If I have two undo tablespace, under what situation oracle will use another

Question

if I have two undo tablespaces, under what situation oracle will use another
one is default undo tablespace, another is created manually
will it use it when the default one is full?
it seem like to be it won't use the second undo tablespace.
thanks
Edited by: user11402556 on Jul 3, 2010 2:18 PM

Answer 1

You can have two Undo Tablespaces created with the CREATE UNDO TABLESPACE syntax.
However, the current database instance will use the Undo Tablespace specified by the instance level parameter undo_tablespace.
When you want to switch the datafiles of an undo tablespace from, say, one mount point to another (or when you want to "resize" an undo tablespace that has grown very large for specific one-off / adhoc jobs), you would create another undo tablespace, reset the instance level parameter to the new undo tablespace and wait for a decent interval (e.g. at least undo_retention or autotuned undo retention) before dropping the old undo tablespace.
So yours is not a hypothetical question. Having two undo tablespaces is a real-world practice -- but the two are concurrently present for only a short duration, while transactions and queries switch from one to the other.
(In RAC, each database instance must have it's own Undo tablespace as well).
Hemant K Chitale