Database Reference
In-Depth Information
Being able to downgrade
One of the aspects rarely taken into consideration is the fact that there might be a point in time after the migration that
your senior management decides that the upgrade-although technically a success-was a failure and you need to get
back to square one from where you started. The most likely reason for this almost worst-case scenario is that a critical
process has not been sufficiently tested.
Whatever the reason, for you to be able to downgrade to the previous release you must not have upgraded the
“compatible” initialization parameter. As soon as you set this to the higher release you are in the world of point-in-time
restores or “forward fixing” (and little sleep). Data that has been entered into the system is then lost, which is most
likely not what you want. Downgrades to the previous release are possible, and most users will consider them for the
database stack. Downgrades of Clusterware, including Oracle Restart are less common and often not needed. In most
rollback-cases the rollback has to be performed as part of the migration weekend, not a number of business days earlier.
If a downgrade is a real possibility in your organization then it has to be tested with the same rigorous quality
assurance as the upgrade. And since every upgrade and downgrade is different, you should test the process
thoroughly, documenting anomalies and the expected outcome so that the DBA in charge during the day is able to see
it through.
Summary
This chapter demonstrated possible methods to upgrade a database. Upgrading a database to version 12c is not very
different from upgrading in previous releases. The chapter walked you through 2 different upgrade paths. The first one
offered full control over the process using the manual method of upgrading. This method can potentially complete in
less time compared to previous versions of the database thanks to the parallel execution of scripts during the upgrade
process. Some database options such as Application Express extend this process significantly though, and not every
step in the migration process can be executed in parallel.
The next upgrade method uses the wizard driven Database Upgrade Assistant. It can make the upgrade a much
smoother process as it runs a lot of the checks prior to the upgrade entirely automatically and repeatedly. Once the
database has been upgraded it is possible to plug the non-CDB into a CDB as part of the consolidation process after
advancing the compatible parameter to 12c.