From 12.2 onwards, each container in an instance can use its own undo table space and this is called local undo mode. In previous release, all containers in an instance shared the same undo table space in multitenant set up.
Today I upgraded, container database from 12.1 -> 12.2 via DBUA and looked at the UNDO setting. Found that we have to manually switch the undo modes for upgraded instances. So here are the steps. Continue reading → Oracle 12R2 – Local Undo Switch