手动修改Oracle sga方法有很多种，这里只介绍其中一种，就是用oracle debug工具
SQL> alter database open read only;

Database altered.

SQL> col scn for 9999999999999999
SQL> select dbms_flashback.get_system_change_number scn from dual;

                  SCN
    —————–
       10999733447365

    SQL> oradebug setmypid
    Statement processed.
    SQL>  oradebug DUMPvar SGA kcsgscn_
    kcslf kcsgscn_ [060012658, 060012688) = 13346AC5 00000A01 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 60012338 00000000

    修改内存地址从0×060012658之后的4个字节，内容为0xffffffff
    SQL> oradebug poke 0×060012658 4 0xffffffff
    BEFORE: [060012658, 06001265C) = 13346AC5
    AFTER:  [060012658, 06001265C) = FFFFFFFF

SQL> oradebug DUMPvar SGA kcsgscn_
kcslf kcsgscn_ [060012658, 060012688) = FFFFFFFF 00000A01 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 60012338 00000000

可以看到已经修改成功
SQL> select dbms_flashback.get_system_change_number scn from dual;

                  SCN
    —————–
       11003706212351

    SQL> select current_scn scn from v$database;

                  SCN
    —————–
                    0

ordebug 手动修改Oracle sga scn