Bug #21302: Result not properly sorted when using an ORDER BY on a second

             table in a join
 The optimizer removes redundant columns in ORDER BY. It is considering 
redundant every reference to const table column, e.g b in :
create table t1 (a int, b int, primary key(a)); 
select 1 from t1 order by b where a = 1

But it must not remove references to const table columns if the 
const table is an outer table because there still can be 2 values :
the const value and NULL. e.g.:
create table t1 (a int, b int, primary key(a));
select t2.b c from t1 left join t1 t2 on (t1.a = t2.a and t2.a = 5) 
  order by c;

mysql-test/r/join_outer.result

@@ -735,7 +735,7 @@ explain select s.*, '*', m.*, (s.match_1_h - m.home) UUX from
 (t2 s left join t1 m on m.match_id = 1) 
 order by m.match_id desc;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	s	ALL	NULL	NULL	NULL	NULL	10	
+1	SIMPLE	s	ALL	NULL	NULL	NULL	NULL	10	Using temporary; Using filesort
 1	SIMPLE	m	const	match_id,match_id_2	match_id	1	const	1	
 explain select s.*, '*', m.*, (s.match_1_h - m.home) UUX from 
 (t2 s left join t1 m on m.match_id = 1) 

mysql-test/r/order_by.result

@@ -852,3 +852,40 @@ b	a
 20	1
 10	2
 DROP TABLE t1;
+CREATE TABLE t1 (a int, b int, PRIMARY KEY  (a));
+INSERT INTO t1 VALUES (1,1), (2,2), (3,3);
+explain SELECT t1.b as a, t2.b as c FROM 
+t1 LEFT JOIN t1 t2 ON (t1.a = t2.a AND t2.a = 2) 
+ORDER BY c;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using temporary; Using filesort
+1	SIMPLE	t2	const	PRIMARY	PRIMARY	4	const	1	
+SELECT t1.b as a, t2.b as c FROM 
+t1 LEFT JOIN t1 t2 ON (t1.a = t2.a AND t2.a = 2) 
+ORDER BY c;
+a	c
+1	NULL
+3	NULL
+2	2
+explain SELECT t1.b as a, t2.b as c FROM 
+t1 JOIN t1 t2 ON (t1.a = t2.a AND t2.a = 2)  
+ORDER BY c;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	const	PRIMARY	PRIMARY	4	const	1	
+1	SIMPLE	t2	const	PRIMARY	PRIMARY	4	const	1	
+CREATE TABLE t2 LIKE t1;
+INSERT INTO t2 SELECT * from t1;
+CREATE TABLE t3 LIKE t1;
+INSERT INTO t3 SELECT * from t1;
+CREATE TABLE t4 LIKE t1;
+INSERT INTO t4 SELECT * from t1;
+INSERT INTO t1 values (0,0),(4,4);
+SELECT t1.*,t2.* FROM t1 LEFT JOIN (t2, t3 LEFT JOIN t4 ON t3.a=t4.a)
+ON (t1.a=t2.a AND t1.b=t3.b) order by t2.b;
+a	b	a	b
+0	0	NULL	NULL
+4	4	NULL	NULL
+1	1	1	1
+2	2	2	2
+3	3	3	3
+DROP TABLE t1,t2,t3,t4;

mysql-test/t/order_by.test

@@ -578,3 +578,35 @@ INSERT INTO t1 VALUES (1,30), (2,20), (1,10), (2,30), (1,20), (2,10);
 DROP TABLE t1;
 
 # End of 4.1 tests
+
+#
+# Bug#21302: Result not properly sorted when using an ORDER BY on a second 
+#             table in a join
+#
+CREATE TABLE t1 (a int, b int, PRIMARY KEY  (a));
+INSERT INTO t1 VALUES (1,1), (2,2), (3,3);
+
+explain SELECT t1.b as a, t2.b as c FROM 
+ t1 LEFT JOIN t1 t2 ON (t1.a = t2.a AND t2.a = 2) 
+ORDER BY c;
+SELECT t1.b as a, t2.b as c FROM 
+ t1 LEFT JOIN t1 t2 ON (t1.a = t2.a AND t2.a = 2) 
+ORDER BY c;
+
+# check that it still removes sort of const table
+explain SELECT t1.b as a, t2.b as c FROM 
+ t1 JOIN t1 t2 ON (t1.a = t2.a AND t2.a = 2)  
+ORDER BY c;
+
+CREATE TABLE t2 LIKE t1;
+INSERT INTO t2 SELECT * from t1;
+CREATE TABLE t3 LIKE t1;
+INSERT INTO t3 SELECT * from t1;
+CREATE TABLE t4 LIKE t1;
+INSERT INTO t4 SELECT * from t1;
+INSERT INTO t1 values (0,0),(4,4);
+
+SELECT t1.*,t2.* FROM t1 LEFT JOIN (t2, t3 LEFT JOIN t4 ON t3.a=t4.a)
+ON (t1.a=t2.a AND t1.b=t3.b) order by t2.b;
+
+DROP TABLE t1,t2,t3,t4;

sql/sql_select.cc

@@ -5976,7 +5976,8 @@ eq_ref_table(JOIN *join, ORDER *start_order, JOIN_TAB *tab)
   if (tab->cached_eq_ref_table)			// If cached
     return tab->eq_ref_table;
   tab->cached_eq_ref_table=1;
-  if (tab->type == JT_CONST)			// We can skip const tables
+  /* We can skip const tables only if not an outer table */
+  if (tab->type == JT_CONST && !tab->first_inner)
     return (tab->eq_ref_table=1);		/* purecov: inspected */
   if (tab->type != JT_EQ_REF || tab->table->maybe_null)
     return (tab->eq_ref_table=0);		// We must use this