join.result, select.result:

  Fixed bug #4976.
join_nested.result, join_nested.test:
  Added a test case for bug #4976.
sql_select.cc:
  Applied conversion from an outer join to an inner join 
  when the on expression does not depend on the outer table.
  It fixed bug #4976.

mysql-test/r/join.result

@@ -59,11 +59,9 @@ id	count(t2.id)
 107	1
 select t1.id,t2.id from t2 left join t1 on t1.id>=74 and t1.id<=0 where t2.id=75 and t1.id is null;
 id	id
-NULL	75
 explain select t1.id,t2.id from t2 left join t1 on t1.id>=74 and t1.id<=0 where t2.id=75 and t1.id is null;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	const	PRIMARY	NULL	NULL	NULL	1	Impossible ON condition
-1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	Using where
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 explain select t1.id, t2.id from t1, t2 where t2.id = t1.id and t1.id <0 and t1.id > 0;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables

mysql-test/r/join_nested.result

@@ -1208,3 +1208,22 @@ SELECT * FROM t1 LEFT JOIN t2 LEFT JOIN t3 ON t2.a=t3.a ON t1.a=t3.a;
 a	a	a
 1	NULL	NULL
 DROP TABLE t1,t2,t3;
+CREATE TABLE t1(a int, key (a));
+CREATE TABLE t2(b int, key (b));
+CREATE TABLE t3(c int, key (c));
+INSERT INTO t1 VALUES (NULL), (0), (1), (2), (3), (4), (5), (6), (7), (8), (9),
+(10), (11), (12), (13), (14), (15), (16), (17), (18), (19);
+INSERT INTO t2 VALUES (NULL), (0), (1), (2), (3), (4), (5), (6), (7), (8), (9),
+(10), (11), (12), (13), (14), (15), (16), (17), (18), (19);
+INSERT INTO t3 VALUES (0), (1), (2), (3), (4), (5);
+EXPLAIN SELECT a, b, c FROM t1 LEFT JOIN (t2, t3) ON c < 3 and b = c;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t3	index	c	c	5	NULL	6	Using where; Using index
+1	SIMPLE	t2	ref	b	b	5	test.t3.c	2	Using where; Using index
+1	SIMPLE	t1	index	NULL	a	5	NULL	21	Using index
+EXPLAIN SELECT a, b, c FROM t1 LEFT JOIN (t2, t3) ON b < 3 and b = c;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	range	b	b	5	NULL	3	Using where; Using index
+1	SIMPLE	t3	ref	c	c	5	test.t2.b	2	Using where; Using index
+1	SIMPLE	t1	index	NULL	a	5	NULL	21	Using index
+DROP TABLE t1,t2,t3;

mysql-test/r/select.result

@@ -2181,10 +2181,10 @@ a	a	a
 select * from (t1 as t2 left join t1 as t3 using (a)) left outer join t1 on t1.a>1;
 a	a	a
 1	1	2
-1	1	3
 2	2	2
-2	2	3
 3	3	2
+1	1	3
+2	2	3
 3	3	3
 select * from t1 left outer join (t1 as t2 left join t1 as t3 using (a)) on t1.a>1;
 a	a	a

mysql-test/t/join_nested.test

@@ -727,3 +727,23 @@ DELETE FROM t2;
 SELECT * FROM t1 LEFT JOIN t2 LEFT JOIN t3 ON t2.a=t3.a ON t1.a=t3.a;
 
 DROP TABLE t1,t2,t3;
+
+#on expression for a nested outer join does not depend on the outer table
+#bug #4976
+
+CREATE TABLE t1(a int, key (a));                                              
+CREATE TABLE t2(b int, key (b));                                               
+CREATE TABLE t3(c int, key (c));                                               
+
+INSERT INTO t1 VALUES (NULL), (0), (1), (2), (3), (4), (5), (6), (7), (8), (9),
+(10), (11), (12), (13), (14), (15), (16), (17), (18), (19);
+
+INSERT INTO t2 VALUES (NULL), (0), (1), (2), (3), (4), (5), (6), (7), (8), (9),
+(10), (11), (12), (13), (14), (15), (16), (17), (18), (19);
+
+INSERT INTO t3 VALUES (0), (1), (2), (3), (4), (5);
+ 
+EXPLAIN SELECT a, b, c FROM t1 LEFT JOIN (t2, t3) ON c < 3 and b = c;
+EXPLAIN SELECT a, b, c FROM t1 LEFT JOIN (t2, t3) ON b < 3 and b = c;
+
+DROP TABLE t1,t2,t3;

sql/sql_select.cc

@@ -6078,7 +6078,9 @@ simplify_joins(JOIN *join, List<TABLE_LIST> *join_list, COND *conds, bool top)
       table->embedding->nested_join->not_null_tables|= not_null_tables;
     }
    
-    if (!table->outer_join || (used_tables & not_null_tables))
+    if (!table->outer_join || (used_tables & not_null_tables) ||
+        (table->outer_join &&
+         !(table->on_expr->used_tables() & ~used_tables)))
     {
       /* 
         For some of the inner tables there are conjunctive predicates