v$archive_gap查不出东西来怎么办

原因可能是：
当备用数据库具有不同的化身（ 查V$DATABASE_INCARNATION ）时，可能会发生这种情况。
每当使用 RESETLOGS 选项（例如闪回、故障转移）打开数据库时，数据库都会创建一个新的化身

例如：

解决方法1：

--v$archive_gap 的替代SQL

select USERENV('Instance'), high.thread#, low.lsq, high.hsq
  from (select a.thread#, rcvsq, min(a.sequence#) - 1 hsq
          from v$archived_log a,
               (select thread#, resetlogs_change#, max(sequence#) rcvsq
                  from v$log_history
                 where (thread#, resetlogs_change#) in
                       (select thread#, max(resetlogs_change#)
                          from v$log_history
                         group by thread#)
                 group by thread#, resetlogs_change#) b
         where a.thread# = b.thread#
           and a.resetlogs_change# = b.resetlogs_change#
           and a.sequence# > rcvsq
         group by a.thread#, rcvsq) high,
       (select srl_lsq.thread#, nvl(lh_lsq.lsq, srl_lsq.lsq) lsq
          from (select thread#, min(sequence#) + 1 lsq
                  from v$log_history, v$datafile
                 where checkpoint_change# <= next_change#
                   and checkpoint_change# >= first_change#
                   and enabled = 'READ WRITE'
                 group by thread#) lh_lsq,
               (select thread#, max(sequence#) + 1 lsq
                  from v$log_history
                 where (select min(checkpoint_change#)
                          from v$datafile
                         where enabled = 'READ WRITE') >= next_change#
                 group by thread#) srl_lsq
         where srl_lsq.thread# = lh_lsq.thread#(+)) low
 where low.thread# = high.thread#
   and lsq < = hsq
   and hsq > rcvsq;

解决方法2：
如果方法 1 的替代 SQL 仍然没有显示 GAP，请重新创建备用控制文件。这将清除 v$archived_log 中的当前归档日志。

在主库上：
alter database create standby controlfile as '/tmp/ctl_for_standby';
scp到备库上
备库上：
shu abort
改名
mv ctl_for_standby control01.dbf
startup mount
recover managed standby database disconnect;

参考
V$ARCHIVE_GAP doesn't show a existing GAP (Doc ID 974730.1)
Steps to recreate a Physical Standby Controlfile (Doc ID 459411.1)