Automatic Collection of Fixed Objects Statistics in 12c

In my previous life as the Optimizer Lady, I wrote a blog on the importance of gathering fixed object statistics, since they were not originally gathered as part of the automatic statistics gather task.

Starting with Oracle Database 12c Release 1, Oracle will automatically gather fixed object statistics as part of automated statistics gathering task, if they have not been previously collected.Does that mean we are off the hook then?

The answer (as always) is it depends!

Let me begin by explaining what we mean by I the term “fixed objects”.

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Should you gather System Statistics?

While at the HotSOS Symposium, last month, I caused quite a stir when I recommended that folks should never gather system statistics.

Why such a stir?

It turns out this goes against what we recommend in the Oracle SQL Tuning Guide, which says “Oracle recommends that you gather system statistics when a physical change occurs in the environment”.

So, who right?

Well in order to figure that out, I spoke with Mohamed Zait, the head of the optimizer development team and Nigel Bayliss, the product manager for the optimizer, upon my return to the office.

After our discussions, Nigel very kindly agreed to write a detailed blog post that explains exactly what system statistics are, how they influence the Optimizer, and provides clear guidance on when, if ever, you should gather system statistics!

What did I learn from all this?

Don’t gather system statistics unless you are in a pure data warehouse environment, with a good IO subsystem (e.g. Exadata) and you want to encourage the Optimizer to pick more full table scans and never says never!