null.test, null.result:

  Added test case for bug #4256.
join_outer.result:
  Fixed bug #4256.
item_cmpfunc.h:
  Fixed inconsistency of values of used_tables_cache and
  const_item_cache for Item_func_isnull objects.
  This inconsistency caused bug #4256.

mysql-test/r/join_outer.result

@@ -91,7 +91,7 @@ grp	a	c	id	a	c	d
 NULL	NULL		NULL	NULL	NULL	NULL
 explain select t1.*,t2.* from t1,t2 where t1.a=t2.a and isnull(t2.a)=1;
 Comment
-Impossible WHERE noticed after reading const tables
+Impossible WHERE
 explain select t1.*,t2.* from t1 left join t2 on t1.a=t2.a where isnull(t2.a)=1;
 table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	ALL	NULL	NULL	NULL	NULL	7	

mysql-test/r/null.result

@@ -109,3 +109,22 @@ a	b	c	d
 	0	0000-00-00 00:00:00	0
 	0	0000-00-00 00:00:00	0
 drop table t1;
+CREATE TABLE t1(i int, KEY(i));
+INSERT INTO t1 VALUES(1);
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+EXPLAIN SELECT * FROM t1 WHERE i=2 OR i IS NULL;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	range	i	i	5	NULL	10	Using where; Using index
+ALTER TABLE t1 CHANGE i i int NOT NULL;
+EXPLAIN SELECT * FROM t1 WHERE i=2 OR i IS NULL;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	range	i	i	4	NULL	7	Using where; Using index
+DROP TABLE t1;

mysql-test/t/null.test

@@ -79,3 +79,26 @@ INSERT INTO t1 (d) values (null),(null);
 select * from t1;
 drop table t1;
 
+# Test case for bug #4256
+
+CREATE TABLE t1(i int, KEY(i));
+
+INSERT INTO t1 VALUES(1);
+
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+INSERT INTO t1 SELECT i*2 FROM t1;
+
+EXPLAIN SELECT * FROM t1 WHERE i=2 OR i IS NULL;
+
+ALTER TABLE t1 CHANGE i i int NOT NULL;
+
+EXPLAIN SELECT * FROM t1 WHERE i=2 OR i IS NULL;
+
+DROP TABLE t1;

sql/item_cmpfunc.h

@@ -469,6 +469,7 @@ class Item_func_isnull :public Item_bool_func
     if (!args[0]->maybe_null)
     {
       used_tables_cache= 0;			/* is always false */
+      const_item_cache= 1;
       cached_value= (longlong) 0;
     }
     else