Refs #17144 -- Removed support for grouping by primary key.

No core backend require the feature anymore as it was only added to
support a MySQL'ism that has been deprecated since then.

django/db/backends/base/features.py

@@ -8,7 +8,6 @@ class BaseDatabaseFeatures:
     gis_enabled = False
     # Oracle can't group by LOB (large object) data types.
     allows_group_by_lob = True
-    allows_group_by_pk = False
     allows_group_by_selected_pks = False
     allows_group_by_refs = True
     empty_fetchmany_value = []

django/db/models/sql/compiler.py

@@ -179,41 +179,10 @@ class SQLCompiler:
         return result
 
     def collapse_group_by(self, expressions, having):
-        # 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.
-        if self.connection.features.allows_group_by_pk:
-            # Determine if the main model's primary key is in the query.
-            pk = None
-            for expr in expressions:
-                # Is this a reference to query's base table primary key? If the
-                # expression isn't a Col-like, then skip the expression.
-                if (
-                    getattr(expr, "target", None) == self.query.model._meta.pk
-                    and getattr(expr, "alias", None) == self.query.base_table
-                ):
-                    pk = expr
-                    break
-            # If the main model's primary key is in the query, group by that
-            # field, HAVING expressions, and expressions associated with tables
-            # that don't have a primary key included in the grouped columns.
-            if pk:
-                pk_aliases = {
-                    expr.alias
-                    for expr in expressions
-                    if hasattr(expr, "target") and expr.target.primary_key
-                }
-                expressions = [pk] + [
-                    expr
-                    for expr in expressions
-                    if expr in having
-                    or (
-                        getattr(expr, "alias", None) is not None
-                        and expr.alias not in pk_aliases
-                    )
-                ]
-        elif self.connection.features.allows_group_by_selected_pks:
+        # If the database supports group by functional dependence reduction,
+        # then the expressions can be reduced to the set of selected table
+        # primary keys as all other columns are functionally dependent on them.
+        if 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