parallel, degrees and instances

create table
PARALLEL ( DEGREE DEFAULT INSTANCES DEFAULT )

The Degree 'default value' is worked out thus :-
number of CPUs (cores) * number_of_thread_per_cpu (from v$parameter)

Instances is the number of nodes in the RAC that you want it to use

We hit a problem with a query going 'massively paralle' because the server had 16 cpus
, the number_of_thread_per_cpu was2 and the number of nodes was 2
We therefore ended up with something close to 64 parallel processes.

unusable columns and exchange partitions

column mismatch during exchange may be caused by unusable columns.

http://itnewscast.com/ora14097-column-type-or-size-mismatch-alter-table-exchange-partition