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Caution – hints

Here’s a little example of why you should be very cautious about implementing undocumented discoveries. If you take a look at the view v$sql_hints in 11.2.0.4 you’ll discover a hint (no_)cluster_by_rowid; and if you look in v$parameter you’ll discover two new parameters _optimizer_cluster_by_rowid and _optimizer_cluster_by_rowid_control.

It doesn’t take much imagination to guess that the parameters and hint have something to do with the costs of accessing compressed data by rowid on an Exadata system (see, for example, this posting) and it’s very easy to check what the hint does:

Upgrades to 11g are finally complete

Just a little slice of reality to cut through all the 12c stuff that is floating around at the moment. I’ve just moved the last of our databases to 11g. Yay! As well as upgrading, we’ve been culling or consolidating old and unused stuff, which has drastically reduced and simplified our Oracle database landscape.

We currently have four projects running databases on HP-UX on Itanium (spit), one project on Solaris and the rest on Oracle Linux under VMware. If I had my way we would kick out HP-UX and Solaris and do everything on Oracle Linux.

We’ve still got one project on 11gR1, but that is being held back intentionally because of some issues with the vendor of the application that runs against it. Hopefully that will soon be on 11gR2 also.

OTN Yathra 2014

I’ve been a little quiet on the blog lately, mostly because I’ve been spending my time freaking out about the arrangements for the OTN Yathra 2014 tour of India. I had to apply for a visa, which in itself was not too bad, but I spent quite a long time without a passport, thanks to the courier not bothering to attempt a delivery. All that time I didn’t know if my visa application had been successful, so I wasn’t sure if I would have to submit it again. By the time that arrived, it was so late that booking the flights became interesting… At one point I wrote to Debra Lilley and said words to the effect of, I think this is just not meant to be!

Well tonight (Tuesday evening) I got my flights confirmed for my trip that I start on Sunday. :)

Row Migration

At one of the presentations I attended at RMOUG this year the presenter claimed that if a row kept increasing in size and had to migrate from block to block as a consequence then each migration of that row would leave a pointer in the previous block so that an indexed access to the row would start at the original table block and have to follow an ever growing chain of pointers to reach the data.

This is not correct, and it’s worth making a little fuss about the error since it’s the sort of thing that can easily become an urban legend that results in people rebuilding tables “for performance” when they don’t need to.

RAC Plans

Recently appeared on Mos – “Bug 18219084 : DIFFERENT EXECUTION PLAN ACROSS RAC INSTANCES”

Now, I’m not going to claim that the following applies to this particular case – but it’s perfectly reasonable to expect to see different plans for the same query on RAC, and it’s perfectly possible for the two different plans to have amazingly different performance characteristics; and in this particular case I can see an obvious reason why the two nodes could have different plans.

Here’s the query reported in the bug:

IOT Catalogue

I’ve just added a catalogue of Richard Foote’s articles on IOTs to the list I made a couple of years ago of Martin Widlake’s articles, so this is just a temporary note to point people to the updated list.

Hotsos Symposium 2014

After missing last year’s Hotsos Symposium (trying to cut my travel as you know :), I will present at and deliver the full-day Training Day at this year’s Hotsos Symposium! It will be my 10th time to attend (and speak at) this awesome conference. So I guess this means more beer than usual. Or maybe less, as I’m getting old. Let’s make it as usual, then :0)

12c fixed subquery

Here’s a simple little demonstration of an enhancement to the optimizer in 12c that may result in some interesting changes in execution plans as cardinality estimates change from “guesses” to accurate estimates.

Minus

Here’s a little script to demonstrate an interesting observation that appeared in my email this morning (that’s morning Denver time):

The only way is automation! (update)

I was a little surprised by the reaction I got to my previous post on this subject. A number of people commented about the problems with automation and many pointed to this very appropriate comic on the subject.

There are one of two conclusions I can draw from this.

  1. My definition of automation of tasks is very much different to other people’s.
  2. It is common for DBAs and middle tier administrators to do everything by hand all the time.

I’m really hoping the answer is option 1, because I think it would be really sad if being a DBA has degenerated to the point where people spend their whole life doing tasks that could be easily scripted.