Insights into Statistics, Query Optimization and the Oracle Optimizer

Upgrade to Oracle Database 12c and Avoid Query Regression

Nigel Bayliss
Product Manager

Those of you that made it to the demo grounds at Oracle Open World this year (it’s still just about 2015) will have had the chance to chat with the Oracle developers and throw lots of difficult questions at them! For everyone in the Optimizer development team it was a great opportunity to get lots of interaction and feedback, so thanks to all of you that took the time to come along and talk to us. We’re not all lucky enough to get a trip out to San Francisco, so I’ve been collating the main topics that came up to steer the subject matter of the next few blog posts. In this way I hope that we can all benefit from these interactions.

I can tell you right away that the number one demo ground question at OOW 2015 was …drum roll… “How can I reduce the risk of query regression when I upgrade to Oracle Database 12c?”. Well, maybe not those words exactly, but words to that effect. There is quite a lot of information out there on this topic, but people seem to struggle to find it… so we put our heads together and we realized that we should publish a 20,000ft view of this topic with pointers down into the detail. That’s the purpose of this post and, for the sake of brevity, I’m going to make the assumption that you are upgrading your Enterprise Edition database from Oracle Database 11g Release 2 to Oracle Database 12c.

The bottom line is this: if you want to mitigate the risk of query regression when you upgrade to Oracle Database 12c, then use SQL Plan Management (SPM). This is the recommended approach for the vast majority of systems out there, where the most critical SQL statements are reusable or, in other words, they are executed more than once.

Here are a couple of common scenarios:

Scenario 1 You want to use all Oracle Database 12c Optimizer features right away, but you need to “repair” any regressed queries quickly and with minimum effort.
Scenario 2 You’re upgrading and want to keep your “tried-and-tested”, Oracle Database 11g execution plans. Nevertheless, you do not want your application to be frozen in time: you want to evolve and use improved execution plans that are available in the new release, and you need to do this in a proven and controlled way.

Scenario 1

This is something you’ll want to think about before your production system goes live, particularly if you have not been able to test a realistic workload on all of your production data. It’s also very relevant if you are running a benchmark or proof of concept, where the time that’s available to resolve problem queries can be pretty limited (I’m using some understatement there!).

Ideally you will have captured SQL plan baselines before you’ve upgraded, because then you’ll have a set of “good” execution plans at-the-ready. It isn’t absolutely necessity to do this, though. As long as you can reproduce or find an example of a good plan, then this can be used to create a SQL plan baseline on-demand. For example, you may find a better plan:

  • By running the problem query in a pre-upgrade environment (remembering that you can export and import SQL plan baselines to copy them between databases).
  • Inside an existing SQL Tuning Set (STS).
  • By reproducing the good plan in the post-upgrade environment using (for example) “alter session set optimizer_features_enabled = 11…”, adjusting other Optimizer database parameters or by using hints. Yes, setting this parameter to an older version will give you the plan of the older version; that’s the whole purpose of it (and if it doesn’t work for you then it usually means that there’s a bug).

The next step is the clever part, but I get the impression that a lot of Oracle experts don’t know that it’s even possible. When you’ve found a good plan and captured the details in a SQL plan baseline, you can use SPM to associate it with a regressed SQL statement without having to change the existing query or the existing application code. For details, take a look in the section, “Creating an accepted plan by modifying the SQL text” in an earlier Optimizer blog entry and also page 24 of SQL Plan Management with Oracle Database 12c. In both cases, an improved SQL execution plan is found using a hint. This plan is associated with a regressed SQL statement so that, in future, the better plan is used.

Scenario 2

You should capture SQL Plan Baselines in your Oracle Database 11g environment and export them so that they can be imported into the upgraded database. If you are upgrading in-place, then existing SQL plan baselines will be available without the need to export and import them. If you neglected to capture baselines in the pre-upgrade environment, then you still have the option to capture 11.2 execution plans in an Oracle Database 12c environment by executing your queries in a session where the database parameter optimizer_features_enabled is set to “” (or whatever version you like).

Once SQL plan baselines are established in the upgraded database, you will enjoy plan stability while you get to know your system in the critical early days after the upgrade. Once you are happy with your shiny new database, you can evolve SQL plan baselines either automatically or at your own pace. Then you will gain advantage of all the new Optimizer features available in Oracle Database 12c.


SPM is an Oracle Database Enterprise Edition (EE) feature. You don’t need to buy an additional license to use it with EE. Optionally, you might choose to use SPM with SQL Tuning Sets (STS). If you do, then you will need to have purchased the Oracle Tuning Pack for Oracle Database (PDF), because STS requires the SQL Tuning Advisor.


Whenever you plan to upgrade, check out Oracle’s Database Upgrade Blog. It’s full of really great information and it will hook you up with the latest advice at the finest level of detail. For example, here are some useful specifics on SPM to get you started.

So now is the time to upgrade, unless you’re a retailer like Amazon who’s heating up its systems for the big Christmas push, or perhaps you’re heading into your end-of-year financial reporting. Nevertheless, even for you, the “now” is pretty close…

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