break on BTIME
select
       btime, event,
       round((time_ms_end-time_ms_beg)/nullif(count_end-count_beg,0),3) avg_ms
from (
select
       to_char(s.snap_time,'DD-MON-YY HH24:MI')  btime,
       e.event,
       total_waits count_end,
       time_waited_micro/1000 time_ms_end,
        lag (e.time_waited_micro/1000)
              OVER(partition by e.event ORDER BY s.snap_id) time_ms_beg,
        lag (e.total_waits)
              OVER(partition by e.event  ORDER BY s.snap_id) count_beg
from
       STATS$SYSTEM_EVENT e,
       STATS$SNAPSHOT s
where
         s.snap_id=e.snap_id-1
   and e.event in
(
                  'db file sequential read',
                  'db file scattered read',
                  'direct path read',
                  'direct path read temp',
                  'direct path write',
                  'direct path write temp',
                  'log file sync',
                  'log file parallel write'
) and s.snap_id between &1 and &2
order by snap_time
)
order by btime
/