybe_reescape(binary), operator, **kw
        )

    def visit_endswith_op_binary(self, binary, operator, **kw):
        return super(BigQueryCompiler, self).visit_endswith_op_binary(
            self._maybe_reescape(binary), operator, **kw
        )

    def visit_notendswith_op_binary(self, binary, operator, **kw):
        return super(BigQueryCompiler, self).visit_notendswith_op_binary(
            self._maybe_reescape(binary), operator, **kw
        )

    ############################################################################

    __placeholder = re.compile(r"%\(([^\]:]+)(:[^\]:]+)?\)s$").match

    __expanded_param = re.compile(fr"\(\[" fr"{__expandng_text}" fr"_[^\]]+\]\)$").match

    __remove_type_parameter = _helpers.substitute_string_re_method(
        r"""
        (STRING|BYTES|NUMERIC|BIGNUMERIC)  # Base type
        \(                                 # Dimensions e.g. '(42)', '(4, 2)':
        \s*\d+\s*                          # First dimension
        (?:,\s*\d+\s*)*                    # Remaining dimensions
        \)
        """,
        repl=r"\1",
        flags=re.VERBOSE | re.IGNORECASE,
    )

    def visit_bindparam(
        self,
        bindparam,
        within_columns_clause=False,
        literal_binds=False,
        skip_bind_expression=False,
        **kwargs,
    ):
        type_ = bindparam.type
        unnest = False
        if (
            bindparam.expanding
            and not isinstance(type_, NullType)
            and not literal_binds
        ):
            # Normally, when performing an IN operation, like:
            #
            #  foo IN (some_sequence)
            #
            # SQAlchemy passes `foo` as a parameter and unpacks
            # `some_sequence` and passes each element as a parameter.
            # This mechanism is refered to as "expanding".  It's
            # inefficient and can't handle large arrays. (It's also
            # very complicated, but that's not the issue we care about
            # here. :) ) BigQuery lets us use arrays directly in this
            # context, we just need to call UNNEST on an array when
            # it's used in IN.
            #
            # So, if we get an `expanding` flag, and if we have a known type
            # (and don't have literal binds, which are implemented in-line in
            # in the SQL), we turn off expanding and we set an unnest flag
            # so that we add an UNNEST() call (below).
            #
            # The NullType/known-type check has to do with some extreme
            # edge cases having to do with empty in-lists that get special
            # hijinks from SQLAlchemy that we don't want to disturb. :)
            #
            # Note that we do *not* want to overwrite the "real" bindparam
            # here, because then we can't do a recompile later (e.g., first
            # print the statment, then execute it).  See issue #357.
            #
            if getattr(bindparam, "expand_op", None) is not None:
                assert bindparam.expand_op.__name__.endswith("in_op")  # in in
                bindparam = bindparam._clone(maintain_key=True)
                bindparam.expanding = False
                unnest = True

        param = super(BigQueryCompiler, self).visit_bindparam(
            bindparam,
            within_columns_clause,
            literal_binds,
            skip_bind_expression,
            **kwargs,
        )

        if literal_binds or isinstance(type_, NullType):
            return param

        if (
            isinstance(type_, Numeric)
            and (type_.precision is None or type_.scale is None)
            and isinstance(bindparam.value, Decimal)
        ):
            t = bindparam.value.as_tuple()

            if type_.precision is None:
                type_.precision = len(t.digits)

            if type_.scale is None and t.exponent < 0:
                type_.scale = -t.exponent

        bq_type = self.dialect.type_compiler.process(type_)
        bq_type = self.__remove_type_parameter(bq_type)

        assert_(param != "%s", f"Unexpected param: {param}")

        if bindparam.expanding:
            assert_(self.__expanded_param(param), f"Unexpected param: {param}")
            param = param.replace(")", f":{bq_type})")

        else:
            m = self.__placeholder(param)
            if m:
                name, type_ = m.groups()
                assert_(type_ is None)
                param = f"%({name}:{bq_type})s"

        if unnest:
            param = f"UNNEST({param})"

        return param

    def visit_getitem_binary(self, binary, operator_, **kw):
        left = self.process(binary.left, **kw)
        right = self.process(binary.right, **kw)
        return f"{left}[OFFSET({right})]"


class BigQueryTypeCompiler(GenericTypeCompiler):
    def visit_INTEGER(self, type_, **kw):
        return "INT64"

    visit_BIGINT = visit_SMALLINT = visit_INTEGER

    def visit_BOOLEAN(self, type_, **kw):
        return "BOOL"

    def visit_FLOAT(self, type_, **kw):
        return "FLOAT64"

    visit_REAL = visit_FLOAT

    def visit_STRING(self, type_, **kw):
        if (type_.length is not None) and isinstance(
            kw.get("type_expression"), Column
        ):  # column def
            return f"STRING({type_.length})"
        return "STRING"

    visit_CHAR = visit_NCHAR = visit_STRING
    visit_VARCHAR = visit_NVARCHAR = visit_TEXT = visit_STRING

    def visit_ARRAY(self, type_, **kw):
        return "ARRAY<{}>".format(self.process(type_.item_type, **kw))

    def visit_BINARY(self, type_, **kw):
        if type_.length is not None:
            return f"BYTES({type_.length})"
        return "BYTES"

    visit_VARBINARY = visit_BLOB = visit_BINARY

    def visit_NUMERIC(self, type_, **kw):
        if (type_.precision is not None) and isinstance(
            kw.get("type_expression"), Column
        ):  # column def
            if type_.scale is not None:
                suffix = f"({type_.precision}, {type_.scale})"
            else:
                suffix = f"({type_.precision})"
        else:
            suffix = ""

        return (
            "BIGNUMERIC"
            if (type_.precision is not None and type_.precision > 38)
            or (type_.scale is not None and type_.scale > 9)
            else "NUMERIC"
        ) + suffix

    visit_DECIMAL = visit_NUMERIC


class BigQueryDDLCompiler(DDLCompiler):

    # BigQuery has no support for foreign keys.
    def visit_foreign_key_constraint(self, constraint):
        return None

    # BigQuery has no support for primary keys.
    def visit_primary_key_constraint(self, constraint):
        return None

    # BigQuery has no support for unique constraints.
    def visit_unique_constraint(self, constraint):
        return None

    def get_column_specification(self, column, **kwargs):
        colspec = super(BigQueryDDLCompiler, self).get_column_specification(
            column, **kwargs
        )
        if column.comment is not None:
            colspec = "{} OPTIONS(description={})".format(
                colspec, process_string_literal(column.comment)
            )
        return colspec

    def post_create_table(self, table):
        bq_opts = table.dialect_options["bigquery"]
        opts = []

        if ("description" in bq_opts) or table.comment:
            description = process_string_literal(
                bq_opts.get("description", table.comment)
            )
            opts.append(f"description={description}")

        if "friendly_name" in bq_opts:
            opts.append(
                "friendly_name={}".format(
                    process_string_literal(bq_opts["friendly_name"])
                )
            )

        if opts:
            return "\nOPTIONS({})".format(", ".join(opts))

        return ""

    def visit_set_table_comment(self, create):
        table_name = self.preparer.format_table(create.element)
        description = self.sql_compiler.render_literal_value(
            create.element.comment, sqlalchemy.sql.sqltypes.String()
        )
        return f"ALTER TABLE {table_name} SET OPTIONS(description={description})"

    def visit_drop_table_comment(self, drop):
        table_name = self.preparer.format_table(drop.element)
        return f"ALTER TABLE {table_name} SET OPTIONS(description=null)"


def process_string_literal(value):
    return repr(value.replace("%", "%%"))


class BQString(String):
    def literal_processor(self, dialect):
        return process_string_literal


class BQBinary(sqlalchemy.sql.sqltypes._Binary):
    @staticmethod
    def __process_bytes_literal(value):
        return repr(value.replace(b"%", b"%%"))

    def literal_processor(self, dialect):
        return self.__process_bytes_literal


class BQClassTaggedStr(sqlalchemy.sql.type_api.TypeEngine):
    """Type that can get literals via str
    """

    @staticmethod
    def process_literal_as_class_tagged_str(value):
        return f"{value.__class__.__name__.upper()} {repr(str(value))}"

    def literal_processor(self, dialect):
        return self.process_literal_as_class_tagged_str


class BQTimestamp(sqlalchemy.sql.type_api.TypeEngine):
    """Type that can get literals via str
    """

    @staticmethod
    def process_timestamp_literal(value):
        return f"TIMESTAMP {process_string_literal(str(value))}"

    def literal_processor(self, dialect):
        return self.process_timestamp_literal


class BQArray(sqlalchemy.sql.sqltypes.ARRAY):
    def literal_processor(self, dialect):

        item_processor = self.item_type._cached_literal_processor(dialect)
        if not item_processor:
            raise NotImplementedError(
                f"Don't know how to literal-quote values of type {self.item_type}"
            )

        def process_array_literal(value):
            return "[" + ", ".join(item_processor(v) for v in value) + "]"

        return process_array_literal


class BigQueryDialect(DefaultDialect):
    name = "bigquery"
    driver = "bigquery"
    preparer = BigQueryIdentifierPreparer
    statement_compiler = BigQueryCompiler
    type_compiler = BigQueryTypeCompiler
    ddl_compiler = BigQueryDDLCompiler
    execution_ctx_cls = BigQueryExecutionContext
    supports_alter = False
    supports_comments = True
    inline_comments = True
    supports_pk_autoincrement = False
    supports_default_values = False
    supports_empty_insert = False
    supports_multivalues_insert = True
    supports_unicode_statements = True
    supports_unicode_binds = True
    supports_native_decimal = True
    description_encoding = None
    supports_native_boolean = True
    supports_simple_order_by_label = True
    postfetch_lastrowid = False
    preexecute_autoincrement_sequences = False

    colspecs = {
        String: BQString,
        sqlalchemy.sql.sqltypes._Binary: BQBinary,
        sqlalchemy.sql.sqltypes.Date: BQClassTaggedStr,
        sqlalchemy.sql.sqltypes.DateTime: BQClassTaggedStr,
        sqlalchemy.sql.sqltypes.Time: BQClassTaggedStr,
        sqlalchemy.sql.sqltypes.TIMESTAMP: BQTimestamp,
        sqlalchemy.sql.sqltypes.ARRAY: BQArray,
    }

    def __init__(
        self,
        arraysize=5000,
        credentials_path=None,
        location=None,
        credentials_info=None,
        list_tables_page_size=1000,
        *args,
        **kwargs,
    ):
        super(BigQueryDialect, self).__init__(*args, **kwargs)
        self.arraysize = arraysize
        self.credentials_path = credentials_path
        self.credentials_info = credentials_info
        self.location = location
        self.dataset_id = None
        self.list_tables_page_size = list_tables_page_size

    @classmethod
    def dbapi(cls):
        return dbapi

    @staticmethod
    def _build_formatted_table_id(table):
        """Build '<dataset_id>.<table_id>' string using given table."""
        return "{}.{}".format(table.reference.dataset_id, table.table_id)

    @staticmethod
    def _add_default_dataset_to_job_config(job_config, project_id, dataset_id):
        # If dataset_id is set, then we know the job_config isn't None
        if dataset_id:
            # If project_id is missing, use default project_id for the current environment
            if not project_id:
                _, project_id = auth.default()

            job_config.default_dataset = "{}.{}".format(project_id, dataset_id)

    def create_connect_args(self, url):
        (
            project_id,
            location,
            dataset_id,
            arraysize,
            credentials_path,
            default_query_job_config,
            list_tables_page_size,
        ) = parse_url(url)

        self.arraysize = arraysize or self.arraysize
        self.list_tables_page_size = list_tables_page_size or self.list_tables_page_size
        self.location = location or self.location
        self.credentials_path = credentials_path or self.credentials_path
        self.dataset_id = dataset_id
        self._add_default_dataset_to_job_config(
            default_query_job_config, project_id, dataset_id
        )
        client = _helpers.create_bigquery_client(
            credentials_path=self.credentials_path,
            credentials_info=self.credentials_info,
            project_id=project_id,
            location=self.location,
            default_query_job_config=default_query_job_config,
        )
        return ([client], {})

    def _get_table_or_view_names(self, connection, table_type, schema=None):
        current_schema = schema or self.dataset_id
        get_table_name = (
            self._build_formatted_table_id
            if self.dataset_id is None
            else operator.attrgetter("table_id")
        )

        client = connection.connection._client
        datasets = client.list_datasets()

        result = []
        for dataset in datasets:
            if current_schema is not None and current_schema != dataset.dataset_id:
                continue

            try:
                tables = client.list_tables(
                    dataset.reference, page_size=self.list_tables_page_size
                )
                for table in tables:
                    if table_type == table.table_type:
                        result.append(get_table_name(table))
            except google.api_core.exceptions.NotFound:
                # It's possible that the dataset was deleted between when we
                # fetched the list of datasets and when we try to list the
                # tables from it. See:
                # https://github.com/googleapis/python-bigquery-sqlalchemy/issues/105
                pass
        return result

    @staticmethod
    def _split_table_name(full_table_name):
        # Split full_table_name to get project, dataset and table name
        dataset = None
        table_name = None
        project = None

        table_name_split = full_table_name.split(".")
        if len(table_name_split) == 1:
            table_name = full_table_name
        elif len(table_name_split) == 2:
            dataset, table_name = table_name_split
        elif len(table_name_split) == 3:
            project, dataset, table_name = table_name_split
        else:
            raise ValueError(
                "Did not understand table_name: {}".format(full_table_name)
            )

        return (project, dataset, table_name)

    def _table_reference(
        self, provided_schema_name, provided_table_name, client_project
    ):
        project_id_from_table, dataset_id_from_table, table_id = self._split_table_name(
            provided_table_name
        )
        project_id_from_schema = None
        dataset_id_from_schema = None
        if provided_schema_name is not None:
            provided_schema_name_split = provided_schema_name.split(".")
            if len(provided_schema_name_split) == 1:
                if dataset_id_from_table:
                    project_id_from_schema = provided_schema_name_split[0]
                else:
                    dataset_id_from_schema = provided_schema_name_split[0]
            elif len(provided_schema_name_split) == 2:
                project_id_from_schema = provided_schema_name_split[0]
                dataset_id_from_schema = provided_schema_name_split[1]
            else:
                raise ValueError(
                    "Did not understand schema: {}".format(provided_schema_name)
                )
        if (
            dataset_id_from_schema
            and dataset_id_from_table
            and dataset_id_from_schema != dataset_id_from_table
        ):
            raise ValueError(
                "dataset_id specified in schema and table_name disagree: "
                "got {} in schema, and {} in table_name".format(
                    dataset_id_from_schema, dataset_id_from_table
                )
            )
        if (
            project_id_from_schema
            and project_id_from_table
            and project_id_from_schema != project_id_from_table
        ):
            raise ValueError(
                "project_id specified in schema and table_name disagree: "
                "got {} in schema, and {} in table_name".format(
                    project_id_from_schema, project_id_from_table
                )
            )
        project_id = project_id_from_schema or project_id_from_table or client_project
        dataset_id = dataset_id_from_schema or dataset_id_from_table or self.dataset_id

        table_ref = TableReference.from_string(
            "{}.{}.{}".format(project_id, dataset_id, table_id)
        )
        return table_ref

    def _get_table(self, connection, table_name, schema=None):
        if isinstance(connection, Engine):
            connection = connection.connect()

        client = connection.connection._client

        table_ref = self._table_reference(schema, table_name, client.project)
        try:
            table = client.get_table(table_ref)
        except NotFound:
            raise NoSuchTableError(table_name)
        return table

    def has_table(self, connection, table_name, schema=None):
        try:
            self._get_table(connection, table_name, schema)
            return True
        except NoSuchTableError:
            return False

    def get_columns(self, connection, table_name, schema=None, **kw):
        table = self._get_table(connection, table_name, schema)
        return _types.get_columns(table.schema)

    def get_table_comment(self, connection, table_name, schema=None, **kw):
        table = self._get_table(connection, table_name, schema)
        return {
            "text": table.description,
        }

    def get_foreign_keys(self, connection, table_name, schema=None, **kw):
        # BigQuery has no support for foreign keys.
        return []

    def get_pk_constraint(self, connection, table_name, schema=None, **kw):
        # BigQuery has no support for primary keys.
        return {"constrained_columns": []}

    def get_indexes(self, connection, table_name, schema=None, **kw):
        table = self._get_table(connection, table_name, schema)
        indexes = []
        if table.time_partitioning:
            indexes.append(
                {
                    "name": "partition",
                    "column_names": [table.time_partitioning.field],
                    "unique": False,
                }
            )
        if table.clustering_fields:
            indexes.append(
                {
                    "name": "clustering",
                    "column_names": table.clustering_fields,
                    "unique": False,
                }
            )
        return indexes

    def get_schema_names(self, connection, **kw):
        if isinstance(connection, Engine):
            connection = connection.connect()

        datasets = connection.connection._client.list_datasets()
        return [d.dataset_id for d in datasets]

    def get_table_names(self, connection, schema=None, **kw):
        if isinstance(connection, Engine):
            connection = connection.connect()

        return self._get_table_or_view_names(connection, "TABLE", schema)

    def get_view_names(self, connection, schema=None, **kw):
        if isinstance(connection, Engine):
            connection = connection.connect()

        return self._get_table_or_view_names(connection, "VIEW", schema)

    def do_rollback(self, dbapi_connection):
        # BigQuery has no support for transactions.
        pass

    def _check_unicode_returns(self, connection, additional_tests=None):
        # requests gives back Unicode strings
        return True

    def get_view_definition(self, connection, view_name, schema=None, **kw):
        if isinstance(connection, Engine):
            connection = connection.connect()
        client = connection.connection._client
        if self.dataset_id:
            view_name = f"{self.dataset_id}.{view_name}"
        view = client.get_table(view_name)
        return view.view_query


class unnest(sqlalchemy.sql.functions.GenericFunction):
    def __init__(self, *args, **kwargs):
        expr = kwargs.pop("expr", None)
        if expr is not None:
            args = (expr,) + args
        if len(args) != 1:
            raise TypeError("The unnest function requires a single argument.")
        arg = args[0]
        if isinstance(arg, sqlalchemy.sql.expression.ColumnElement):
            if not isinstance(arg.type, sqlalchemy.sql.sqltypes.ARRAY):
                raise TypeError("The argument to unnest must have an ARRAY type.")
            self.type = arg.type.item_type
        super().__init__(*args, **kwargs)


dialect = BigQueryDialect

try:
    import alembic  # noqa
except ImportError:
    pass
else:
    from alembic.ddl import impl

    class SqlalchemyBigqueryImpl(impl.DefaultImpl):
        __dialect__ = "bigquery"