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Upgrade Argh

Time for another of those little surprises that catch you out after the upgrade.
Take a look at this “Top N” from a standard AWR report, from an instance running 11.2.0.2

Top 5 Timed Foreground Events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                           Avg
                                                          wait   % DB
Event                                 Waits     Time(s)   (ms)   time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
direct path read                  3,464,056       6,593      2   33.5 User I/O
DB CPU                                            3,503          17.8
db flash cache single block ph    2,293,604       3,008      1   15.3 User I/O
db file sequential read             200,779       2,294     11   11.6 User I/O
enq: TC - contention                     82       1,571  19158    8.0 Other

System Stats

A quick collation – and warning – for 11.2

Bottom line – be careful about what you do with system stats on 11.2

Footnote: the MOS link is a search string  producing a list of references. I set it up like that because one of the articles referencing the bug is called “Things to consider before upgrade to 11.2.0.2″ and it’s worth reading.

Addendum: one of the people on the two-day course I’ve just run in Berlin sent me a link for a quick note on how to set your own values for the system stats if you hit this bug. It’s actually quite a reasonable thing to do whether or not you hit the bug given the way that gathering the stats can produce unsuitable figures anyway:  setting system stats. (I’ve also added their company blog to the links on the right, they have a number interesting items and post fairly regularly.)