V$EVENT_HISTOGRAM displays a histogram of the number of waits, the maximum wait, and total wait time on an event basis. The histogram has buckets of time intervals from < 1 ms, < 2 ms, < 4 ms, < 8 ms, ... < 221 ms, < 222 ms, and >= 222 ms.
The histogram will not be filled unless the TIMED_STATISTICS initialization parameter is set to true.
Name of the Event
Amount of time the bucket represents (in milliseconds). If the duration = num, then this column represents waits of duration < num that are not included in any smaller bucket.
Number of waits of the duration belonging to the bucket of the histogram
Indicates the last time the bucket was updated (the ending timestamp of the last wait falling into the bucket's duration)