V$FILE_HISTOGRAM displays a histogram of all single block reads on a per-file basis. The histogram has buckets of time intervals from < 1 ms, < 2 ms, < 4 ms, < 8 ms, ... < 2^21 ms, < 2^22 ms, >= 2^22 ms.
The histogram will not be filled unless the TIMED_STATISTICS initialization parameter is set to true.
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