Fix for bug#5088:

 * When executing EXPLAIN, do the same as for the query: convert join type to JT_CONST if keyuse array covers all key parts and all of them are constants.
 * In remove_const, don't remove conditions that depend on some-const-table and current-table.

mysql-test/r/join_outer.result

@@ -634,7 +634,7 @@ insert into t2 values (10,1),(20,2),(30,3);
 explain select * from t2 left join t1 on t1.fooID = t2.fooID and t1.fooID = 30;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	index	NULL	PRIMARY	4	NULL	3	Using index
-1	SIMPLE	t1	eq_ref	PRIMARY	PRIMARY	2	const	1	Using where; Using index
+1	SIMPLE	t1	const	PRIMARY	PRIMARY	2	const	1	Using where; Using index
 select * from t2 left join t1 on t1.fooID = t2.fooID and t1.fooID = 30;
 fooID	barID	fooID
 10	1	NULL
@@ -682,3 +682,72 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	2	
 1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	2	
 drop table t1, t2, t3;
+create table t1 (
+match_id tinyint(3) unsigned not null auto_increment,
+home tinyint(3) unsigned default '0',
+unique key match_id (match_id),
+key match_id_2 (match_id)
+);
+insert into t1 values("1", "2");
+create table t2 (
+player_id tinyint(3) unsigned default '0',
+match_1_h tinyint(3) unsigned default '0',
+key player_id (player_id)
+);
+insert into t2 values("1", "5");
+insert into t2 values("2", "9");
+insert into t2 values("3", "3");
+insert into t2 values("4", "7");
+insert into t2 values("5", "6");
+insert into t2 values("6", "8");
+insert into t2 values("7", "4");
+insert into t2 values("8", "12");
+insert into t2 values("9", "11");
+insert into t2 values("10", "10");
+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	m	const	match_id,match_id_2	match_id	1	const	1	Using where
+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 UUX desc;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+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	Using where
+select s.*, '*', m.*, (s.match_1_h - m.home) UUX from 
+(t2 s left join t1 m on m.match_id = 1) 
+order by UUX desc;
+player_id	match_1_h	*	match_id	home	UUX
+8	12	*	1	2	10
+9	11	*	1	2	9
+10	10	*	1	2	8
+2	9	*	1	2	7
+6	8	*	1	2	6
+4	7	*	1	2	5
+5	6	*	1	2	4
+1	5	*	1	2	3
+7	4	*	1	2	2
+3	3	*	1	2	1
+explain select s.*, '*', m.*, (s.match_1_h - m.home) UUX from 
+t2 s straight_join t1 m where m.match_id = 1 
+order by UUX desc;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+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	Using where
+select s.*, '*', m.*, (s.match_1_h - m.home) UUX from 
+t2 s straight_join t1 m where m.match_id = 1 
+order by UUX desc;
+player_id	match_1_h	*	match_id	home	UUX
+8	12	*	1	2	10
+9	11	*	1	2	9
+10	10	*	1	2	8
+2	9	*	1	2	7
+6	8	*	1	2	6
+4	7	*	1	2	5
+5	6	*	1	2	4
+1	5	*	1	2	3
+7	4	*	1	2	2
+3	3	*	1	2	1
+drop table t1, t2;

mysql-test/t/join_outer.test

@@ -451,3 +451,53 @@ select * from t1 left join t2  on  b1 = a1 left join t3  on  c1 = a1  and  b1 is
 explain select * from t1 left join t2  on  b1 = a1 left join t3  on  c1 = a1  and  b1 is null;
 
 drop table t1, t2, t3;
+
+# Test for BUG#5088
+
+create table t1 (
+  match_id tinyint(3) unsigned not null auto_increment,
+  home tinyint(3) unsigned default '0',
+  unique key match_id (match_id),
+  key match_id_2 (match_id)
+);
+
+insert into t1 values("1", "2");
+
+create table t2 (
+  player_id tinyint(3) unsigned default '0',
+  match_1_h tinyint(3) unsigned default '0',
+  key player_id (player_id)
+);
+
+insert into t2 values("1", "5");
+insert into t2 values("2", "9");
+insert into t2 values("3", "3");
+insert into t2 values("4", "7");
+insert into t2 values("5", "6");
+insert into t2 values("6", "8");
+insert into t2 values("7", "4");
+insert into t2 values("8", "12");
+insert into t2 values("9", "11");
+insert into t2 values("10", "10");
+
+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;
+  
+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 UUX desc;
+
+select s.*, '*', m.*, (s.match_1_h - m.home) UUX from 
+  (t2 s left join t1 m on m.match_id = 1) 
+  order by UUX desc;
+
+explain select s.*, '*', m.*, (s.match_1_h - m.home) UUX from 
+  t2 s straight_join t1 m where m.match_id = 1 
+  order by UUX desc;
+
+select s.*, '*', m.*, (s.match_1_h - m.home) UUX from 
+  t2 s straight_join t1 m where m.match_id = 1 
+  order by UUX desc;
+
+drop table t1, t2;

sql/sql_select.cc

@@ -3215,6 +3215,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j, KEYUSE *org_keyuse,
 
   store_key **ref_key= j->ref.key_copy;
   byte *key_buff=j->ref.key_buff, *null_ref_key= 0;
+  bool keyuse_uses_no_tables= true;
   if (ftkey)
   {
     j->ref.items[0]=((Item_func*)(keyuse->val))->key_item();
@@ -3234,6 +3235,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j, KEYUSE *org_keyuse,
 
       uint maybe_null= test(keyinfo->key_part[i].null_bit);
       j->ref.items[i]=keyuse->val;		// Save for cond removal
+      keyuse_uses_no_tables= keyuse_uses_no_tables & !keyuse->used_tables;
       if (!keyuse->used_tables &&
 	  !(join->select_options & SELECT_DESCRIBE))
       {					// Compare against constant
@@ -3273,7 +3275,7 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j, KEYUSE *org_keyuse,
     j->type= null_ref_key ? JT_REF_OR_NULL : JT_REF;
     j->ref.null_ref_key= null_ref_key;
   }
-  else if (ref_key == j->ref.key_copy)
+  else if (keyuse_uses_no_tables)
   {
     /*
       This happen if we are using a constant expression in the ON part
@@ -4062,7 +4064,7 @@ remove_const(JOIN *join,ORDER *first_order, COND *cond, bool *simple_order)
 	}
 	if ((ref=order_tables & (not_const_tables ^ first_table)))
 	{
-	  if (only_eq_ref_tables(join,first_order,ref))
+	  if (!(order_tables & first_table) && only_eq_ref_tables(join,first_order,ref))
 	  {
 	    DBUG_PRINT("info",("removing: %s", order->item[0]->full_name()));
 	    continue;