Is this a Bug or Feature of 10g?

Question

Below SQL works fine on my 9i db, SELECT returns 3 rows.
After migration to 10g same SELECT returns 0 rows.
I can see why 10g maybe returning 0 rows, maybe because t2 table definition says NOT NULL, but select looks for NULL on the column.
Anyone know the TRUE reason behind this behavior?
==
My 9i version is 9.2.0.5.0 (Redhat)
My 10g version is 10.2.0.1.0 (Redhat)
create table t1 (
     id number primary key,
     name varchar2(32)
insert into t1 values (1, 'Apple');
insert into t1 values (2, 'Pear');
insert into t1 values (3, 'Banana');
create table t2 (
     id number primary key,
     foo_exp number not null
select * from t1 where id NOT IN(select id from t2 where foo_exp is null);

Answer 1

Assuming that the documentation correctly stated that a select with NOT IN and item-less subquery list (no rows returned) should return all rows, and it does makes sense, I simply tried:
SQL> select * from t1 where id NOT IN (select id from t2 where foo_exp is null);
no rows selected
Execution Plan
Plan hash value: 3155678272
| Id  | Operation           | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT    |             |     1 |    35 |     0   (0)|          |
|*  1 |  FILTER             |             |       |       |            |          |
|   2 |   NESTED LOOPS ANTI |             |     2 |    70 |     3   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| T1          |     3 |    27 |     3   (0)| 00:00:01 |
|*  4 |    INDEX UNIQUE SCAN| SYS_C006836 |     1 |       |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter(NULL IS NOT NULL)
   4 - access("ID"="ID")The filter operation 1 kills the result set/row source.
A look on MetaLink quickly discovered that this is a known bug in 10.2, and (as you've actually found out) it is fixed in 10.2.0.3. See Note:4622729.8 about the Bug, caught from Patch Set list of bug fixes Note:391116.1.
So, it's upgrade time! :)