Archive for April 2013

E4 2013 – Exadata Conference Call for Papers Closing

 

Just a quick note to remind you that the call for papers for E4 is closing in a few days (on April 30). ┬áSo if you have anything you think is interesting related to Exadata that you’d like to share we’d love to hear from you. By the way, you don’t have to be a world renowned speaker on Oracle technology to get accepted. You just need to have an interesting story to tell about your experience. Implementations, migrations and consolidation stories are all worthy of consideration. Any interaction between Exadata and Big Data platforms will also be of great interest to the attendees. Of course the more details you can provide the better.

Here’s a link to the submission page:

Submit an Abstract for Enkitec’s Extreme Exadata Expo

Feel free to shoot me an email if you have ideas for a talk that you want to discuss before formally submitting an abstract.

Bad Advice?

I got an email a few days ago . . .

Subject: Exadata / your blogs / you aren’t saying to put HINTS on everything, Are you…

. . . A peer of mine found a couple of your blogs listed below which discuss Profiles and Hints. The position he is inferring upon you is that you are stating unequivocally that we should create lots of indexes, add hints, create profiles and outlines on our Exadata machine. Having read your book and achieve actual results in line with that book, I know that pretty much the opposite is true. . . .

Here’s what I said in my reply (edited a bit to clean up typos and to be a bit more precise):

I do not advocate widespread usage of hints or hint based mechanisms (Outlines, Patches, Profiles). However, because I have written quite a bit about using these techniques, I fear I may have inadvertently left people with the impression that I think hints should be used everywhere. I do not. I view them as expedient ways to change plans or lock in specific plans that I want, but in most cases I use them as bandaids to temporarily cover up other problems. I haven’t really written a blog post specifically on my view of the proper usage of these techniques, so maybe I should do that. When I do presentations on hint based mechanisms to control execution plans, I always have a slide with a picture of a vial of morphine. I use that metaphor to caution people not to over use them. There is a presentation on my blog in the whitepapers/presentations section called “Controlling Execution Plans” (or something to that effect) that you can download which has that slide in it. Also I wrote a chapter on controlling execution plans in a book called Pro Oracle SQL that probably has some prose on when and why to use them (and not use them). But in general – I think they should only be used to give temporary relief until underlying problems can be fixed or in cases where the optimizer just can’t deal with a specific situation (like correlated columns for example) or in cases where you’re using a packaged app that has badly written code that you can’t change. The newest version of the optimizer (which should be released soon) actually stores plans and not just hints with Baselines, so I will leave out the case where you want to manage plans for stability sake.

Since you mentioned Exadata I’ll add that the issue of mixed workload DB’s on Exadata and when the optimizer uses indexes vs when it doesn’t is not straight forward. In general, the current version of the optimizer doesn’t know it’s running on Exadata so it tends to cost FTS’s too high, meaning it often picks indexes when a full scan would be faster. There are some things that can be done to push the optimizer towards full scans. Think of anything that pushes the optimizer in that direction – large db_file_multiblock_read_count for example. Also, System Stats has recently been updated to have an “Exadata” mode which basically just updates the MBRC setting to 128 which does the same sort of thing. If that’s not enough to get most of the statements behaving the way you want then exceptional measures can be taken such as making indexes invisible and using alter session for specific processing to enable the optimizer to see invisible indexes, or doing away with some indexes altogether. Hints (or hint based mechanisms) can be used as well. But my goal would be to get the optimizer to pick the right plan on it’s own in the vast majority of cases before resorting to the more extreme measures.

Hope this helps.

Here’s a copy of the slide I referenced in the email and have been using recently:

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