Fixed #32632, Fixed #32657 -- Removed flawed support for Subquery deconstruction.

Subquery deconstruction support required implementing complex and
expensive equality rules for sql.Query objects for little benefit as
the latter cannot themselves be made deconstructible to their reference
to model classes.

Making Expression @deconstructible and not BaseExpression allows
interested parties to conform to the "expression" API even if they are
not deconstructible as it's only a requirement for expressions allowed
in Model fields and meta options (e.g. constraints, indexes).

Thanks Phillip Cutter for the report.

This also fixes a performance regression in bbf141bcdc31f1324048af9233583a523ac54c94.

6 files changed, 31 insertions, 65 deletions

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 528d988e85..0bbf8cfade 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -147,7 +147,6 @@ class Combinable:
         )
 
 
-@deconstructible
 class BaseExpression:
     """Base class for all query expressions."""
 
@@ -389,6 +388,11 @@ class BaseExpression:
             return self.output_field.select_format(compiler, sql, params)
         return sql, params
 
+
+@deconstructible
+class Expression(BaseExpression, Combinable):
+    """An expression that can be combined with other expressions."""
+
     @cached_property
     def identity(self):
         constructor_signature = inspect.signature(self.__init__)
@@ -409,7 +413,7 @@ class BaseExpression:
         return tuple(identity)
 
     def __eq__(self, other):
-        if not isinstance(other, BaseExpression):
+        if not isinstance(other, Expression):
             return NotImplemented
         return other.identity == self.identity
 
@@ -417,11 +421,6 @@ class BaseExpression:
         return hash(self.identity)
 
 
-class Expression(BaseExpression, Combinable):
-    """An expression that can be combined with other expressions."""
-    pass
-
-
 _connector_combinators = {
     connector: [
         (fields.IntegerField, fields.IntegerField, fields.IntegerField),
@@ -1103,7 +1102,7 @@ class Case(Expression):
         return super().get_group_by_cols(alias)
 
 
-class Subquery(Expression):
+class Subquery(BaseExpression, Combinable):
     """
     An explicit subquery. It may contain OuterRef() references to the outer
     query which will be resolved when it is applied to that query.
@@ -1117,16 +1116,6 @@ class Subquery(Expression):
         self.extra = extra
         super().__init__(output_field)
 
-    def __getstate__(self):
-        state = super().__getstate__()
-        args, kwargs = state['_constructor_args']
-        if args:
-            args = (self.query, *args[1:])
-        else:
-            kwargs['queryset'] = self.query
-        state['_constructor_args'] = args, kwargs
-        return state
-
     def get_source_expressions(self):
         return [self.query]
 
@@ -1203,6 +1192,7 @@ class Exists(Subquery):
         return sql, params
 
 
+@deconstructible
 class OrderBy(BaseExpression):
     template = '%(expression)s %(ordering)s'
     conditional = False
diff --git a/django/db/models/query_utils.py b/django/db/models/query_utils.py
index 43c93ce455..188b640850 100644
--- a/django/db/models/query_utils.py
+++ b/django/db/models/query_utils.py
@@ -5,6 +5,7 @@ Factored out from django.db.models.query to avoid making the main module very
 large and/or so that they can be used by other modules without getting into
 circular import difficulties.
 """
+import copy
 import functools
 import inspect
 from collections import namedtuple
@@ -43,14 +44,11 @@ class Q(tree.Node):
         if not(isinstance(other, Q) or getattr(other, 'conditional', False) is True):
             raise TypeError(other)
 
-        # If the other Q() is empty, ignore it and just use `self`.
-        if not other:
+        if not self:
+            return other.copy() if hasattr(other, 'copy') else copy.copy(other)
+        elif isinstance(other, Q) and not other:
             _, args, kwargs = self.deconstruct()
             return type(self)(*args, **kwargs)
-        # Or if this Q is empty, ignore it and just use `other`.
-        elif not self:
-            _, args, kwargs = other.deconstruct()
-            return type(other)(*args, **kwargs)
 
         obj = type(self)()
         obj.connector = conn
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index be08c55c7c..b505af4efa 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -36,7 +36,6 @@ from django.db.models.sql.where import (
     AND, OR, ExtraWhere, NothingNode, WhereNode,
 )
 from django.utils.functional import cached_property
-from django.utils.hashable import make_hashable
 from django.utils.tree import Node
 
 __all__ = ['Query', 'RawQuery']
@@ -250,14 +249,6 @@ class Query(BaseExpression):
         for alias in self.alias_map:
             return alias
 
-    @property
-    def identity(self):
-        identity = (
-            (arg, make_hashable(value))
-            for arg, value in self.__dict__.items()
-        )
-        return (self.__class__, *identity)
-
     def __str__(self):
         """
         Return the query as a string of SQL with the parameter values
diff --git a/docs/releases/3.2.1.txt b/docs/releases/3.2.1.txt
index 1c8776a0bf..545c9adce3 100644
--- a/docs/releases/3.2.1.txt
+++ b/docs/releases/3.2.1.txt
@@ -68,3 +68,7 @@ Bugfixes
 
 * Fixed a regression in Django 3.2 where the calling process environment would
   not be passed to the ``dbshell`` command on PostgreSQL (:ticket:`32687`).
+
+* Fixed a performance regression in Django 3.2 when building complex filters
+  with subqueries (:ticket:`32632`). As a side-effect the private API to check
+  ``django.db.sql.query.Query`` equality is removed.
diff --git a/tests/queries/test_q.py b/tests/queries/test_q.py
index 84ae6e90a5..bd39b2dc32 100644
--- a/tests/queries/test_q.py
+++ b/tests/queries/test_q.py
@@ -1,4 +1,5 @@
-from django.db.models import Exists, F, OuterRef, Q
+from django.db.models import BooleanField, Exists, F, OuterRef, Q
+from django.db.models.expressions import RawSQL
 from django.test import SimpleTestCase
 
 from .models import Tag
@@ -50,6 +51,16 @@ class QTests(SimpleTestCase):
         with self.assertRaisesMessage(TypeError, str(obj)):
             q & obj
 
+    def test_combine_negated_boolean_expression(self):
+        tagged = Tag.objects.filter(category=OuterRef('pk'))
+        tests = [
+            Q() & ~Exists(tagged),
+            Q() | ~Exists(tagged),
+        ]
+        for q in tests:
+            with self.subTest(q=q):
+                self.assertIs(q.negated, True)
+
     def test_deconstruct(self):
         q = Q(price__gt=F('discounted_price'))
         path, args, kwargs = q.deconstruct()
@@ -101,10 +112,10 @@ class QTests(SimpleTestCase):
         self.assertEqual(kwargs, {})
 
     def test_deconstruct_boolean_expression(self):
-        tagged = Tag.objects.filter(category=OuterRef('pk'))
-        q = Q(Exists(tagged))
+        expr = RawSQL('1 = 1', BooleanField())
+        q = Q(expr)
         _, args, kwargs = q.deconstruct()
-        self.assertEqual(args, (Exists(tagged),))
+        self.assertEqual(args, (expr,))
         self.assertEqual(kwargs, {})
 
     def test_reconstruct(self):
diff --git a/tests/queries/test_query.py b/tests/queries/test_query.py
index 5db9d96163..523fa607f0 100644
--- a/tests/queries/test_query.py
+++ b/tests/queries/test_query.py
@@ -150,31 +150,3 @@ class TestQuery(SimpleTestCase):
         msg = 'Cannot filter against a non-conditional expression.'
         with self.assertRaisesMessage(TypeError, msg):
             query.build_where(Func(output_field=CharField()))
-
-    def test_equality(self):
-        self.assertNotEqual(
-            Author.objects.all().query,
-            Author.objects.filter(item__name='foo').query,
-        )
-        self.assertEqual(
-            Author.objects.filter(item__name='foo').query,
-            Author.objects.filter(item__name='foo').query,
-        )
-        self.assertEqual(
-            Author.objects.filter(item__name='foo').query,
-            Author.objects.filter(Q(item__name='foo')).query,
-        )
-
-    def test_hash(self):
-        self.assertNotEqual(
-            hash(Author.objects.all().query),
-            hash(Author.objects.filter(item__name='foo').query)
-        )
-        self.assertEqual(
-            hash(Author.objects.filter(item__name='foo').query),
-            hash(Author.objects.filter(item__name='foo').query),
-        )
-        self.assertEqual(
-            hash(Author.objects.filter(item__name='foo').query),
-            hash(Author.objects.filter(Q(item__name='foo')).query),
-        )