Perform an "ALTER SEQUENCE ... NOCACHE" to unload the unused cached sequence numbers
from the Oracle library cache. This way, no cached numbers will be lost.
If you then select from the USER_SEQUENCES dictionary view, you will see the correct
high water mark value that would be returned for the next NEXTVALL call.
Afterwards, perform an "ALTER SEQUENCE ... CACHE" to restore caching.
You can use the above technique to prevent sequence number loss before a SHUTDOWN ABORT,
or any other operation that would cause gaps in sequence values.