Refs #30188 -- Prevented double annotation of subquery when aggregated over.

Thanks Can Sarıgöl for the suggested trimming approach.

django/db/models/sql/query.py

@@ -383,8 +383,12 @@ class Query(BaseExpression):
                 new_expr, col_cnt = self.rewrite_cols(expr, col_cnt)
                 new_exprs.append(new_expr)
             elif isinstance(expr, (Col, Subquery)) or (expr.contains_aggregate and not expr.is_summary):
-                # Reference to column. Make sure the referenced column
-                # is selected.
+                # Reference to column, subquery, or another aggregate. Make
+                # sure the expression is selected and reuse its alias if so.
+                for col_alias, selected_annotation in self.annotation_select.items():
+                    if selected_annotation == expr:
+                        break
+                else:
                     col_cnt += 1
                     col_alias = '__col%d' % col_cnt
                     self.annotations[col_alias] = expr

tests/expressions/tests.py

@@ -553,6 +553,7 @@ class BasicExpressionsTests(TestCase):
 
     @skipUnlessDBFeature('supports_subqueries_in_group_by')
     def test_aggregate_subquery_annotation(self):
+        with self.assertNumQueries(1) as ctx:
             aggregate = Company.objects.annotate(
                 ceo_salary=Subquery(
                     Employee.objects.filter(
@@ -563,6 +564,9 @@ class BasicExpressionsTests(TestCase):
                 ceo_salary_gt_20=Count('pk', filter=Q(ceo_salary__gt=20)),
             )
         self.assertEqual(aggregate, {'ceo_salary_gt_20': 1})
+        # Aggregation over a subquery annotation doesn't annotate the subquery
+        # twice in the inner query.
+        self.assertLessEqual(ctx.captured_queries[0]['sql'].count('SELECT'), 4,)
 
     def test_explicit_output_field(self):
         class FuncA(Func):