There are two undo modes in oracle 12.2 Multitenant database

1. Local undo mode
2. Shared undo mode

Local undo mode:

In this mode, each container ( i.e PDB ) in multitenant will have their own active undo tablespace.

Share Undo mode:

In this mode, There will be only one undo tablespace for the instance.

NOTE – In previous releases, undo was in shared mode.
NOTE – From oracle 12.2, Oracle recommends to use local undo mode only.

How to check the current undo mode:

Convert local undo mode to shared undo mode:

After converting to shared undo mode, the individual undo tablespaces won’t be dropped automatically.
We have to drop them manually, by connecting to the respective database.

Convert shared undo mode to local undo mode:

Once local undo is enabled, individual undo tablespaces will be created automatically for each PDB.

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