Fixed #19102 -- Fixed fast-path delete for modified SELECT clause cases

There was a bug introduced in #18676 which caused fast-path deletes
implemented as "DELETE WHERE pk IN <subquery>" to fail if the SELECT
clause contained additional stuff (for example extra() and annotate()).

Thanks to Trac alias pressureman for spotting this regression.

django/db/models/sql/query.py

@@ -431,13 +431,9 @@ class Query(object):
 
     def has_results(self, using):
         q = self.clone()
+        q.clear_select_clause()
         q.add_extra({'a': 1}, None, None, None, None, None)
-        q.select = []
-        q.select_fields = []
-        q.default_cols = False
-        q.select_related = False
-        q.set_extra_mask(('a',))
-        q.set_aggregate_mask(())
+        q.set_extra_mask(['a'])
         q.clear_ordering(True)
         q.set_limits(high=1)
         compiler = q.get_compiler(using=using)
@@ -1626,6 +1622,17 @@ class Query(object):
         """
         return not self.low_mark and self.high_mark is None
 
+    def clear_select_clause(self):
+        """
+        Removes all fields from SELECT clause.
+        """
+        self.select = []
+        self.select_fields = []
+        self.default_cols = False
+        self.select_related = False
+        self.set_extra_mask(())
+        self.set_aggregate_mask(())
+
     def clear_select_fields(self):
         """
         Clears the list of fields to select (but not extra_select columns).

django/db/models/sql/subqueries.py

@@ -70,8 +70,9 @@ class DeleteQuery(Query):
                 self.delete_batch(values, using)
                 return
             else:
-                values = innerq
+                innerq.clear_select_clause()
                 innerq.select = [(self.get_initial_alias(), pk.column)]
+                values = innerq
             where = self.where_class()
             where.add((Constraint(None, pk.column, pk), 'in', values), AND)
             self.where = where
@@ -237,11 +238,8 @@ class DateQuery(Query):
                 % field.name
         alias = result[3][-1]
         select = Date((alias, field.column), lookup_type)
+        self.clear_select_clause()
         self.select = [select]
-        self.select_fields = [None]
-        self.select_related = False # See #7097.
-        self.aggregates = SortedDict() # See 18056.
-        self.set_extra_mask([])
         self.distinct = True
         self.order_by = order == 'ASC' and [1] or [-1]