Suppose we have a job submitted, interval is sysdate + 1 / 24 / 60, but the job is going to take 5 minutes, how will Oracle behave?
Looks like the sequence of events will be the following:
T0, Job #1 starts, the next run time is going to be sysdate(T0) + 1 minute.
T0 + 1 minute, The next job #2 starts, but is blocked by a QS lock, id2 in v$lock will tell the job id.
T0 + 5 minute, Job #1 completes, set sys.job$.next_date=greatest(T0 + 1/24/60, sysdate), effectively changing the this_date for job #2 to be sysdate. Job #2 starts.
Another issue is T0 is loosely guarenteed and could be off by couple of seconds, and even if the job completes within interval, the start time could drift if one uses sysdate only. To avoid drifting, use some calculation based on trunc(sysdate) instead of sysdate.
dbms_job, pended and drifting
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