I don’t have a clear answer for that. There is definitely an overhead in SQL Trace but I am not able to provide a concrete figure. Turning it on will affect the time measured so the time of a SQL with SQL Trace turned on is not comparable to another SQL without SQL Trace.
The Oracle documentation stated this:
“The additional overhead of running the SQL Trace facility against an application with performance problems is normally insignificant compared with the inherent overhead caused by the application’s inefficiency.”
This is a fair statement. If your SQL has performance problem. The overhead is relative insignificant and should be worth spending. But on the other side, you won’t want to turn on SQL Trace without the need as there is an overhead to collect the statistics and also to write to the trace file. That is why we have it in the Options so user can turn it on only if needed.
One thing worth to note is that the SQL Trace is on session level so it would have the minimal impact on other sessions.
Hope this answered your question.