Refs #31150 -- Enabled implicit GROUP BY aliases.

This ensures implicit grouping from aggregate function annotations
groups by uncollapsed selected aliases if supported.

The feature is disabled on Oracle because it doesn't support it.

django/db/models/sql/compiler.py

@@ -123,6 +123,7 @@ class SQLCompiler:
         if self.query.group_by is None:
             return []
         expressions = []
+        allows_group_by_refs = self.connection.features.allows_group_by_refs
         if self.query.group_by is not True:
             # If the group by is set to a list (by .values() call most likely),
             # then we need to add everything in it to the GROUP BY clause.
@@ -132,20 +133,21 @@ class SQLCompiler:
             for expr in self.query.group_by:
                 if not hasattr(expr, "as_sql"):
                     expr = self.query.resolve_ref(expr)
-                if not self.connection.features.allows_group_by_refs and isinstance(
-                    expr, Ref
-                ):
+                if not allows_group_by_refs and isinstance(expr, Ref):
                     expr = expr.source
                 expressions.append(expr)
         # Note that even if the group_by is set, it is only the minimal
         # set to group by. So, we need to add cols in select, order_by, and
         # having into the select in any case.
         ref_sources = {expr.source for expr in expressions if isinstance(expr, Ref)}
-        for expr, _, _ in select:
+        aliased_exprs = {}
+        for expr, _, alias in select:
             # Skip members of the select clause that are already included
             # by reference.
             if expr in ref_sources:
                 continue
+            if alias:
+                aliased_exprs[expr] = alias
             cols = expr.get_group_by_cols()
             for col in cols:
                 expressions.append(col)
@@ -163,6 +165,8 @@ class SQLCompiler:
         expressions = self.collapse_group_by(expressions, having_group_by)
 
         for expr in expressions:
+            if allows_group_by_refs and (alias := aliased_exprs.get(expr)):
+                expr = Ref(alias, expr)
             try:
                 sql, params = self.compile(expr)
             except EmptyResultSet:

tests/aggregation/tests.py

@@ -1639,12 +1639,12 @@ class AggregateTestCase(TestCase):
             )
             .annotate(count=Count("authors"))
         )
-        self.assertSequenceEqual(books_qs, [book])
-        # FIXME: GROUP BY doesn't need to include a subquery with
-        # non-multivalued JOINs, see Col.possibly_multivalued (refs #31150):
-        # with self.assertNumQueries(1) as ctx:
-        #     self.assertSequenceEqual(books_qs, [book])
-        # self.assertEqual(ctx[0]['sql'].count('SELECT'), 2)
+        with self.assertNumQueries(1) as ctx:
+            self.assertSequenceEqual(books_qs, [book])
+        # Outerquery SELECT, annotation SELECT, and WHERE SELECT but GROUP BY
+        # selected alias, if allowed.
+        if connection.features.allows_group_by_refs:
+            self.assertEqual(ctx[0]["sql"].count("SELECT"), 3)
 
     @skipUnlessDBFeature("supports_subqueries_in_group_by")
     def test_aggregation_nested_subquery_outerref(self):