Parallel not working in peak hours

Dear Exprerts,
I put the parallel on select statement to spool the file.
select /*+ parallel(a,16) full(a) */ * from temp a;In non peak hours it took 1 hours to complete but in peak hours it took 6 hours to compete.
In peak hours also query was running in 16 session.
What can be the reason?
Regards
Rahul Malik

Any number of reasons - contention for CPU, slower IO....
What's the point in guessing?
Trace it.
If you're on a recent version of Oracle and licensed for Diagnostic+Tuning pack, look at the active session history data or use real time sql monitoring.
But is there a conceptual issue here?
Parallel query is a great feature for using more of the available resources to do the work. So if your database is normally idle then great - you can use some of those otherwise idle resources to divide and conquer. If during these "peak hours" there are more sessions, more concurrent queries and/or just more contention for these resources, parallel isn't going to be as effective, in fact it can just make things worse.

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