ORACLE如何根据ITL找到前镜像的

SQL> select * from emp where sal=800;
EMPNO ENAME      JOB         MGR HIREDATE          SAL      COMM DEPTNO
----- ---------- --------- ----- ----------- --------- --------- ------
 7369 SMITH      CLERK      7902 12/17/1980     800.00               20
1 rows selected
SQL> select dbms_rowid.rowid_relative_fno(rowid)file_id,dbms_rowid.rowid_block_number(rowid)block_id from emp  where sal=800;
   FILE_ID   BLOCK_ID
---------- ----------
         5        204
SQL> update emp set sal=1000 where sal=800;
1 row updated
SQL> alter system dump datafile 5 block 204;
系统已更改。
SQL> select  d.value||'/'||lower(rtrim(i.instance, chr(0)))||'_ora_'||p.spid||'.trc' trace_file_name
  2              from
  3              ( select p.spid
  4              from sys.v$mystat m,sys.v$session s,sys.v$process p
  5              where m.statistic# = 1 and s.sid = m.sid and p.addr = s.paddr) p,
  6              ( select t.instance from sys.v$thread  t,sys.v$parameter  v
  7              where v.name = 'thread' and (v.value = 0 or t.thread# = to_number(v.value))) i,
  8              (