Fixed #28211 -- Prevented ORing an empty Q() from reducing query join efficiency.

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
@@ -61,6 +62,14 @@ class Q(tree.Node):
     def _combine(self, other, conn):
         if not isinstance(other, Q):
             raise TypeError(other)
+
+        # If the other Q() is empty, ignore it and just use `self`.
+        if not other:
+            return copy.deepcopy(self)
+        # Or if this Q is empty, ignore it and just use `other`.
+        elif not self:
+            return copy.deepcopy(other)
+
         obj = type(self)()
         obj.connector = conn
         obj.add(self, conn)

tests/queries/test_q.py

@@ -11,6 +11,14 @@ class QTests(SimpleTestCase):
     def test_combine_and_both_empty(self):
         self.assertEqual(Q() & Q(), Q())
 
+    def test_combine_or_empty(self):
+        q = Q(x=1)
+        self.assertEqual(q | Q(), q)
+        self.assertEqual(Q() | q, q)
+
+    def test_combine_or_both_empty(self):
+        self.assertEqual(Q() | Q(), Q())
+
     def test_deconstruct(self):
         q = Q(price__gt=F('discounted_price'))
         path, args, kwargs = q.deconstruct()