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I suspect that the Tuning Advisor is wrong here, sys_op_undescend and sys_op_descend are internal Oracle functions that the optimizer typically applies to indexes that were created as descending:
so those functions are applied in order for the optimizer to be able to use that index.
create index i on t (col) descending
You should actually look at the explain plan for that query to see what is actually doing. Richard Foote has a good explanation here http://richardfoote.wordpress.com/category/descending-indexes/