Fixed #19259 -- Added group by selected primary keys support.

django/db/backends/base/features.py

@@ -6,6 +6,7 @@ from django.utils.functional import cached_property
 class BaseDatabaseFeatures(object):
     gis_enabled = False
     allows_group_by_pk = False
+    allows_group_by_selected_pks = False
     # True if django.db.backends.utils.typecast_timestamp is used on values
     # returned from dates() calls.
     needs_datetime_string_cast = True

django/db/backends/postgresql_psycopg2/features.py

@@ -3,6 +3,7 @@ from django.db.utils import InterfaceError
 
 
 class DatabaseFeatures(BaseDatabaseFeatures):
+    allows_group_by_selected_pks = True
     needs_datetime_string_cast = False
     can_return_id_from_insert = True
     has_real_datatype = True

django/db/models/sql/compiler.py

@@ -136,10 +136,7 @@ class SQLCompiler(object):
         # If the DB can group by primary key, then group by the primary key of
         # query's main model. Note that for PostgreSQL the GROUP BY clause must
         # include the primary key of every table, but for MySQL it is enough to
-        # have the main table's primary key. Currently only the MySQL form is
-        # implemented.
-        # MySQLism: however, columns in HAVING clause must be added to the
-        # GROUP BY.
+        # have the main table's primary key.
         if self.connection.features.allows_group_by_pk:
             # The logic here is: if the main model's primary key is in the
             # query, then set new_expressions to that field. If that happens,
@@ -150,7 +147,18 @@ class SQLCompiler(object):
                         getattr(expr.output_field, 'model') == self.query.model):
                     pk = expr
             if pk:
+                # MySQLism: Columns in HAVING clause must be added to the GROUP BY.
                 expressions = [pk] + [expr for expr in expressions if expr in having]
+        elif self.connection.features.allows_group_by_selected_pks:
+            # Filter out all expressions associated with a table's primary key
+            # present in the grouped columns. This is done by identifying all
+            # tables that have their primary key included in the grouped
+            # columns and removing non-primary key columns referring to them.
+            pks = {expr for expr in expressions if hasattr(expr, 'target') and expr.target.primary_key}
+            aliases = {expr.alias for expr in pks}
+            expressions = [
+                expr for expr in expressions if expr in pks or getattr(expr, 'alias', None) not in aliases
+            ]
         return expressions
 
     def get_select(self):