Fix for bug #1335: filesort is missing in EXPLAIN if ORDER BY NULL is used

mysql-test/r/group_by.result:
  test results fixed
mysql-test/t/group_by.test:
  added few explicit tests for bug #1335

mysql-test/r/group_by.result

@@ -287,7 +287,7 @@ table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	ALL	NULL	NULL	NULL	NULL	8	Using filesort
 explain select sql_big_result spid,sum(userid) from t1 group by spid desc order by null;
 table	type	possible_keys	key	key_len	ref	rows	Extra
-t1	ALL	NULL	NULL	NULL	NULL	8	
+t1	ALL	NULL	NULL	NULL	NULL	8	Using filesort
 select sql_big_result spid,sum(userid) from t1 group by spid desc;
 spid	sum(userid)
 7	3
@@ -597,3 +597,18 @@ count(*)	category
 1	3
 1	4
 drop table t1;
+userid	count(*)
+3	5
+2	1
+1	2
+EXPLAIN SELECT userid,count(*) FROM t1 GROUP BY userid DESC;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ALL	NULL	NULL	NULL	NULL	8	Using temporary; Using filesort
+i	COUNT(DISTINCT(i))
+1	1
+2	1
+4	4
+explain SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ALL	NULL	NULL	NULL	NULL	6	Using filesort
+DROP TABLE t1;

mysql-test/t/group_by.test

@@ -423,3 +423,31 @@ select id, sum(qty) as sqty, count(qty) as cqty from t1 group by id having sum(q
 select count(*), case interval(qty,2,3,4,5,6,7,8) when -1 then NULL when 0 then "zero" when 1 then "one" when 2 then "two" end as category from t1 group by category;
 select count(*), interval(qty,2,3,4,5,6,7,8) as category from t1 group by category;
 drop table t1;
+#
+# Tests for bug #1355: 'Using filesort' is missing in EXPLAIN when ORDER BY
+# NULL is used.
+#
+--disable_query_log
+CREATE TABLE t1 (
+  userid int(10) unsigned,
+  score smallint(5) unsigned,
+  key (score)
+);
+INSERT INTO t1 VALUES (1,1),(2,2),(1,1),(3,3),(3,3),(3,3),(3,3),(3,3);
+# Here we select unordered GROUP BY into a temporary talbe, 
+# and then sort it with filesort (GROUP BY in MySQL 
+# implies sorted order of results)
+SELECT userid,count(*) FROM t1 GROUP BY userid DESC;
+--enable_query_log
+EXPLAIN SELECT userid,count(*) FROM t1 GROUP BY userid DESC;
+--disable_query_log
+DROP TABLE t1;
+CREATE TABLE t1 (
+  i int(11) default NULL,
+  j int(11) default NULL
+);
+INSERT INTO t1 VALUES (1,2),(2,3),(4,5),(3,5),(1,5),(23,5);
+SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
+--enable_query_log
+explain SELECT i, COUNT(DISTINCT(i)) FROM t1 GROUP BY j ORDER BY NULL;
+DROP TABLE t1;

sql/sql_select.cc

@@ -696,8 +696,18 @@ mysql_select(THD *thd,TABLE_LIST *tables,List<Item> &fields,COND *conds,
 
   if (select_options & SELECT_DESCRIBE)
   {
-    if (!order && !no_order)
-      order=group;
+    /*
+      Check if we managed to optimize ORDER BY away and don't use temporary
+      table to resolve ORDER BY: in that case, we only may need to do
+      filesort for GROUP BY.
+    */
+    if (!order && !no_order && (!skip_sort_order || !need_tmp))
+    {
+      /* Reset 'order' to 'group' and reinit variables describing 'order' */
+      order= group;
+      simple_order= simple_group;
+      skip_sort_order= 0;
+    }
     if (order &&
 	(join.const_tables == join.tables ||
 	 ((simple_order || skip_sort_order) &&