group_by.result:

  WL3527: disable wrong optimization.
sql_select.cc:
  WL#3527: disable wrong optimization.

mysql-test/r/group_by.result

@@ -1067,7 +1067,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	256	Using index
 EXPLAIN SELECT a FROM t1 IGNORE INDEX FOR ORDER BY (PRIMARY,i2) ORDER BY a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	256	Using index
+1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	256	Using index; Using filesort
 EXPLAIN SELECT a FROM t1 IGNORE INDEX FOR ORDER BY (PRIMARY)
 IGNORE INDEX FOR GROUP BY (i2) GROUP BY a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

sql/sql_select.cc

@@ -12393,24 +12393,12 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
   DBUG_ENTER("test_if_skip_sort_order");
   LINT_INIT(ref_key_parts);
 
-  /* Check which keys can be used to resolve ORDER BY. */
-  usable_keys= table->keys_in_use_for_query;
-
   /*
     Keys disabled by ALTER TABLE ... DISABLE KEYS should have already
     been taken into account.
   */
   usable_keys= *map;
 
-  /* 
-    If there is a covering index, and we have IGNORE INDEX FOR GROUP/ORDER
-    and this index is used for the JOIN part, then we have to ignore the
-    IGNORE INDEX FOR GROUP/ORDER
-  */  
-  if (table->key_read ||
-      (table->covering_keys.is_set(tab->index) && !table->no_keyread))
-    usable_keys.set_bit (tab->index);
-
   for (ORDER *tmp_order=order; tmp_order ; tmp_order=tmp_order->next)
   {
     Item *item= (*tmp_order->item)->real_item();