I have an index on the two columns used in this query. Why would the optimizer choose 
an index full scan over an index fast full scan? 

My question isnt why an index is used, but the type of index scan? 

select *
    from (select col1, col2,
          dense_rank()
          over (partition by col1
                order by col2 desc)tab
          from mytable)
   where tab = 1

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