Hi Nicole,
As you aware, all the columns in a columnar table are sorted ( dictionary tables). An INNER JOIN(in generic terms) would perform more efficiently if the key columns of the tables are sorted in the same order(eg: PO number in item table and details table are sorted ascending) . In our case, it is sorted internally .Another thing is, you should not be concerned about the performance here, as there is no choice for you to use something else instead of INNER JOINS if you are supposed to do it 
Sreehari