Oracle outer join syntax (+): query with checking for null of non-null column uses wrong query plan and returns wrong result

create table t1 (a int default NULL);

create table t2 (a int not null);

insert into t1 values (1), (2), (3), (4), (5), (6), (NULL);

insert into t2 values (1), (4), (5), (6), (7);

select t1.*,t2.* from t1,t2 where t1.a=t2.a and isnull(t2.a)=1;

explain select t1.*,t2.* from t1,t2 where t1.a=t2.a and isnull(t2.a)=1;

select t1.*,t2.* from t1 left join t2 on t1.a=t2.a where isnull(t2.a)=1;

explain select t1.*,t2.* from t1 left join t2 on t1.a=t2.a where isnull(t2.a)=1;

select t1.*,t2.* from t1,t2 where t1.a=t2.a(+) and isnull(t2.a)=1;

explain extended select t1.*,t2.* from t1,t2 where t1.a=t2.a(+) and isnull(t2.a)=1;

drop table t1,t2;

select t1.*,t2.* from t1,t2 where t1.a=t2.a and isnull(t2.a)=1;

a	a

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

select t1.*,t2.* from t1 left join t2 on t1.a=t2.a where isnull(t2.a)=1;

a	a

2	NULL

3	NULL

NULL	NULL

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	

1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	Using where; Using join buffer (flat, BNL join)

select t1.*,t2.* from t1,t2 where t1.a=t2.a(+) and isnull(t2.a)=1;

a	a

explain extended 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	filtered	Extra

1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE

Warnings:

Note	1003	select "test"."t1"."a" AS "a","test"."t2"."a" AS "a" from "test"."t1" left join "test"."t2" on(multiple equal("test"."t1"."a", "test"."t2"."a")) where 0