Issue with using N'...' values in a where clause against a function index

Question

We have a table that is defined with non Unicode columns with a UPPER(..) function index on the index column (to allow searching in any alphabetic case)
e.g.
create table my_table
index_column varchar2(20),
desc_column varchar2(40)
create index my_table_idx as on my_table(UPPER(index_column));
there is approx. > 10 million rows in this table
The issue we have is that when we do the following select
select index_column, desc_column from my_table
where upper(index_column) = 'SOME VALUE'; this statement runs in approx 0.03 seconds which is great
But we also have some statements that run as
select index_column, desc_column from my_table
where upper(index_column) = N'SOME VALUE'; notice the N'...' string (unicode) value used. This ends up doing a full table scan (> 5 seconds)
So... the question is how can i make this select statement passing in a Unicode string value hit this function based UPPER index? Is there anyway?
I have tried these extra indexes - to no avail
create index my_table_idx as on my_table(UPPER(CAST(index_column as nvarchar2(20)));
and
create index my_table_idx as on my_table(UPPER(COMPOSE(index_column)));
I assumed Oracle should have done and implicit conversion back to a non unicode string value and then passed that value into the UPPER(...) function index, but it appears as though Oracle isn't recognizing that the column is a different type (varchar2 vs nvarchar2)
Any help greatly appreciated?

Answer 1

Horrible amount of irrelevant tags, and the only thing relevant , the four digit version, is of course not mentioned.
Also the characterset of the database is relevant.
As far as I know in 11g and higher one no longer needs the N'<string construct>'
As to implicit conversion
assume <number_column> = '9'
Oracle always converts this into
to_char(<number_column>='9'
Same applies to your case.
Try leaving out the N, check whether it works and whether your function based index is used.
Sybrand Bakker
Senior Oracle DBA