AWR Reports: Don’t Trust the Numbers

After implementing a recent change, and I pulled of a quick AWR report to compare performance.  At first glance I was impressed by significant improvement in “log file parallel write” event, which had halved from 2 millisecond to 1 millisecond, but something about the numbers didn’t quite look right.

Before:
before

After:
after

Let’s calculate the average wait times ourselves:
Before:  2,346s/1,482,747=0.00158219844s=1.58 ms.
After:  2,658s/1,785,172=0.00148893215s=1.49 ms.

So AWR rounds to the nearest millisecond, and this has turned an actual 6% (=(1.58-1.49)/1.58*100)  improvement into a 100% improvement.

Note that in Oracle 12.2 AWR handles this much better by reporting wait events in microseconds, one more reason to upgrade!

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