Added supports_select_union skips in queries and aggregation tests.

tests/aggregation/tests.py

@@ -2365,6 +2365,7 @@ class AggregateAnnotationPruningTests(TestCase):
         ).aggregate(count=Count("id", filter=Q(id__in=[F("max_book_author"), 0])))
         self.assertEqual(aggregates, {"count": 1})
 
+    @skipUnlessDBFeature("supports_select_union")
     def test_aggregate_combined_queries(self):
         # Combined queries could have members in their values select mask while
         # others have them in their annotation mask which makes annotation

tests/queries/test_explain.py

@@ -19,8 +19,11 @@ class ExplainTests(TestCase):
             Tag.objects.filter(name="test").prefetch_related("children"),
             Tag.objects.filter(name="test").annotate(Count("children")),
             Tag.objects.filter(name="test").values_list("name"),
-            Tag.objects.order_by().union(Tag.objects.order_by().filter(name="test")),
         ]
+        if connection.features.supports_select_union:
+            querysets.append(
+                Tag.objects.order_by().union(Tag.objects.order_by().filter(name="test"))
+            )
         if connection.features.has_select_for_update:
             querysets.append(Tag.objects.select_for_update().filter(name="test"))
         supported_formats = connection.features.supported_explain_formats
@@ -96,6 +99,7 @@ class ExplainTests(TestCase):
                     option = "{} {}".format(name.upper(), "true" if value else "false")
                     self.assertIn(option, captured_queries[0]["sql"])
 
+    @skipUnlessDBFeature("supports_select_union")
     def test_multi_page_text_explain(self):
         if "TEXT" not in connection.features.supported_explain_formats:
             self.skipTest("This backend does not support TEXT format.")

tests/queries/tests.py

@@ -1375,6 +1375,7 @@ class Queries1Tests(TestCase):
         self.assertCountEqual(items_after, [self.i2, self.i3, self.i4])
         self.assertCountEqual(items_before, items_after)
 
+    @skipUnlessDBFeature("supports_select_union")
     def test_union_values_subquery(self):
         items = Item.objects.filter(creator=OuterRef("pk"))
         item_authors = Author.objects.annotate(is_creator=Exists(items)).order_by()