[20180627]truncate table的另类恢复.txt

Posted 2020-11-21 lfree

[20180627]truncate table的另类恢复.txt

--//前几天看链接http://www.xifenfei.com/2018/06/truncate-table-recovery.html
--//提到几种方式不影响数据的恢复,突然看到reuse storage选项,感觉在truncate时使用该参数时存储空间不会回收,
--//感觉这样应该还是会降低高水位标志.

--//使用reuse storage选项,存储空间不会回收,另外在truncate时,数据段号data_object_id会发生变化,自己想想是否
--//可以通过修改相应数据库的段号指向新的段号来实现恢复.自己动手测试看看.

1.环境:
[email protected]> @ ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

create table empx as select * from emp;

[email protected]> select rowid,empx.* from empx where rownum=1;
ROWID                   EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
------------------ ---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
AAAWFOAAEAAAALbAAA       7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20

[email protected]> @ rowid AAAWFOAAEAAAALbAAA
    OBJECT       FILE      BLOCK        ROW ROWID_DBA            DBA                  TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
     90446          4        731          0  0x10002DB           4,731                alter system dump datafile 4 block 731 ;

[email protected]> select object_id,data_object_id from dba_objects where owner=user and object_name=‘EMPX‘;
 OBJECT_ID DATA_OBJECT_ID
---------- --------------
     90446          90446

[email protected]> column PARTITION_NAME noprint
[email protected]> select * from dba_extents where owner=user and segment_name=‘EMPX‘;
OWNER  SEGMENT_NAME         SEGMENT_TYPE       TABLESPACE_NAME                 EXTENT_ID    FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------ -------------------- ------------------ ------------------------------ ---------- ---------- ---------- ---------- ---------- ------------
SCOTT  EMPX                 TABLE              USERS                                   0          4        728      65536          8            4

[email protected]> truncate table empx reuse storage;
Table truncated.

[email protected]> select object_id,data_object_id from dba_objects where owner=user and object_name=‘EMPX‘;
 OBJECT_ID DATA_OBJECT_ID
---------- --------------
     90446          90447

--//可以发现truncate后data_object_id=90447发生变化.

[email protected]> select * from dba_extents where owner=user and segment_name=‘EMPX‘;
OWNER  SEGMENT_NAME         SEGMENT_TYPE       TABLESPACE_NAME                 EXTENT_ID    FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------ -------------------- ------------------ ------------------------------ ---------- ---------- ---------- ---------- ---------- ------------
SCOTT  EMPX                 TABLE              USERS                                   0          4        728      65536          8            4

SCOTT[email protected]> alter system checkpoint;
System altered.

2.看看是否通过修改相应块的段号:

BBED> set dba 4,728
        DBA             0x010002d8 (16777944 4,728)

BBED> p /d ktbbh.ktbbhsid.
union ktbbhsid, 4 bytes                     @24
   ub4 ktbbhsg1                             @24       90446
   ub4 ktbbhod1                             @24       90446
--//可以发现truncate后数据段ID还是原来的90446,如果有数据插入段号就发生变化.我想通过修改段号实现数据恢复.

BBED> assign dba 4,731 ktbbh.ktbbhsid.ktbbhod1=90447;
Warning: contents of previous BIFILE will be lost. Proceed? (Y/N) y
ub4 ktbbhod1                                @24       0x0001614f

BBED> sum apply dba 4,731
Check value for File 4, Block 731:
current = 0x9cc2, required = 0x9cc2

3.检查是否能正常显示:
[email protected]> alter system flush buffer_cache;
System altered.

[email protected]> select rowid,empx.* from empx ;
no rows selected

--//不行!!看看直接使用rowid是否可以查询.
[email protected]> select rowid,empx.* from empx where rowid=dbms_rowid.rowid_create (1, 90447, 4, 731, 0) ;
ROWID                   EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
------------------ ---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
AAAWFPAAEAAAALbAAA       7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20

--//可以发现使用rowid可以查询对应记录.如果写成如下:
[email protected]> select rowid,empx.* from empx where rowid between dbms_rowid.rowid_create (1, 90447, 4, 731, 0) and dbms_rowid.rowid_create (1, 90447, 4, 731, 1);
no rows selected

--//因为执行计划是这样:
Plan hash value: 4633905
-------------------------------------------------------------------------------------
| Id  | Operation                   | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |      |        |       |     2 (100)|          |
|*  1 |  TABLE ACCESS BY ROWID RANGE| EMPX |      1 |    99 |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1 / [email protected]$1
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access(ROWID>=CHARTOROWID("DBMS_ROWID"."ROWID_CREATE"(1,90447,4,731,0)
              ) AND ROWID<=CHARTOROWID("DBMS_ROWID"."ROWID_CREATE"(1,90447,4,731,1)))
--//执行计划TABLE ACCESS BY ROWID RANGE,注意范围这个关键字.这样要访问段头,因为是truncate表后降低了高水位标识,
--//这样oracle认为要访问的数据块在高水位之上,这样就没有结果集.
--//而前面的执行计划是TABLE ACCESS BY USER ROWID.直接通过rowid访问块.越过了数据段头的访问.
[email protected]> @ &r/dpc ‘‘ ‘‘
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  3vzm2qyuft1dh, child number 0
-------------------------------------
select rowid,empx.* from empx where rowid=dbms_rowid.rowid_create (1, 90447, 4, 731, 0)
Plan hash value: 3191763077
------------------------------------------------------------------------------------
| Id  | Operation                  | Name | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |      |        |       |     1 (100)|          |
|   1 |  TABLE ACCESS BY USER ROWID| EMPX |      1 |    99 |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1 / [email protected]$1

3.有了以上思路,应该可以恢复全部记录.
--//实际上就是当段头损坏.

[email protected]> CREATE TABLE empy tablespace tea  AS SELECT * FROM empx where  1=0;
Table created.

--//网上找到的脚本,参考链接:http://www.xifenfei.com/3898.html
set serveroutput on
set concat off
DECLARE
 nrows number;
 rid rowid;
 dobj number;
 ROWSPERBLOCK number;
BEGIN
 ROWSPERBLOCK:=736;  --估算最大的一个块中记录条数,8K最多736条记录1块.
 nrows:=0;

 select data_object_id  into dobj
 from dba_objects
 where owner = ‘SCOTT‘
 and object_name = ‘EMPX‘
-- and subobject_name = ‘<table partition>‘  Add this condition if table is partitioned
 ;

 for i in (select relative_fno, block_id, block_id+blocks-1 totblocks
           from dba_extents
           where owner = ‘SCOTT‘
             and segment_name = ‘EMPX‘
-- and partition_name = ‘<table partition>‘ Add this condition if table is partitioned
-- and file_id != <OFFLINED DATAFILE> This condition is only used if a datafile needs to be skipped due to ORA-376 (A)
          order by extent_id)
 loop
   for br in i.block_id..i.totblocks loop
    for j in 1..ROWSPERBLOCK loop
    begin
      rid := dbms_rowid.ROWID_CREATE(1,dobj,i.relative_fno, br , j-1);
      insert into empy
      select /*+ ROWID(A) */ *
      from empx A
      where rowid = rid;
      if sql%rowcount = 1 then nrows:=nrows+1; end if;
      if (mod(nrows,10000)=0) then commit; end if;
    exception when others then null;
    end;
    end loop;
  end loop;
 end loop;
 COMMIT;
 dbms_output.put_line(‘Total rows:‘||to_char(nrows));
END;
/

--//执行后:
[email protected]> select * from empy;
     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
      7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20
      7499 ALLEN      SALESMAN        7698 1981-02-20 00:00:00       1600        300         30
      7521 WARD       SALESMAN        7698 1981-02-22 00:00:00       1250        500         30
      7566 JONES      MANAGER         7839 1981-04-02 00:00:00       2975                    20
      7654 MARTIN     SALESMAN        7698 1981-09-28 00:00:00       1250       1400         30
      7698 BLAKE      MANAGER         7839 1981-05-01 00:00:00       2850                    30
      7782 CLARK      MANAGER         7839 1981-06-09 00:00:00       2450                    10
      7788 SCOTT      ANALYST         7566 1987-04-19 00:00:00       3000                    20
      7839 KING       PRESIDENT            1981-11-17 00:00:00       5000                    10
      7844 TURNER     SALESMAN        7698 1981-09-08 00:00:00       1500          0         30
      7876 ADAMS      CLERK           7788 1987-05-23 00:00:00       1100                    20
      7900 JAMES      CLERK           7698 1981-12-03 00:00:00        950                    30
      7902 FORD       ANALYST         7566 1981-12-03 00:00:00       3000                    20
      7934 MILLER     CLERK           7782 1982-01-23 00:00:00       1300                    10
14 rows selected.

4.你可以将这个恢复不实际,我仅仅恢复一个块.而且是1条1条的恢复,应该效率很低.多个数据块就比较麻烦了,我仅仅提供1个解决方法.
--//有时间写一个大一点的表的恢复.总之通过修改对于数据块的段号恢复是可行的.