文档解释
ORA-19955: only one open thread is allowed to change the DBID
Cause: The operation failed because there were active threads in the database. The most likely cause is that the database crashed the last time it was shut down.
Action: Ensure that all threads are closed before retrying the operation. Start and open the database to perform crash recovery, then shut down with the NORMAL or IMMEDIATE options to close it cleanly. Finally, try running the utility again.
