Bug #21456: SELECT DISTINCT(x) produces incorrect results when using order by

GROUP BY/DISTINCT pruning optimization must be done before ORDER BY 
optimization because ORDER BY may be removed when GROUP BY/DISTINCT
sorts as a side effect, e.g. in 
  SELECT DISTINCT <non-key-col>,<pk> FROM t1
  ORDER BY <non-key-col> DISTINCT
must be removed before ORDER BY as if done the other way around
it will remove both.

mysql-test/r/distinct.result

@@ -555,3 +555,14 @@ EXPLAIN SELECT DISTINCT a,b,d FROM t2 GROUP BY c,b,d;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	
 DROP TABLE t1,t2;
+CREATE TABLE t1 (a int primary key, b int);
+INSERT INTO t1 (a,b) values (1,1), (2,3), (3,2);
+explain SELECT DISTINCT a, b FROM t1 ORDER BY b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using filesort
+SELECT DISTINCT a, b FROM t1 ORDER BY b;
+a	b
+1	1
+3	2
+2	3
+DROP TABLE t1;

mysql-test/t/distinct.test

@@ -378,4 +378,15 @@ EXPLAIN SELECT DISTINCT a,b,d FROM t2 GROUP BY c,b,d;
 
 DROP TABLE t1,t2;
 
+#
+# Bug 21456: SELECT DISTINCT(x) produces incorrect results when using order by
+#
+CREATE TABLE t1 (a int primary key, b int);
+
+INSERT INTO t1 (a,b) values (1,1), (2,3), (3,2);
+
+explain SELECT DISTINCT a, b FROM t1 ORDER BY b;
+SELECT DISTINCT a, b FROM t1 ORDER BY b;
+DROP TABLE t1;
+
 # End of 4.1 tests

sql/sql_select.cc

@@ -648,6 +648,36 @@ JOIN::optimize()
     if (!order && org_order)
       skip_sort_order= 1;
   }
+  /*
+     Check if we can optimize away GROUP BY/DISTINCT.
+     We can do that if there are no aggregate functions and the
+     fields in DISTINCT clause (if present) and/or columns in GROUP BY
+     (if present) contain direct references to all key parts of
+     an unique index (in whatever order).
+     Note that the unique keys for DISTINCT and GROUP BY should not
+     be the same (as long as they are unique).
+
+     The FROM clause must contain a single non-constant table.
+  */
+  if (tables - const_tables == 1 && (group_list || select_distinct) &&
+      !tmp_table_param.sum_func_count)
+  {
+    if (group_list &&
+       list_contains_unique_index(join_tab[const_tables].table,
+                                 find_field_in_order_list,
+                                 (void *) group_list))
+    {
+      group_list= 0;
+      group= 0;
+    }
+    if (select_distinct &&
+       list_contains_unique_index(join_tab[const_tables].table,
+                                 find_field_in_item_list,
+                                 (void *) &fields_list))
+    {
+      select_distinct= 0;
+    }
+  }
   if (group_list || tmp_table_param.sum_func_count)
   {
     if (! hidden_group_fields && rollup.state == ROLLUP::STATE_NONE)
@@ -717,36 +747,6 @@ JOIN::optimize()
     if (old_group_list && !group_list)
       select_distinct= 0;
   }
-  /*
-     Check if we can optimize away GROUP BY/DISTINCT.
-     We can do that if there are no aggregate functions and the
-     fields in DISTINCT clause (if present) and/or columns in GROUP BY
-     (if present) contain direct references to all key parts of
-     an unique index (in whatever order).
-     Note that the unique keys for DISTINCT and GROUP BY should not
-     be the same (as long as they are unique).
-
-     The FROM clause must contain a single non-constant table.
-  */
-  if (tables - const_tables == 1 && (group_list || select_distinct) &&
-      !tmp_table_param.sum_func_count)
-  {
-    if (group_list &&
-       list_contains_unique_index(join_tab[const_tables].table,
-                                 find_field_in_order_list,
-                                 (void *) group_list))
-    {
-      group_list= 0;
-      group= 0;
-    }
-    if (select_distinct &&
-       list_contains_unique_index(join_tab[const_tables].table,
-                                 find_field_in_item_list,
-                                 (void *) &fields_list))
-    {
-      select_distinct= 0;
-    }
-  }
   if (!group_list && group)
   {
     order=0;					// The output has only one row