在做异地恢复数据库试验的时候，报错 ORA-38856，具体情况如下：
源库为Oracle RAC，2节点数据库 版本 11.2.0.4
目标库为单实例ORACLE， 版本 11.2.0.4
源库Rman备份为0级全备
在restore成功以后，recover不成功，需要用到原来的online log，重建控制文件，
然后 alter database open resetlogs，报错
SQL> alter database open resetlogs;
alter database open resetlogs
*
第 1 行出现错误:
ORA-38856: 无法将实例 UNNAMED_INSTANCE_2 (重做线程 2) 标记为启用
是一个Bug：
Oracle bug, 4355382 ORA-38856: FAILED TO OPEN DATABASE WITH RESETLOGS WHEN USING RAC BACKUP, exists in the
Oracle 10g release 2.0 that affects backups taken from a RAC database. The problem is related to the number of threads
used by the source database and an expectation that the cloned database must have an identical number of threads.
解决办法
1. Set the following parameter in the auxiliary init.ora file: _ no_recovery_through_resetlogs=TRUE.
2. Open the database in resetlogs mode.
3. Remove _no_recovery_through_resetlogs=TRUE from init.ora.
4. Restart database.