join_outer.result, null.result, null.test:

  Fixed inconsistency of values of used_tables_cache
  and const_item_cache for Item_func_isnull objects (bug #1990).
item_cmpfunc.h:
  Fixed inconsistency of values of used_tables_cache
  and const_item_cache for Item_func_isnull objects (bug #1990).

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;
 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
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 explain select t1.*,t2.* from t1 left join t2 on t1.a=t2.a where isnull(t2.a)=1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	7	

mysql-test/r/null.result

@@ -142,3 +142,14 @@ 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 (a int not null, b int not null, index idx(a));
+insert into t1 values
+(1,1), (2,2), (3,3), (4,4), (5,5), (6,6),
+(7,7), (8,8), (9,9), (10,10), (11,11), (12,12);
+explain select * from t1 where a between 2 and 3;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	idx	idx	4	NULL	2	Using where
+explain select * from t1 where a between 2 and 3 or b is null;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	idx	idx	4	NULL	2	Using where
+drop table t1;

mysql-test/t/null.test

@@ -86,3 +86,14 @@ INSERT INTO t1 (d) values (null),(null);
 select * from t1;
 drop table t1;
 
+#
+# Test to check elimination of IS NULL predicate for a non-nullable attribute
+# (bug #1990)  
+#
+create table t1 (a int not null, b int not null, index idx(a));
+insert into t1 values
+  (1,1), (2,2), (3,3), (4,4), (5,5), (6,6),
+  (7,7), (8,8), (9,9), (10,10), (11,11), (12,12);
+explain select * from t1 where a between 2 and 3;
+explain select * from t1 where a between 2 and 3 or b is null;
+drop table t1;

sql/item_cmpfunc.h

@@ -753,6 +753,7 @@ public:
     if (!args[0]->maybe_null)
     {
       used_tables_cache= 0;			/* is always false */
+      const_item_cache= 1;
       cached_value= (longlong) 0;
     }
     else