In 12c databases, ORA-12805, ORA-07445 [qertqistart] or ORA-600  errors may occur depending on bug 25694206. User operation cannot be completed due to the closing of parallel processes.
The problem is corrected in version 126.96.36.199, and the following workraound can be applied to the problem.
To resolve the problem, the parameter “_px_wif_extend_distribution_keys” must be set to false.
SQL> alter system set "_px_wif_extend_distribution_keys"=false scope=both sid='*';