Index not used on view when table stats exist

Question

Hello,
I would be grateful if someone comes with ideas on the following problem I'm currently facing.
I have a table with XMLTYPE data type column:
sql-->desc ACFBNK_STMT008
RECID     NOT NULL     VARCHAR2(200)
XMLRECORD XMLTYPE
I have a view V_ACFBNK_STMT008 on that table, in which the view columns are defined as extracted tags values from the XMLTYPE field, e.g. for the view field N_BOOKING_DATE:
numcast(extractValue(xmlrecord,'/row/c25')) "N_BOOKING_DATE"
(note: numcast is just a simple function that returns TO_NUMBER of its input argument)
I have also a function-based index on this field of the table:
CREATE INDEX train4.NIX_ACFBNK_STMT008_C25
ON train4.ACFBNK_STMT008("TRAIN4"."NUMCAST"(extractValue(xmlrecord,'/row/c25')))
And so, I'm executing on the view the following SQL statement:
SELECT RECID FROM V_ACFBNK_STMT008 WHERE (N_BOOKING_DATE > TO_NUMBER('20070725'));
Now, the problem comes: when statistics exist on the view base table (that is ACFBNK_STMT008) then the above statement is not using the index and is making a "table access full". When I delete the statistics for the table then the SQL runs fast with an "index range scan".
Which is further strange - when I change the ">" operand with a "=" the SQL statement correctly captures the index regardless of whether or not statistics exist.
I've tried to manually rewrite the SQL and include the "numcast" function in it:
SELECT RECID FROM TRAIN4.V_ACFBNK_STMT008 WHERE ( N_BOOKING_DATE>train4.numcast(TO_NUMBER( '20010725' ) ));
And in this way the index is used OK even with statistics existing!
But regretfully I don't have a way to change the application and the SQL, so the only things I can change is the view and/or the index.
Thank you in advance,
Evgeni
P.S.
I've tried gathering statistics in both the following ways but still the problem persists:
sql-->analyze table train4.ACFBNK_STMT008 compute statistics;
sql-->exec dbms_stats.gather_table_stats(ownname=>'TRAIN4', tabname=>'ACFBNK_STMT008', CASCADE=>TRUE, partname=>NULL);

Answer 1

Oh, and I forgot to mention: I cannot change the view definition as well (for example, to remove the "numcast"), since every now and then the application would recreate it automatically with the same code. :(