Linguistic indexes

Question

I'm using oracle 8.1.6.
I'd like to improve the performance of sorting using linguistic indexes.
I'have found an example on how to do this in the Oracle8i National Language Support Guide
(http://otn.oracle.com/docs/products/oracle8i/doc_library/817_doc/server.817/a76966/ch2.htm#92022)
Here it is:
ALTER SESSION SET query_rewrite_enabled=true;
ALTER SESSION SET NLS_COMP = ANSI;
ALTER SESSION SET NLS_SORT='FRENCH';
CREATE TABLE test(col VARCHAR(20) NOT NULL);
CREATE INDEX test_idx ON test(NLSSORT(col, 'NLS_SORT=FRENCH'));
SELECT * FROM test WHERE NLSSORT(col) IS NOT NULL ORDER BY col;
It produces the required output, but unfortunately it uses the following execution plan:
SELECT STATEMENT, GOAL = CHOOSE
SORT ORDER BY
TABLE ACCESS FULL PAN TEST
So the optimezer ignores to use the index.
Yes, I've read the entire section, the oprimizer is CBO and I've analyzed the index and the table.
Any idea how to make it work using the index?
Thanks in advance.
Franci

Answer 1

Unfortunately your hints doesn't work. I think there is something wrong or misconfigured at my index, so the cbo is not allowed to use it.
I have made some deeper research. These are the facts:
If I run the example using a binary index and NLS_SORT set to binary, then I got the proper execution plan:
SELECT * FROM test
WHERE NLSSORT(col) IS NOT NULL
ORDER BY col
SELECT STATEMENT, GOAL = CHOOSE                    
INDEX FULL SCAN     TEST_IDX_BIN
Fine this is what I want.     
But, if I enter 120000 records to this table and run the same select statement, I got the following execution plan:
SELECT STATEMENT, GOAL = CHOOSE                    
SORT ORDER BY               
TABLE ACCESS FULL     TEST
Remember, there is 120000 records in the table, of course analyzed. So why the hell the CBO think ordering at runtime is a better solution?
If i switch to RBO and run the same query, i got the proper plan:
SELECT STATEMENT, GOAL = RULE                         INDEX FULL SCAN     TEST_IDX_BIN          
And here comes the best, the magic! :
if I modify my query, remove the where clause     
SELECT * FROM test
ORDER BY col
then I got the proper plan again in CBO mode:
SELECT STATEMENT, GOAL = CHOOSE                    
INDEX FULL SCAN     TEST_IDX_BIN
Somebody should explain me what the hell is going on!
So you can see, I found a way how to do a binary ordering, but linguistic ordering is still a question. I would really apriciate if someone could help me out.
I use pl/sql developer in single session mode. Can this be a problem? I don't think so, but who knows!
Thanks
Franci