cheapest way to access every 15th row in table ordered by timestamp column

11.2.0.2 on Linux 5

We have a lab instrument recording information every 2 seconds into a table.

The scientists pull that data into excel for analysis (I hope to look at this 
excel spreadsheet in the very near future now that I've read chapter 9 of Pro 
Oracle SQL).

They are requesting a view that picks out every 15th row of data going back 30 
days.

Can anyone think of a more efficient way to do it than this:

SELECT x.col,
              x.t_stamp
      FROM ( SELECT col,
                                    t_stamp
                                   rownum rn
                         FROM t
                        WHERE t_stamp >= TRUNC( SYSDATE - 30 )) x
     WHERE MOD( x.rn, 15 ) = 0;


It feels like there should be a better way to do it.

chris

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