Last January I wrote a post with the following content:
Upgrading critical applications can be very difficult. One of the main problems is that for reasons of availability, long downtimes cannot be periodically scheduled. Therefore, for such applications, it is desirable to implement online upgrades. This requires that the application in question, as well as any software used by the application (e.g. the database engine) all support online upgrades. Oracle has recognized this problem for years. Unfortunately, up to and including Database 11g Release 1, only a limited number of features have been implemented for that purpose. As of Oracle Database 11g Release 2, this situation has changed greatly. With edition-based redefinition, Oracle Database offers real support for implementing online upgrades. The aim of the paper Edition-Based Redefinition, that I just put online here, is to provide an overview of this new feature.
I updated the aforementioned paper to provide information about the capability, as of 11.2.0.2, to specify a default edition at the service level. At the same time I added a note about the SHOW EDITION command.