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+# postgresql/ext.py
+# Copyright (C) 2005-2022 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
+
+from .array import ARRAY
+from ... import util
+from ...sql import coercions
+from ...sql import elements
+from ...sql import expression
+from ...sql import functions
+from ...sql import roles
+from ...sql import schema
+from ...sql.schema import ColumnCollectionConstraint
+
+
+class aggregate_order_by(expression.ColumnElement):
+    """Represent a PostgreSQL aggregate order by expression.
+
+    E.g.::
+
+        from sqlalchemy.dialects.postgresql import aggregate_order_by
+        expr = func.array_agg(aggregate_order_by(table.c.a, table.c.b.desc()))
+        stmt = select(expr)
+
+    would represent the expression::
+
+        SELECT array_agg(a ORDER BY b DESC) FROM table;
+
+    Similarly::
+
+        expr = func.string_agg(
+            table.c.a,
+            aggregate_order_by(literal_column("','"), table.c.a)
+        )
+        stmt = select(expr)
+
+    Would represent::
+
+        SELECT string_agg(a, ',' ORDER BY a) FROM table;
+
+    .. versionadded:: 1.1
+
+    .. versionchanged:: 1.2.13 - the ORDER BY argument may be multiple terms
+
+    .. seealso::
+
+        :class:`_functions.array_agg`
+
+    """
+
+    __visit_name__ = "aggregate_order_by"
+
+    stringify_dialect = "postgresql"
+    inherit_cache = False
+
+    def __init__(self, target, *order_by):
+        self.target = coercions.expect(roles.ExpressionElementRole, target)
+        self.type = self.target.type
+
+        _lob = len(order_by)
+        if _lob == 0:
+            raise TypeError("at least one ORDER BY element is required")
+        elif _lob == 1:
+            self.order_by = coercions.expect(
+                roles.ExpressionElementRole, order_by[0]
+            )
+        else:
+            self.order_by = elements.ClauseList(
+                *order_by, _literal_as_text_role=roles.ExpressionElementRole
+            )
+
+    def self_group(self, against=None):
+        return self
+
+    def get_children(self, **kwargs):
+        return self.target, self.order_by
+
+    def _copy_internals(self, clone=elements._clone, **kw):
+        self.target = clone(self.target, **kw)
+        self.order_by = clone(self.order_by, **kw)
+
+    @property
+    def _from_objects(self):
+        return self.target._from_objects + self.order_by._from_objects
+
+
+class ExcludeConstraint(ColumnCollectionConstraint):
+    """A table-level EXCLUDE constraint.
+
+    Defines an EXCLUDE constraint as described in the `PostgreSQL
+    documentation`__.
+
+    __ https://www.postgresql.org/docs/current/static/sql-createtable.html#SQL-CREATETABLE-EXCLUDE
+
+    """  # noqa
+
+    __visit_name__ = "exclude_constraint"
+
+    where = None
+    inherit_cache = False
+
+    create_drop_stringify_dialect = "postgresql"
+
+    @elements._document_text_coercion(
+        "where",
+        ":class:`.ExcludeConstraint`",
+        ":paramref:`.ExcludeConstraint.where`",
+    )
+    def __init__(self, *elements, **kw):
+        r"""
+        Create an :class:`.ExcludeConstraint` object.
+
+        E.g.::
+
+            const = ExcludeConstraint(
+                (Column('period'), '&&'),
+                (Column('group'), '='),
+                where=(Column('group') != 'some group'),
+                ops={'group': 'my_operator_class'}
+            )
+
+        The constraint is normally embedded into the :class:`_schema.Table`
+        construct
+        directly, or added later using :meth:`.append_constraint`::
+
+            some_table = Table(
+                'some_table', metadata,
+                Column('id', Integer, primary_key=True),
+                Column('period', TSRANGE()),
+                Column('group', String)
+            )
+
+            some_table.append_constraint(
+                ExcludeConstraint(
+                    (some_table.c.period, '&&'),
+                    (some_table.c.group, '='),
+                    where=some_table.c.group != 'some group',
+                    name='some_table_excl_const',
+                    ops={'group': 'my_operator_class'}
+                )
+            )
+
+        :param \*elements:
+
+          A sequence of two tuples of the form ``(column, operator)`` where
+          "column" is a SQL expression element or a raw SQL string, most
+          typically a :class:`_schema.Column` object,
+          and "operator" is a string
+          containing the operator to use.   In order to specify a column name
+          when a  :class:`_schema.Column` object is not available,
+          while ensuring
+          that any necessary quoting rules take effect, an ad-hoc
+          :class:`_schema.Column` or :func:`_expression.column`
+          object should be
+          used.
+
+        :param name:
+          Optional, the in-database name of this constraint.
+
+        :param deferrable:
+          Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
+          issuing DDL for this constraint.
+
+        :param initially:
+          Optional string.  If set, emit INITIALLY <value> when issuing DDL
+          for this constraint.
+
+        :param using:
+          Optional string.  If set, emit USING <index_method> when issuing DDL
+          for this constraint. Defaults to 'gist'.
+
+        :param where:
+          Optional SQL expression construct or literal SQL string.
+          If set, emit WHERE <predicate> when issuing DDL
+          for this constraint.
+
+        :param ops:
+          Optional dictionary.  Used to define operator classes for the
+          elements; works the same way as that of the
+          :ref:`postgresql_ops <postgresql_operator_classes>`
+          parameter specified to the :class:`_schema.Index` construct.
+
+          .. versionadded:: 1.3.21
+
+          .. seealso::
+
+            :ref:`postgresql_operator_classes` - general description of how
+            PostgreSQL operator classes are specified.
+
+        """
+        columns = []
+        render_exprs = []
+        self.operators = {}
+
+        expressions, operators = zip(*elements)
+
+        for (expr, column, strname, add_element), operator in zip(
+            coercions.expect_col_expression_collection(
+                roles.DDLConstraintColumnRole, expressions
+            ),
+            operators,
+        ):
+            if add_element is not None:
+                columns.append(add_element)
+
+            name = column.name if column is not None else strname
+
+            if name is not None:
+                # backwards compat
+                self.operators[name] = operator
+
+            render_exprs.append((expr, name, operator))
+
+        self._render_exprs = render_exprs
+
+        ColumnCollectionConstraint.__init__(
+            self,
+            *columns,
+            name=kw.get("name"),
+            deferrable=kw.get("deferrable"),
+            initially=kw.get("initially")
+        )
+        self.using = kw.get("using", "gist")
+        where = kw.get("where")
+        if where is not None:
+            self.where = coercions.expect(roles.StatementOptionRole, where)
+
+        self.ops = kw.get("ops", {})
+
+    def _set_parent(self, table, **kw):
+        super(ExcludeConstraint, self)._set_parent(table)
+
+        self._render_exprs = [
+            (
+                expr if isinstance(expr, elements.ClauseElement) else colexpr,
+                name,
+                operator,
+            )
+            for (expr, name, operator), colexpr in util.zip_longest(
+                self._render_exprs, self.columns
+            )
+        ]
+
+    def _copy(self, target_table=None, **kw):
+        elements = [
+            (
+                schema._copy_expression(expr, self.parent, target_table),
+                self.operators[expr.name],
+            )
+            for expr in self.columns
+        ]
+        c = self.__class__(
+            *elements,
+            name=self.name,
+            deferrable=self.deferrable,
+            initially=self.initially,
+            where=self.where,
+            using=self.using
+        )
+        c.dispatch._update(self.dispatch)
+        return c
+
+
+def array_agg(*arg, **kw):
+    """PostgreSQL-specific form of :class:`_functions.array_agg`, ensures
+    return type is :class:`_postgresql.ARRAY` and not
+    the plain :class:`_types.ARRAY`, unless an explicit ``type_``
+    is passed.
+
+    .. versionadded:: 1.1
+
+    """
+    kw["_default_array_type"] = ARRAY
+    return functions.func.array_agg(*arg, **kw)