Bug #21174: Index degrades sort performance and

             optimizer does not honor IGNORE INDEX
 - Allow an index to be used for sorting the table 
   instead of filesort only if it is not disabled by
   IGNORE INDEX.

mysql-test/r/group_by.result

@@ -821,3 +821,12 @@ a	b	real_b
 68	France	France
 DROP VIEW v1;
 DROP TABLE t1,t2;
+CREATE TABLE t1 (a INT, b INT, KEY(a));
+INSERT INTO t1 VALUES (1, 1), (2, 2), (3,3), (4,4);
+EXPLAIN SELECT a, SUM(b) FROM t1 GROUP BY a LIMIT 2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	a	5	NULL	4	
+EXPLAIN SELECT a, SUM(b) FROM t1 IGNORE INDEX (a) GROUP BY a LIMIT 2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	Using temporary; Using filesort
+DROP TABLE t1;

mysql-test/t/group_by.test

@@ -655,3 +655,15 @@ where t2.b=v1.a GROUP BY t2.b;
 
 DROP VIEW v1;
 DROP TABLE t1,t2;
+
+#
+# Bug #21174: Index degrades sort performance and 
+#             optimizer does not honor IGNORE INDEX
+#
+CREATE TABLE t1 (a INT, b INT, KEY(a));
+INSERT INTO t1 VALUES (1, 1), (2, 2), (3,3), (4,4);
+
+EXPLAIN SELECT a, SUM(b) FROM t1 GROUP BY a LIMIT 2; 
+EXPLAIN SELECT a, SUM(b) FROM t1 IGNORE INDEX (a) GROUP BY a LIMIT 2;
+
+DROP TABLE t1;

sql/sql_select.cc

@@ -11498,6 +11498,8 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
     We must not try to use disabled keys.
   */
   usable_keys= table->s->keys_in_use;
+  /* we must not consider keys that are disabled by IGNORE INDEX */
+  usable_keys.intersect(table->keys_in_use_for_query);
 
   for (ORDER *tmp_order=order; tmp_order ; tmp_order=tmp_order->next)
   {