Why doesn't this query fail, when converting to_date

Question

This query runs and returns no rows.

SELECT DISTINCT TO_CHAR(START_DATE,'MM/DD/YYYY'),                             
                TO_CHAR(END_DATE,  'MM/DD/YYYY')                              
FROM                                                                          
(                                   
    SELECT START_DATE, END_DATE, END_DATE - START_DATE
    FROM
    (
        SELECT TO_DATE(SUBSTR(B,  1, 10), 'MM/DD/YYYY') START_DATE,               
               TO_DATE(SUBSTR(B, 14, 10), 'MM/DD/YYYY') END_DATE                  
        FROM (SELECT 'test date' b from dual)
    )
    WHERE END_DATE - START_DATE != 6
)

This small piece fails to run, due to the conversion error. [1]: ORA-01858: a non-numeric character was found where a numeric was expected

SELECT TO_DATE(SUBSTR(B,  1, 10), 'MM/DD/YYYY') START_DATE,               
       TO_DATE(SUBSTR(B, 14, 10), 'MM/DD/YYYY') END_DATE                  
FROM (SELECT 'test date' b from dual)

My expectation here was that the conversion error would cause an Oracle exception causing the program to fail out. There is something I don't know, or am not thinking about correctly.

Can someone point my nose in the correct direction on this one?

Thanks. Evil.

EDIT - NULL is handled differently with to_date. Oracle: Avoiding NULL value in to_date

EDIT - Plan

3 SELECT STATEMENT  ALL_ROWS
Cost: 3  Bytes: 0  Cardinality: 1  
Partition #: 0          
    2 FILTER  
    Cost: 0  Bytes: 0  Cardinality: 0  
    Partition #: 0      
        1 FAST DUAL  
        Cost: 2  Bytes: 0  Cardinality: 1  
        Partition #: 0 

EDIT - I am running 10g.

Answer 1

OK, I figured this out because it was driving me crazy. If you simplify the query to:

SELECT START_DATE, END_DATE, END_DATE - START_DATE
FROM
(
    SELECT TO_DATE(SUBSTR(B,  1, 10), 'MM/DD/YYYY') START_DATE,               
           TO_DATE(SUBSTR(B, 14, 10), 'MM/DD/YYYY') END_DATE                  
    FROM (SELECT 'test date' b from dual)
)
WHERE END_DATE - START_DATE != 6

The resulting execution plan (on 11gR2) looks like this:

Execution Plan
----------------------------------------------------------
Plan hash value: 4034615273

-----------------------------------------------------------------
| Id  | Operation        | Name | Rows  | Cost (%CPU)| Time     |
-----------------------------------------------------------------
|   0 | SELECT STATEMENT |      |     1 |     2   (0)| 00:00:01 |
|*  1 |  FILTER          |      |       |            |          |
|   2 |   FAST DUAL      |      |     1 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(TO_DATE(NULL)-TO_DATE('test date','MM/DD/YYYY')<>6)

So, the CBO has determined that the substring for END_DATE is null, and hence, the to_date will result in a null value also. The optimizer then does not evaluate the START_DATE expression, and the invalid data formatting error is not encountered. You can see the same behavior if you execute:

SELECT TO_DATE (NULL) - TO_DATE ('test date', 'MM/DD/YYYY') FROM DUAL

It will not fail.