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This query returns the top 5 wait events for the last hour from the v$active_session_history view.
Be careful, this view is part of the diagnostic pack, you should not query this view if you not licensed for it.
Top 5 wait events from v$active_session_history
select * from (
count(sample_time) as EST_SECS_IN_WAIT
where sample_time between sysdate - interval '1' hour and sysdate
group by WAIT_CLASS,EVENT
order by count(sample_time) desc
where rownum <6
This is obviously an approximation, because v$active_session_history contains only 1 second samples, and who knows what happens during each second sample. If you compare the time given by this query with information from v$system_event it will not exactly match but you should be close if you choose a sufficiently long period.