Tales From A Lazy Fat DBA

Happy Republic Day!

Hi Mates,

I see a lot of confusion, mix-ups and perplexity in between the DBAs on few of the metrics under ‘Instance Efficiency % section’ in AWR reports. Specially for one of the metric “Parse CPU to Parse Elapsd %”. In today’s post i will try to explain the metrics in detail which will help you to understand it in depth and clear the muddiness.

Few of the lines you might have read about this metric i.e.
“we should always look for as low as possible numbers for this metric …”
“Try to achieve the impossible value of zero for this one “
– Even some of the Metalink notes are misleading too.

Scenario:

Below is the snippet from one of my test box.

Note: Just taking a look at the instance efficiency ratios can be very dangerous and i advise to first start with Load Profile, top 5 waits and there on …

Okay, so my definition of this statistic differ from what you judge after reading the name of the metric – This datum signals the delay/wait in parsing of SQL queries during the snap interval.
In our example the value is 1.37% this means that for every CPU second spend parsing we spent about 72.99 (100/1.37) Seconds of clock time. It can happen due to various reasons i.e Latch or any contention between the sessions etc.

The ideal value for this stat should also be 100% like rest of the ratios (Yes, that’s correct!!).

Let’s see how this value was calculated, what all it considers while deducing that final figure of 1.37%.
It takes the “parse time cpu/parse time elapsed * 100” to get the ‘Parse CPU to Parse Elapsed’ figure.

Statistic	                              Total	per Second     per Trans
------------------------------------------------------------------------------------------------
parse time cpu                                  398	      0.11	0.01
parse time elapsed                           29,055	      8.07	0.81

So, in short each time there is a drop in this metric from 100%, means the database was waiting for something which slowed down the parse times.
If you want to dig in deep then you should trace the session using 10046 tracing with level 8 to see where the other % of parse time is being spent and leaving this for readers to test.

Hope It Helps
Prashant Dixit