Fixed #31792 -- Made Exists() reuse QuerySet.exists() optimizations.

The latter is already optimized to limit the number of results, avoid
selecting unnecessary fields, and drop ordering if possible without
altering the semantic of the query.

tests/expressions/tests.py

@@ -22,7 +22,7 @@ from django.db.models.functions import (
 from django.db.models.sql import constants
 from django.db.models.sql.datastructures import Join
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
-from django.test.utils import Approximate, isolate_apps
+from django.test.utils import Approximate, CaptureQueriesContext, isolate_apps
 from django.utils.functional import SimpleLazyObject
 
 from .models import (
@@ -1738,6 +1738,26 @@ class ValueTests(TestCase):
             Value(object()).output_field
 
 
+class ExistsTests(TestCase):
+    def test_optimizations(self):
+        with CaptureQueriesContext(connection) as context:
+            list(Experiment.objects.values(exists=Exists(
+                Experiment.objects.order_by('pk'),
+            )).order_by())
+        captured_queries = context.captured_queries
+        self.assertEqual(len(captured_queries), 1)
+        captured_sql = captured_queries[0]['sql']
+        self.assertNotIn(
+            connection.ops.quote_name(Experiment._meta.pk.column),
+            captured_sql,
+        )
+        self.assertIn(
+            connection.ops.limit_offset_sql(None, 1),
+            captured_sql,
+        )
+        self.assertNotIn('ORDER BY', captured_sql)
+
+
 class FieldTransformTests(TestCase):
 
     @classmethod