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diff --git a/lib/sqlalchemy/sql/ddl.py b/lib/sqlalchemy/sql/ddl.py
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+# sql/ddl.py
+# Copyright (C) 2009-2022 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: https://www.opensource.org/licenses/mit-license.php
+"""
+Provides the hierarchy of DDL-defining schema items as well as routines
+to invoke them for a create/drop call.
+
+"""
+
+from . import roles
+from .base import _bind_or_error
+from .base import _generative
+from .base import Executable
+from .base import SchemaVisitor
+from .elements import ClauseElement
+from .. import exc
+from .. import util
+from ..util import topological
+
+
+class _DDLCompiles(ClauseElement):
+    _hierarchy_supports_caching = False
+    """disable cache warnings for all _DDLCompiles subclasses. """
+
+    def _compiler(self, dialect, **kw):
+        """Return a compiler appropriate for this ClauseElement, given a
+        Dialect."""
+
+        return dialect.ddl_compiler(dialect, self, **kw)
+
+    def _compile_w_cache(self, *arg, **kw):
+        raise NotImplementedError()
+
+
+class DDLElement(roles.DDLRole, Executable, _DDLCompiles):
+    """Base class for DDL expression constructs.
+
+    This class is the base for the general purpose :class:`.DDL` class,
+    as well as the various create/drop clause constructs such as
+    :class:`.CreateTable`, :class:`.DropTable`, :class:`.AddConstraint`,
+    etc.
+
+    :class:`.DDLElement` integrates closely with SQLAlchemy events,
+    introduced in :ref:`event_toplevel`.  An instance of one is
+    itself an event receiving callable::
+
+        event.listen(
+            users,
+            'after_create',
+            AddConstraint(constraint).execute_if(dialect='postgresql')
+        )
+
+    .. seealso::
+
+        :class:`.DDL`
+
+        :class:`.DDLEvents`
+
+        :ref:`event_toplevel`
+
+        :ref:`schema_ddl_sequences`
+
+    """
+
+    _execution_options = Executable._execution_options.union(
+        {"autocommit": True}
+    )
+
+    target = None
+    on = None
+    dialect = None
+    callable_ = None
+
+    def _execute_on_connection(
+        self, connection, multiparams, params, execution_options
+    ):
+        return connection._execute_ddl(
+            self, multiparams, params, execution_options
+        )
+
+    @util.deprecated_20(
+        ":meth:`.DDLElement.execute`",
+        alternative="All statement execution in SQLAlchemy 2.0 is performed "
+        "by the :meth:`_engine.Connection.execute` method of "
+        ":class:`_engine.Connection`, "
+        "or in the ORM by the :meth:`.Session.execute` method of "
+        ":class:`.Session`.",
+    )
+    def execute(self, bind=None, target=None):
+        """Execute this DDL immediately.
+
+        Executes the DDL statement in isolation using the supplied
+        :class:`.Connectable` or
+        :class:`.Connectable` assigned to the ``.bind``
+        property, if not supplied. If the DDL has a conditional ``on``
+        criteria, it will be invoked with None as the event.
+
+        :param bind:
+          Optional, an ``Engine`` or ``Connection``. If not supplied, a valid
+          :class:`.Connectable` must be present in the
+          ``.bind`` property.
+
+        :param target:
+          Optional, defaults to None.  The target :class:`_schema.SchemaItem`
+          for the execute call.   This is equivalent to passing the
+          :class:`_schema.SchemaItem` to the :meth:`.DDLElement.against`
+          method and then invoking :meth:`_schema.DDLElement.execute`
+          upon the resulting :class:`_schema.DDLElement` object.  See
+          :meth:`.DDLElement.against` for further detail.
+
+        """
+
+        if bind is None:
+            bind = _bind_or_error(self)
+
+        if self._should_execute(target, bind):
+            return bind.execute(self.against(target))
+        else:
+            bind.engine.logger.info("DDL execution skipped, criteria not met.")
+
+    @_generative
+    def against(self, target):
+        """Return a copy of this :class:`_schema.DDLElement` which will include
+        the given target.
+
+        This essentially applies the given item to the ``.target`` attribute
+        of the returned :class:`_schema.DDLElement` object.  This target
+        is then usable by event handlers and compilation routines in order to
+        provide services such as tokenization of a DDL string in terms of a
+        particular :class:`_schema.Table`.
+
+        When a :class:`_schema.DDLElement` object is established as an event
+        handler for the :meth:`_events.DDLEvents.before_create` or
+        :meth:`_events.DDLEvents.after_create` events, and the event
+        then occurs for a given target such as a :class:`_schema.Constraint`
+        or :class:`_schema.Table`, that target is established with a copy
+        of the :class:`_schema.DDLElement` object using this method, which
+        then proceeds to the :meth:`_schema.DDLElement.execute` method
+        in order to invoke the actual DDL instruction.
+
+        :param target: a :class:`_schema.SchemaItem` that will be the subject
+         of a DDL operation.
+
+        :return: a copy of this :class:`_schema.DDLElement` with the
+         ``.target`` attribute assigned to the given
+         :class:`_schema.SchemaItem`.
+
+        .. seealso::
+
+            :class:`_schema.DDL` - uses tokenization against the "target" when
+            processing the DDL string.
+
+        """
+
+        self.target = target
+
+    @_generative
+    def execute_if(self, dialect=None, callable_=None, state=None):
+        r"""Return a callable that will execute this
+        :class:`_ddl.DDLElement` conditionally within an event handler.
+
+        Used to provide a wrapper for event listening::
+
+            event.listen(
+                        metadata,
+                        'before_create',
+                        DDL("my_ddl").execute_if(dialect='postgresql')
+                    )
+
+        :param dialect: May be a string or tuple of strings.
+          If a string, it will be compared to the name of the
+          executing database dialect::
+
+            DDL('something').execute_if(dialect='postgresql')
+
+          If a tuple, specifies multiple dialect names::
+
+            DDL('something').execute_if(dialect=('postgresql', 'mysql'))
+
+        :param callable\_: A callable, which will be invoked with
+          four positional arguments as well as optional keyword
+          arguments:
+
+            :ddl:
+              This DDL element.
+
+            :target:
+              The :class:`_schema.Table` or :class:`_schema.MetaData`
+              object which is the
+              target of this event. May be None if the DDL is executed
+              explicitly.
+
+            :bind:
+              The :class:`_engine.Connection` being used for DDL execution
+
+            :tables:
+              Optional keyword argument - a list of Table objects which are to
+              be created/ dropped within a MetaData.create_all() or drop_all()
+              method call.
+
+            :state:
+              Optional keyword argument - will be the ``state`` argument
+              passed to this function.
+
+            :checkfirst:
+             Keyword argument, will be True if the 'checkfirst' flag was
+             set during the call to ``create()``, ``create_all()``,
+             ``drop()``, ``drop_all()``.
+
+          If the callable returns a True value, the DDL statement will be
+          executed.
+
+        :param state: any value which will be passed to the callable\_
+          as the ``state`` keyword argument.
+
+        .. seealso::
+
+            :class:`.DDLEvents`
+
+            :ref:`event_toplevel`
+
+        """
+        self.dialect = dialect
+        self.callable_ = callable_
+        self.state = state
+
+    def _should_execute(self, target, bind, **kw):
+        if isinstance(self.dialect, util.string_types):
+            if self.dialect != bind.engine.name:
+                return False
+        elif isinstance(self.dialect, (tuple, list, set)):
+            if bind.engine.name not in self.dialect:
+                return False
+        if self.callable_ is not None and not self.callable_(
+            self, target, bind, state=self.state, **kw
+        ):
+            return False
+
+        return True
+
+    def __call__(self, target, bind, **kw):
+        """Execute the DDL as a ddl_listener."""
+
+        if self._should_execute(target, bind, **kw):
+            return bind.execute(self.against(target))
+
+    def bind(self):
+        if self._bind:
+            return self._bind
+
+    def _set_bind(self, bind):
+        self._bind = bind
+
+    bind = property(bind, _set_bind)
+
+    def _generate(self):
+        s = self.__class__.__new__(self.__class__)
+        s.__dict__ = self.__dict__.copy()
+        return s
+
+
+class DDL(DDLElement):
+    """A literal DDL statement.
+
+    Specifies literal SQL DDL to be executed by the database.  DDL objects
+    function as DDL event listeners, and can be subscribed to those events
+    listed in :class:`.DDLEvents`, using either :class:`_schema.Table` or
+    :class:`_schema.MetaData` objects as targets.
+    Basic templating support allows
+    a single DDL instance to handle repetitive tasks for multiple tables.
+
+    Examples::
+
+      from sqlalchemy import event, DDL
+
+      tbl = Table('users', metadata, Column('uid', Integer))
+      event.listen(tbl, 'before_create', DDL('DROP TRIGGER users_trigger'))
+
+      spow = DDL('ALTER TABLE %(table)s SET secretpowers TRUE')
+      event.listen(tbl, 'after_create', spow.execute_if(dialect='somedb'))
+
+      drop_spow = DDL('ALTER TABLE users SET secretpowers FALSE')
+      connection.execute(drop_spow)
+
+    When operating on Table events, the following ``statement``
+    string substitutions are available::
+
+      %(table)s  - the Table name, with any required quoting applied
+      %(schema)s - the schema name, with any required quoting applied
+      %(fullname)s - the Table name including schema, quoted if needed
+
+    The DDL's "context", if any, will be combined with the standard
+    substitutions noted above.  Keys present in the context will override
+    the standard substitutions.
+
+    """
+
+    __visit_name__ = "ddl"
+
+    @util.deprecated_params(
+        bind=(
+            "2.0",
+            "The :paramref:`_ddl.DDL.bind` argument is deprecated and "
+            "will be removed in SQLAlchemy 2.0.",
+        ),
+    )
+    def __init__(self, statement, context=None, bind=None):
+        """Create a DDL statement.
+
+        :param statement:
+          A string or unicode string to be executed.  Statements will be
+          processed with Python's string formatting operator using
+          a fixed set of string substitutions, as well as additional
+          substitutions provided by the optional :paramref:`.DDL.context`
+          parameter.
+
+          A literal '%' in a statement must be escaped as '%%'.
+
+          SQL bind parameters are not available in DDL statements.
+
+        :param context:
+          Optional dictionary, defaults to None.  These values will be
+          available for use in string substitutions on the DDL statement.
+
+        :param bind:
+          Optional. A :class:`.Connectable`, used by
+          default when ``execute()`` is invoked without a bind argument.
+
+
+        .. seealso::
+
+            :class:`.DDLEvents`
+
+            :ref:`event_toplevel`
+
+        """
+
+        if not isinstance(statement, util.string_types):
+            raise exc.ArgumentError(
+                "Expected a string or unicode SQL statement, got '%r'"
+                % statement
+            )
+
+        self.statement = statement
+        self.context = context or {}
+
+        self._bind = bind
+
+    def __repr__(self):
+        return "<%s@%s; %s>" % (
+            type(self).__name__,
+            id(self),
+            ", ".join(
+                [repr(self.statement)]
+                + [
+                    "%s=%r" % (key, getattr(self, key))
+                    for key in ("on", "context")
+                    if getattr(self, key)
+                ]
+            ),
+        )
+
+
+class _CreateDropBase(DDLElement):
+    """Base class for DDL constructs that represent CREATE and DROP or
+    equivalents.
+
+    The common theme of _CreateDropBase is a single
+    ``element`` attribute which refers to the element
+    to be created or dropped.
+
+    """
+
+    @util.deprecated_params(
+        bind=(
+            "2.0",
+            "The :paramref:`_ddl.DDLElement.bind` argument is "
+            "deprecated and "
+            "will be removed in SQLAlchemy 2.0.",
+        ),
+    )
+    def __init__(
+        self,
+        element,
+        bind=None,
+        if_exists=False,
+        if_not_exists=False,
+        _legacy_bind=None,
+    ):
+        self.element = element
+        if bind:
+            self.bind = bind
+        elif _legacy_bind:
+            self.bind = _legacy_bind
+        self.if_exists = if_exists
+        self.if_not_exists = if_not_exists
+
+    @property
+    def stringify_dialect(self):
+        return self.element.create_drop_stringify_dialect
+
+    def _create_rule_disable(self, compiler):
+        """Allow disable of _create_rule using a callable.
+
+        Pass to _create_rule using
+        util.portable_instancemethod(self._create_rule_disable)
+        to retain serializability.
+
+        """
+        return False
+
+
+class CreateSchema(_CreateDropBase):
+    """Represent a CREATE SCHEMA statement.
+
+    The argument here is the string name of the schema.
+
+    """
+
+    __visit_name__ = "create_schema"
+
+    def __init__(self, name, quote=None, **kw):
+        """Create a new :class:`.CreateSchema` construct."""
+
+        self.quote = quote
+        super(CreateSchema, self).__init__(name, **kw)
+
+
+class DropSchema(_CreateDropBase):
+    """Represent a DROP SCHEMA statement.
+
+    The argument here is the string name of the schema.
+
+    """
+
+    __visit_name__ = "drop_schema"
+
+    def __init__(self, name, quote=None, cascade=False, **kw):
+        """Create a new :class:`.DropSchema` construct."""
+
+        self.quote = quote
+        self.cascade = cascade
+        super(DropSchema, self).__init__(name, **kw)
+
+
+class CreateTable(_CreateDropBase):
+    """Represent a CREATE TABLE statement."""
+
+    __visit_name__ = "create_table"
+
+    @util.deprecated_params(
+        bind=(
+            "2.0",
+            "The :paramref:`_ddl.CreateTable.bind` argument is deprecated and "
+            "will be removed in SQLAlchemy 2.0.",
+        ),
+    )
+    def __init__(
+        self,
+        element,
+        bind=None,
+        include_foreign_key_constraints=None,
+        if_not_exists=False,
+    ):
+        """Create a :class:`.CreateTable` construct.
+
+        :param element: a :class:`_schema.Table` that's the subject
+         of the CREATE
+        :param on: See the description for 'on' in :class:`.DDL`.
+        :param bind: See the description for 'bind' in :class:`.DDL`.
+        :param include_foreign_key_constraints: optional sequence of
+         :class:`_schema.ForeignKeyConstraint` objects that will be included
+         inline within the CREATE construct; if omitted, all foreign key
+         constraints that do not specify use_alter=True are included.
+
+         .. versionadded:: 1.0.0
+
+        :param if_not_exists: if True, an IF NOT EXISTS operator will be
+         applied to the construct.
+
+         .. versionadded:: 1.4.0b2
+
+        """
+        super(CreateTable, self).__init__(
+            element, _legacy_bind=bind, if_not_exists=if_not_exists
+        )
+        self.columns = [CreateColumn(column) for column in element.columns]
+        self.include_foreign_key_constraints = include_foreign_key_constraints
+
+
+class _DropView(_CreateDropBase):
+    """Semi-public 'DROP VIEW' construct.
+
+    Used by the test suite for dialect-agnostic drops of views.
+    This object will eventually be part of a public "view" API.
+
+    """
+
+    __visit_name__ = "drop_view"
+
+
+class CreateColumn(_DDLCompiles):
+    """Represent a :class:`_schema.Column`
+    as rendered in a CREATE TABLE statement,
+    via the :class:`.CreateTable` construct.
+
+    This is provided to support custom column DDL within the generation
+    of CREATE TABLE statements, by using the
+    compiler extension documented in :ref:`sqlalchemy.ext.compiler_toplevel`
+    to extend :class:`.CreateColumn`.
+
+    Typical integration is to examine the incoming :class:`_schema.Column`
+    object, and to redirect compilation if a particular flag or condition
+    is found::
+
+        from sqlalchemy import schema
+        from sqlalchemy.ext.compiler import compiles
+
+        @compiles(schema.CreateColumn)
+        def compile(element, compiler, **kw):
+            column = element.element
+
+            if "special" not in column.info:
+                return compiler.visit_create_column(element, **kw)
+
+            text = "%s SPECIAL DIRECTIVE %s" % (
+                    column.name,
+                    compiler.type_compiler.process(column.type)
+                )
+            default = compiler.get_column_default_string(column)
+            if default is not None:
+                text += " DEFAULT " + default
+
+            if not column.nullable:
+                text += " NOT NULL"
+
+            if column.constraints:
+                text += " ".join(
+                            compiler.process(const)
+                            for const in column.constraints)
+            return text
+
+    The above construct can be applied to a :class:`_schema.Table`
+    as follows::
+
+        from sqlalchemy import Table, Metadata, Column, Integer, String
+        from sqlalchemy import schema
+
+        metadata = MetaData()
+
+        table = Table('mytable', MetaData(),
+                Column('x', Integer, info={"special":True}, primary_key=True),
+                Column('y', String(50)),
+                Column('z', String(20), info={"special":True})
+            )
+
+        metadata.create_all(conn)
+
+    Above, the directives we've added to the :attr:`_schema.Column.info`
+    collection
+    will be detected by our custom compilation scheme::
+
+        CREATE TABLE mytable (
+                x SPECIAL DIRECTIVE INTEGER NOT NULL,
+                y VARCHAR(50),
+                z SPECIAL DIRECTIVE VARCHAR(20),
+            PRIMARY KEY (x)
+        )
+
+    The :class:`.CreateColumn` construct can also be used to skip certain
+    columns when producing a ``CREATE TABLE``.  This is accomplished by
+    creating a compilation rule that conditionally returns ``None``.
+    This is essentially how to produce the same effect as using the
+    ``system=True`` argument on :class:`_schema.Column`, which marks a column
+    as an implicitly-present "system" column.
+
+    For example, suppose we wish to produce a :class:`_schema.Table`
+    which skips
+    rendering of the PostgreSQL ``xmin`` column against the PostgreSQL
+    backend, but on other backends does render it, in anticipation of a
+    triggered rule.  A conditional compilation rule could skip this name only
+    on PostgreSQL::
+
+        from sqlalchemy.schema import CreateColumn
+
+        @compiles(CreateColumn, "postgresql")
+        def skip_xmin(element, compiler, **kw):
+            if element.element.name == 'xmin':
+                return None
+            else:
+                return compiler.visit_create_column(element, **kw)
+
+
+        my_table = Table('mytable', metadata,
+                    Column('id', Integer, primary_key=True),
+                    Column('xmin', Integer)
+                )
+
+    Above, a :class:`.CreateTable` construct will generate a ``CREATE TABLE``
+    which only includes the ``id`` column in the string; the ``xmin`` column
+    will be omitted, but only against the PostgreSQL backend.
+
+    """
+
+    __visit_name__ = "create_column"
+
+    def __init__(self, element):
+        self.element = element
+
+
+class DropTable(_CreateDropBase):
+    """Represent a DROP TABLE statement."""
+
+    __visit_name__ = "drop_table"
+
+    @util.deprecated_params(
+        bind=(
+            "2.0",
+            "The :paramref:`_ddl.DropTable.bind` argument is "
+            "deprecated and "
+            "will be removed in SQLAlchemy 2.0.",
+        ),
+    )
+    def __init__(self, element, bind=None, if_exists=False):
+        """Create a :class:`.DropTable` construct.
+
+        :param element: a :class:`_schema.Table` that's the subject
+         of the DROP.
+        :param on: See the description for 'on' in :class:`.DDL`.
+        :param bind: See the description for 'bind' in :class:`.DDL`.
+        :param if_exists: if True, an IF EXISTS operator will be applied to the
+         construct.
+
+         .. versionadded:: 1.4.0b2
+
+        """
+        super(DropTable, self).__init__(
+            element, _legacy_bind=bind, if_exists=if_exists
+        )
+
+
+class CreateSequence(_CreateDropBase):
+    """Represent a CREATE SEQUENCE statement."""
+
+    __visit_name__ = "create_sequence"
+
+
+class DropSequence(_CreateDropBase):
+    """Represent a DROP SEQUENCE statement."""
+
+    __visit_name__ = "drop_sequence"
+
+
+class CreateIndex(_CreateDropBase):
+    """Represent a CREATE INDEX statement."""
+
+    __visit_name__ = "create_index"
+
+    @util.deprecated_params(
+        bind=(
+            "2.0",
+            "The :paramref:`_ddl.CreateIndex.bind` argument is "
+            "deprecated and "
+            "will be removed in SQLAlchemy 2.0.",
+        ),
+    )
+    def __init__(self, element, bind=None, if_not_exists=False):
+        """Create a :class:`.Createindex` construct.
+
+        :param element: a :class:`_schema.Index` that's the subject
+         of the CREATE.
+        :param on: See the description for 'on' in :class:`.DDL`.
+        :param bind: See the description for 'bind' in :class:`.DDL`.
+        :param if_not_exists: if True, an IF NOT EXISTS operator will be
+         applied to the construct.
+
+         .. versionadded:: 1.4.0b2
+
+        """
+        super(CreateIndex, self).__init__(
+            element, _legacy_bind=bind, if_not_exists=if_not_exists
+        )
+
+
+class DropIndex(_CreateDropBase):
+    """Represent a DROP INDEX statement."""
+
+    __visit_name__ = "drop_index"
+
+    @util.deprecated_params(
+        bind=(
+            "2.0",
+            "The :paramref:`_ddl.DropIndex.bind` argument is "
+            "deprecated and "
+            "will be removed in SQLAlchemy 2.0.",
+        ),
+    )
+    def __init__(self, element, bind=None, if_exists=False):
+        """Create a :class:`.DropIndex` construct.
+
+        :param element: a :class:`_schema.Index` that's the subject
+         of the DROP.
+        :param on: See the description for 'on' in :class:`.DDL`.
+        :param bind: See the description for 'bind' in :class:`.DDL`.
+        :param if_exists: if True, an IF EXISTS operator will be applied to the
+         construct.
+
+         .. versionadded:: 1.4.0b2
+
+        """
+        super(DropIndex, self).__init__(
+            element, _legacy_bind=bind, if_exists=if_exists
+        )
+
+
+class AddConstraint(_CreateDropBase):
+    """Represent an ALTER TABLE ADD CONSTRAINT statement."""
+
+    __visit_name__ = "add_constraint"
+
+    def __init__(self, element, *args, **kw):
+        super(AddConstraint, self).__init__(element, *args, **kw)
+        element._create_rule = util.portable_instancemethod(
+            self._create_rule_disable
+        )
+
+
+class DropConstraint(_CreateDropBase):
+    """Represent an ALTER TABLE DROP CONSTRAINT statement."""
+
+    __visit_name__ = "drop_constraint"
+
+    def __init__(self, element, cascade=False, **kw):
+        self.cascade = cascade
+        super(DropConstraint, self).__init__(element, **kw)
+        element._create_rule = util.portable_instancemethod(
+            self._create_rule_disable
+        )
+
+
+class SetTableComment(_CreateDropBase):
+    """Represent a COMMENT ON TABLE IS statement."""
+
+    __visit_name__ = "set_table_comment"
+
+
+class DropTableComment(_CreateDropBase):
+    """Represent a COMMENT ON TABLE '' statement.
+
+    Note this varies a lot across database backends.
+
+    """
+
+    __visit_name__ = "drop_table_comment"
+
+
+class SetColumnComment(_CreateDropBase):
+    """Represent a COMMENT ON COLUMN IS statement."""
+
+    __visit_name__ = "set_column_comment"
+
+
+class DropColumnComment(_CreateDropBase):
+    """Represent a COMMENT ON COLUMN IS NULL statement."""
+
+    __visit_name__ = "drop_column_comment"
+
+
+class DDLBase(SchemaVisitor):
+    def __init__(self, connection):
+        self.connection = connection
+
+
+class SchemaGenerator(DDLBase):
+    def __init__(
+        self, dialect, connection, checkfirst=False, tables=None, **kwargs
+    ):
+        super(SchemaGenerator, self).__init__(connection, **kwargs)
+        self.checkfirst = checkfirst
+        self.tables = tables
+        self.preparer = dialect.identifier_preparer
+        self.dialect = dialect
+        self.memo = {}
+
+    def _can_create_table(self, table):
+        self.dialect.validate_identifier(table.name)
+        effective_schema = self.connection.schema_for_object(table)
+        if effective_schema:
+            self.dialect.validate_identifier(effective_schema)
+        return not self.checkfirst or not self.dialect.has_table(
+            self.connection, table.name, schema=effective_schema
+        )
+
+    def _can_create_index(self, index):
+        effective_schema = self.connection.schema_for_object(index.table)
+        if effective_schema:
+            self.dialect.validate_identifier(effective_schema)
+        return not self.checkfirst or not self.dialect.has_index(
+            self.connection,
+            index.table.name,
+            index.name,
+            schema=effective_schema,
+        )
+
+    def _can_create_sequence(self, sequence):
+        effective_schema = self.connection.schema_for_object(sequence)
+
+        return self.dialect.supports_sequences and (
+            (not self.dialect.sequences_optional or not sequence.optional)
+            and (
+                not self.checkfirst
+                or not self.dialect.has_sequence(
+                    self.connection, sequence.name, schema=effective_schema
+                )
+            )
+        )
+
+    def visit_metadata(self, metadata):
+        if self.tables is not None:
+            tables = self.tables
+        else:
+            tables = list(metadata.tables.values())
+
+        collection = sort_tables_and_constraints(
+            [t for t in tables if self._can_create_table(t)]
+        )
+
+        seq_coll = [
+            s
+            for s in metadata._sequences.values()
+            if s.column is None and self._can_create_sequence(s)
+        ]
+
+        event_collection = [t for (t, fks) in collection if t is not None]
+        metadata.dispatch.before_create(
+            metadata,
+            self.connection,
+            tables=event_collection,
+            checkfirst=self.checkfirst,
+            _ddl_runner=self,
+        )
+
+        for seq in seq_coll:
+            self.traverse_single(seq, create_ok=True)
+
+        for table, fkcs in collection:
+            if table is not None:
+                self.traverse_single(
+                    table,
+                    create_ok=True,
+                    include_foreign_key_constraints=fkcs,
+                    _is_metadata_operation=True,
+                )
+            else:
+                for fkc in fkcs:
+                    self.traverse_single(fkc)
+
+        metadata.dispatch.after_create(
+            metadata,
+            self.connection,
+            tables=event_collection,
+            checkfirst=self.checkfirst,
+            _ddl_runner=self,
+        )
+
+    def visit_table(
+        self,
+        table,
+        create_ok=False,
+        include_foreign_key_constraints=None,
+        _is_metadata_operation=False,
+    ):
+        if not create_ok and not self._can_create_table(table):
+            return
+
+        table.dispatch.before_create(
+            table,
+            self.connection,
+            checkfirst=self.checkfirst,
+            _ddl_runner=self,
+            _is_metadata_operation=_is_metadata_operation,
+        )
+
+        for column in table.columns:
+            if column.default is not None:
+                self.traverse_single(column.default)
+
+        if not self.dialect.supports_alter:
+            # e.g., don't omit any foreign key constraints
+            include_foreign_key_constraints = None
+
+        self.connection.execute(
+            # fmt: off
+            CreateTable(
+                table,
+                include_foreign_key_constraints=  # noqa
+                    include_foreign_key_constraints,  # noqa
+            )
+            # fmt: on
+        )
+
+        if hasattr(table, "indexes"):
+            for index in table.indexes:
+                self.traverse_single(index, create_ok=True)
+
+        if self.dialect.supports_comments and not self.dialect.inline_comments:
+            if table.comment is not None:
+                self.connection.execute(SetTableComment(table))
+
+            for column in table.columns:
+                if column.comment is not None:
+                    self.connection.execute(SetColumnComment(column))
+
+        table.dispatch.after_create(
+            table,
+            self.connection,
+            checkfirst=self.checkfirst,
+            _ddl_runner=self,
+            _is_metadata_operation=_is_metadata_operation,
+        )
+
+    def visit_foreign_key_constraint(self, constraint):
+        if not self.dialect.supports_alter:
+            return
+        self.connection.execute(AddConstraint(constraint))
+
+    def visit_sequence(self, sequence, create_ok=False):
+        if not create_ok and not self._can_create_sequence(sequence):
+            return
+        self.connection.execute(CreateSequence(sequence))
+
+    def visit_index(self, index, create_ok=False):
+        if not create_ok and not self._can_create_index(index):
+            return
+        self.connection.execute(CreateIndex(index))
+
+
+class SchemaDropper(DDLBase):
+    def __init__(
+        self, dialect, connection, checkfirst=False, tables=None, **kwargs
+    ):
+        super(SchemaDropper, self).__init__(connection, **kwargs)
+        self.checkfirst = checkfirst
+        self.tables = tables
+        self.preparer = dialect.identifier_preparer
+        self.dialect = dialect
+        self.memo = {}
+
+    def visit_metadata(self, metadata):
+        if self.tables is not None:
+            tables = self.tables
+        else:
+            tables = list(metadata.tables.values())
+
+        try:
+            unsorted_tables = [t for t in tables if self._can_drop_table(t)]
+            collection = list(
+                reversed(
+                    sort_tables_and_constraints(
+                        unsorted_tables,
+                        filter_fn=lambda constraint: False
+                        if not self.dialect.supports_alter
+                        or constraint.name is None
+                        else None,
+                    )
+                )
+            )
+        except exc.CircularDependencyError as err2:
+            if not self.dialect.supports_alter:
+                util.warn(
+                    "Can't sort tables for DROP; an "
+                    "unresolvable foreign key "
+                    "dependency exists between tables: %s; and backend does "
+                    "not support ALTER.  To restore at least a partial sort, "
+                    "apply use_alter=True to ForeignKey and "
+                    "ForeignKeyConstraint "
+                    "objects involved in the cycle to mark these as known "
+                    "cycles that will be ignored."
+                    % (", ".join(sorted([t.fullname for t in err2.cycles])))
+                )
+                collection = [(t, ()) for t in unsorted_tables]
+            else:
+                util.raise_(
+                    exc.CircularDependencyError(
+                        err2.args[0],
+                        err2.cycles,
+                        err2.edges,
+                        msg="Can't sort tables for DROP; an "
+                        "unresolvable foreign key "
+                        "dependency exists between tables: %s.  Please ensure "
+                        "that the ForeignKey and ForeignKeyConstraint objects "
+                        "involved in the cycle have "
+                        "names so that they can be dropped using "
+                        "DROP CONSTRAINT."
+                        % (
+                            ", ".join(
+                                sorted([t.fullname for t in err2.cycles])
+                            )
+                        ),
+                    ),
+                    from_=err2,
+                )
+
+        seq_coll = [
+            s
+            for s in metadata._sequences.values()
+            if self._can_drop_sequence(s)
+        ]
+
+        event_collection = [t for (t, fks) in collection if t is not None]
+
+        metadata.dispatch.before_drop(
+            metadata,
+            self.connection,
+            tables=event_collection,
+            checkfirst=self.checkfirst,
+            _ddl_runner=self,
+        )
+
+        for table, fkcs in collection:
+            if table is not None:
+                self.traverse_single(
+                    table,
+                    drop_ok=True,
+                    _is_metadata_operation=True,
+                    _ignore_sequences=seq_coll,
+                )
+            else:
+                for fkc in fkcs:
+                    self.traverse_single(fkc)
+
+        for seq in seq_coll:
+            self.traverse_single(seq, drop_ok=seq.column is None)
+
+        metadata.dispatch.after_drop(
+            metadata,
+            self.connection,
+            tables=event_collection,
+            checkfirst=self.checkfirst,
+            _ddl_runner=self,
+        )
+
+    def _can_drop_table(self, table):
+        self.dialect.validate_identifier(table.name)
+        effective_schema = self.connection.schema_for_object(table)
+        if effective_schema:
+            self.dialect.validate_identifier(effective_schema)
+        return not self.checkfirst or self.dialect.has_table(
+            self.connection, table.name, schema=effective_schema
+        )
+
+    def _can_drop_index(self, index):
+        effective_schema = self.connection.schema_for_object(index.table)
+        if effective_schema:
+            self.dialect.validate_identifier(effective_schema)
+        return not self.checkfirst or self.dialect.has_index(
+            self.connection,
+            index.table.name,
+            index.name,
+            schema=effective_schema,
+        )
+
+    def _can_drop_sequence(self, sequence):
+        effective_schema = self.connection.schema_for_object(sequence)
+        return self.dialect.supports_sequences and (
+            (not self.dialect.sequences_optional or not sequence.optional)
+            and (
+                not self.checkfirst
+                or self.dialect.has_sequence(
+                    self.connection, sequence.name, schema=effective_schema
+                )
+            )
+        )
+
+    def visit_index(self, index, drop_ok=False):
+        if not drop_ok and not self._can_drop_index(index):
+            return
+
+        self.connection.execute(DropIndex(index))
+
+    def visit_table(
+        self,
+        table,
+        drop_ok=False,
+        _is_metadata_operation=False,
+        _ignore_sequences=(),
+    ):
+        if not drop_ok and not self._can_drop_table(table):
+            return
+
+        table.dispatch.before_drop(
+            table,
+            self.connection,
+            checkfirst=self.checkfirst,
+            _ddl_runner=self,
+            _is_metadata_operation=_is_metadata_operation,
+        )
+
+        self.connection.execute(DropTable(table))
+
+        # traverse client side defaults which may refer to server-side
+        # sequences. noting that some of these client side defaults may also be
+        # set up as server side defaults (see https://docs.sqlalchemy.org/en/
+        # latest/core/defaults.html#associating-a-sequence-as-the-server-side-
+        # default), so have to be dropped after the table is dropped.
+        for column in table.columns:
+            if (
+                column.default is not None
+                and column.default not in _ignore_sequences
+            ):
+                self.traverse_single(column.default)
+
+        table.dispatch.after_drop(
+            table,
+            self.connection,
+            checkfirst=self.checkfirst,
+            _ddl_runner=self,
+            _is_metadata_operation=_is_metadata_operation,
+        )
+
+    def visit_foreign_key_constraint(self, constraint):
+        if not self.dialect.supports_alter:
+            return
+        self.connection.execute(DropConstraint(constraint))
+
+    def visit_sequence(self, sequence, drop_ok=False):
+
+        if not drop_ok and not self._can_drop_sequence(sequence):
+            return
+        self.connection.execute(DropSequence(sequence))
+
+
+def sort_tables(
+    tables,
+    skip_fn=None,
+    extra_dependencies=None,
+):
+    """Sort a collection of :class:`_schema.Table` objects based on
+    dependency.
+
+    This is a dependency-ordered sort which will emit :class:`_schema.Table`
+    objects such that they will follow their dependent :class:`_schema.Table`
+    objects.
+    Tables are dependent on another based on the presence of
+    :class:`_schema.ForeignKeyConstraint`
+    objects as well as explicit dependencies
+    added by :meth:`_schema.Table.add_is_dependent_on`.
+
+    .. warning::
+
+        The :func:`._schema.sort_tables` function cannot by itself
+        accommodate automatic resolution of dependency cycles between
+        tables, which are usually caused by mutually dependent foreign key
+        constraints. When these cycles are detected, the foreign keys
+        of these tables are omitted from consideration in the sort.
+        A warning is emitted when this condition occurs, which will be an
+        exception raise in a future release.   Tables which are not part
+        of the cycle will still be returned in dependency order.
+
+        To resolve these cycles, the
+        :paramref:`_schema.ForeignKeyConstraint.use_alter` parameter may be
+        applied to those constraints which create a cycle.  Alternatively,
+        the :func:`_schema.sort_tables_and_constraints` function will
+        automatically return foreign key constraints in a separate
+        collection when cycles are detected so that they may be applied
+        to a schema separately.
+
+        .. versionchanged:: 1.3.17 - a warning is emitted when
+           :func:`_schema.sort_tables` cannot perform a proper sort due to
+           cyclical dependencies.  This will be an exception in a future
+           release.  Additionally, the sort will continue to return
+           other tables not involved in the cycle in dependency order
+           which was not the case previously.
+
+    :param tables: a sequence of :class:`_schema.Table` objects.
+
+    :param skip_fn: optional callable which will be passed a
+     :class:`_schema.ForeignKey` object; if it returns True, this
+     constraint will not be considered as a dependency.  Note this is
+     **different** from the same parameter in
+     :func:`.sort_tables_and_constraints`, which is
+     instead passed the owning :class:`_schema.ForeignKeyConstraint` object.
+
+    :param extra_dependencies: a sequence of 2-tuples of tables which will
+     also be considered as dependent on each other.
+
+    .. seealso::
+
+        :func:`.sort_tables_and_constraints`
+
+        :attr:`_schema.MetaData.sorted_tables` - uses this function to sort
+
+
+    """
+
+    if skip_fn is not None:
+
+        def _skip_fn(fkc):
+            for fk in fkc.elements:
+                if skip_fn(fk):
+                    return True
+            else:
+                return None
+
+    else:
+        _skip_fn = None
+
+    return [
+        t
+        for (t, fkcs) in sort_tables_and_constraints(
+            tables,
+            filter_fn=_skip_fn,
+            extra_dependencies=extra_dependencies,
+            _warn_for_cycles=True,
+        )
+        if t is not None
+    ]
+
+
+def sort_tables_and_constraints(
+    tables, filter_fn=None, extra_dependencies=None, _warn_for_cycles=False
+):
+    """Sort a collection of :class:`_schema.Table`  /
+    :class:`_schema.ForeignKeyConstraint`
+    objects.
+
+    This is a dependency-ordered sort which will emit tuples of
+    ``(Table, [ForeignKeyConstraint, ...])`` such that each
+    :class:`_schema.Table` follows its dependent :class:`_schema.Table`
+    objects.
+    Remaining :class:`_schema.ForeignKeyConstraint`
+    objects that are separate due to
+    dependency rules not satisfied by the sort are emitted afterwards
+    as ``(None, [ForeignKeyConstraint ...])``.
+
+    Tables are dependent on another based on the presence of
+    :class:`_schema.ForeignKeyConstraint` objects, explicit dependencies
+    added by :meth:`_schema.Table.add_is_dependent_on`,
+    as well as dependencies
+    stated here using the :paramref:`~.sort_tables_and_constraints.skip_fn`
+    and/or :paramref:`~.sort_tables_and_constraints.extra_dependencies`
+    parameters.
+
+    :param tables: a sequence of :class:`_schema.Table` objects.
+
+    :param filter_fn: optional callable which will be passed a
+     :class:`_schema.ForeignKeyConstraint` object,
+     and returns a value based on
+     whether this constraint should definitely be included or excluded as
+     an inline constraint, or neither.   If it returns False, the constraint
+     will definitely be included as a dependency that cannot be subject
+     to ALTER; if True, it will **only** be included as an ALTER result at
+     the end.   Returning None means the constraint is included in the
+     table-based result unless it is detected as part of a dependency cycle.
+
+    :param extra_dependencies: a sequence of 2-tuples of tables which will
+     also be considered as dependent on each other.
+
+    .. versionadded:: 1.0.0
+
+    .. seealso::
+
+        :func:`.sort_tables`
+
+
+    """
+
+    fixed_dependencies = set()
+    mutable_dependencies = set()
+
+    if extra_dependencies is not None:
+        fixed_dependencies.update(extra_dependencies)
+
+    remaining_fkcs = set()
+    for table in tables:
+        for fkc in table.foreign_key_constraints:
+            if fkc.use_alter is True:
+                remaining_fkcs.add(fkc)
+                continue
+
+            if filter_fn:
+                filtered = filter_fn(fkc)
+
+                if filtered is True:
+                    remaining_fkcs.add(fkc)
+                    continue
+
+            dependent_on = fkc.referred_table
+            if dependent_on is not table:
+                mutable_dependencies.add((dependent_on, table))
+
+        fixed_dependencies.update(
+            (parent, table) for parent in table._extra_dependencies
+        )
+
+    try:
+        candidate_sort = list(
+            topological.sort(
+                fixed_dependencies.union(mutable_dependencies),
+                tables,
+            )
+        )
+    except exc.CircularDependencyError as err:
+        if _warn_for_cycles:
+            util.warn(
+                "Cannot correctly sort tables; there are unresolvable cycles "
+                'between tables "%s", which is usually caused by mutually '
+                "dependent foreign key constraints.  Foreign key constraints "
+                "involving these tables will not be considered; this warning "
+                "may raise an error in a future release."
+                % (", ".join(sorted(t.fullname for t in err.cycles)),)
+            )
+        for edge in err.edges:
+            if edge in mutable_dependencies:
+                table = edge[1]
+                if table not in err.cycles:
+                    continue
+                can_remove = [
+                    fkc
+                    for fkc in table.foreign_key_constraints
+                    if filter_fn is None or filter_fn(fkc) is not False
+                ]
+                remaining_fkcs.update(can_remove)
+                for fkc in can_remove:
+                    dependent_on = fkc.referred_table
+                    if dependent_on is not table:
+                        mutable_dependencies.discard((dependent_on, table))
+        candidate_sort = list(
+            topological.sort(
+                fixed_dependencies.union(mutable_dependencies),
+                tables,
+            )
+        )
+
+    return [
+        (table, table.foreign_key_constraints.difference(remaining_fkcs))
+        for table in candidate_sort
+    ] + [(None, list(remaining_fkcs))]