SQL & PL/SQL

I have a problem with something I can't understand about indexes in Oracle 11g.

We can create test data with:

create table test2(field1 varchar2(100),field2 varchar2(100),field3 number,field4 varchar2(100));

create index test2_idx1 on test2(upper(field1));

create index test2_idx1b on test2(field1);

create index test2_idx2 on test2(field3);

DECLARE
j NUMBER :=1;
BEGIN
FOR i IN 1..500000
LOOP
INSERT
INTO test2
(field1,field2, field3, field4)
VALUES
('field1='||i,'a', j, 'i' );

IF (i mod 1000)=0 THEN
j := j+1;
END IF;
END LOOP;
COMMIT;
END;


EXEC DBMS_STATS.GATHER_TABLE_STATS ('system', 'test2');

Then I make some explain plans which result I can't understand

Query 1:

SELECT * FROM test2 WHERE field3=1;

Explain plan:

Explain plan for query 01

------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1003 | 28084 | 10 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST2 | 1003 | 28084 | 10 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | TEST2_IDX2 | 1003 | | 5 (0)| 00:00:01 |
------------------------------------------------------------------------------------------

Everything OK here. Index is used.

Query 2:

SELECT * FROM test2 WHERE upper(field1)='FIELD1=1';

Explain plan

------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 28 | 4 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST2 | 1 | 28 | 4 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | TEST2_IDX1 | 1 | | 3 (0)| 00:00:01 |
------------------------------------------------------------------------------------------

Everything OK again. Index is used.

Query 3:

SELECT /*+ USE_CONCAT */ * FROM test2 WHERE field1='FIELD1=1' OR field3=1;

Explain plan

--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1004 | 28112 | 14 (0)| 00:00:01 |
| 1 | CONCATENATION | | | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| TEST2 | 1 | 28 | 4 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | TEST2_IDX1B | 1 | | 3 (0)| 00:00:01 |
|* 4 | TABLE ACCESS BY INDEX ROWID| TEST2 | 1003 | 28084 | 10 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | TEST2_IDX2 | 1003 | | 5 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

Indenxes are used in concatenation. No problem again.

Query 4:

SELECT /*+ USE_CONCAT */ * FROM test2 WHERE upper(field1)='FIELD1=1' OR field3=1;

Explain plan

-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1004 | 28112 | 641 (4)| 00:00:08 |
| 1 | CONCATENATION | | | | | |
|* 2 | TABLE ACCESS FULL | TEST2 | 1 | 28 | 631 (4)| 00:00:08 |
|* 3 | TABLE ACCESS BY INDEX ROWID| TEST2 | 1003 | 28084 | 10 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | TEST2_IDX2 | 1003 | | 5 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------

Here my problem arises. Why is test2_idx1 not being used? Is it because it is a function index? Is there any workaround in this cases?

Thanks a lot in advance.