实验环境：OEL 5.7 + Oracle 10.2.0.5
Tips：该参数仅在特殊恢复场景下使用，需要在专业Oracle工程师指导下进行操作。
1.隐藏参数说明
查询隐藏参数"_allow_resetlogs_corruption"及说明：
set linesize 333
col name for a35
col description for a66
col value for a30
SELECT i.ksppinm name,
i.ksppdesc description,
CV.ksppstvl VALUE
FROM sys.x$ksppi i, sys.x$ksppcv CV
WHERE i.inst_id = USERENV ('Instance')
AND CV.inst_id = USERENV ('Instance')
AND i.indx = CV.indx
AND i.ksppinm LIKE '%&keyword%'
ORDER BY 1;
Enter value for keyword: allow_resetlog
old 8: AND i.ksppinm LIKE '%&keyword%'
new 8: AND i.ksppinm LIKE '%allow_resetlog%'
NAME DESCRIPTION VALUE
----------------------------------- ------------------------------------------------------------------ ------------------------------
_allow_resetlogs_corruption allow resetlogs even if it will cause corruption FALSE
通过这个隐藏参数非常规恢复的库，原则建议还是要重建库的。其实在alert日志中也会看到有这样的建议：
Wed Dec 26 00:00:41 CST 2018
alter database open resetlogs
Wed Dec 26 00:00:41 CST 2018
RESETLOGS is being done without consistancy checks. This may result
in a corrupted database. The database should be recreated.
2.故障场景再现
模拟常规开库失败的场景：
SQL> select checkpoint_change# from v$datafile_header;
CHECKPOINT_CHANGE#
------------------
10013731555
10014045643
10014045643
10014045643
10014045643
10014045643
10014045643
10014045643
10014045643
9 rows selected.
SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-01589: must use RESETLOGS or NORESETLOGS option for database open
SQL> alter database open resetlogs;
alter database open resetlogs
*
ERROR at line 1:
ORA-01194: file 1 needs more recovery to be consistent
ORA-01110: data file 1: '+ZHAOJINGYU/jy/datafile/system.256.839673875'
说明：这个环境是模拟数据文件1丢失，最终从备份restore出来一个旧的文件，但由于种种原因，总之没有后续的归档去做recover，导致无法追平。
此时就可尝试使用_allow_resetlogs_corruption隐藏参数强制开库：
SQL> alter system set "_allow_resetlogs_corruption" = true scope=spfile;
SQL> shutdown immediate
SQL> startup mount
SQL> alter database open resetlogs;
此时再去查询数据文件头的SCN已经一致：
SQL> select checkpoint_change# from v$datafile_header;
CHECKPOINT_CHANGE#
--------------------
10014022016
10014022016
10014022016
10014022016
10014022016
10014022016
10014022016
10014022016
10014022016
9 rows selected.
注意处理完毕后及时改回这个隐藏参数为false：
alter system set "_allow_resetlogs_corruption" = false scope=spfile;
其他注意事项：如果开库遇到ORA-600 [2662]类错误，可以参考之前随笔：
最终通过推进SCN的手段来解决ORA-600 [2662]类问题。
其实这个场景其实可能会遇到各种问题，都属于非常规恢复范畴，后续我会计划去继续测试验证一些常见场景及解决方案。