Fixed #31507 -- Added QuerySet.exists() optimizations to compound queries.

tests/queries/test_qs_combinators.py

@@ -3,6 +3,7 @@ import operator
 from django.db import DatabaseError, NotSupportedError, connection
 from django.db.models import Exists, F, IntegerField, OuterRef, Value
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
+from django.test.utils import CaptureQueriesContext
 
 from .models import Number, ReservedName
 
@@ -257,7 +258,19 @@ class QuerySetSetOperationTests(TestCase):
     def test_exists_union(self):
         qs1 = Number.objects.filter(num__gte=5)
         qs2 = Number.objects.filter(num__lte=5)
-        self.assertIs(qs1.union(qs2).exists(), True)
+        with CaptureQueriesContext(connection) as context:
+            self.assertIs(qs1.union(qs2).exists(), True)
+        captured_queries = context.captured_queries
+        self.assertEqual(len(captured_queries), 1)
+        captured_sql = captured_queries[0]['sql']
+        self.assertNotIn(
+            connection.ops.quote_name(Number._meta.pk.column),
+            captured_sql,
+        )
+        self.assertEqual(
+            captured_sql.count(connection.ops.limit_offset_sql(None, 1)),
+            3 if connection.features.supports_slicing_ordering_in_compound else 1
+        )
 
     def test_exists_union_empty_result(self):
         qs = Number.objects.filter(pk__in=[])