[20180627]truncate table的另類恢復.txt

--//前幾天看鏈接http://www.xifenfei.com/2018/06/truncate-table-recovery.html
--//提到幾種方式不影響數據的恢復,突然看到reuse storage選項,感覺在truncate時使用該參數時存儲空間不會回收,
--//感覺這樣應該還是會降低高水位標誌.

--//使用reuse storage選項,存儲空間不會回收,另外在truncate時,數據段號data_object_id會發生變化,自己想想是否
--//可以通過修改相應數據庫的段號指向新的段號來實現恢復.自己動手測試看看.

1.環境:
SCOTT@book> @ 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;

SCOTT@book> 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

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

SCOTT@book> select object_id,data_object_id from dba_objects where owner=user and object_name=‘EMPX‘;
OBJECT_ID DATA_OBJECT_ID
---------- --------------
90446 90446

SCOTT@book> column PARTITION_NAME noprint
SCOTT@book> 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@book> truncate table empx reuse storage;
Table truncated.

SCOTT@book> select object_id,data_object_id from dba_objects where owner=user and object_name=‘EMPX‘;
OBJECT_ID DATA_OBJECT_ID
---------- --------------
90446 90447

--//可以發現truncate後data_object_id=90447發生變化.

SCOTT@book> 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@book> alter system checkpoint;
System altered.

2.看看是否通過修改相應塊的段號:

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
--//可以發現truncate後數據段ID還是原來的90446,如果有數據插入段號就發生變化.我想通過修改段號實現數據恢復.

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.檢查是否能正常顯示:
SCOTT@book> alter system flush buffer_cache;
System altered.

SCOTT@book> select rowid,empx.* from empx ;
no rows selected

--//不行!!看看直接使用rowid是否可以查詢.
SCOTT@book> 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

--//可以發現使用rowid可以查詢對應記錄.如果寫成如下:
SCOTT@book> 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

--//因為執行計劃是這樣:
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 / EMPX@SEL$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)))
--//執行計劃TABLE ACCESS BY ROWID RANGE,註意範圍這個關鍵字.這樣要訪問段頭,因為是truncate表後降低了高水位標識,
--//這樣oracle認為要訪問的數據塊在高水位之上,這樣就沒有結果集.
--//而前面的執行計劃是TABLE ACCESS BY USER ROWID.直接通過rowid訪問塊.越過了數據段頭的訪問.
SCOTT@book> @ &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 / EMPX@SEL$1

3.有了以上思路,應該可以恢復全部記錄.
--//實際上就是當段頭損壞.

SCOTT@book> CREATE TABLE empy tablespace tea AS SELECT * FROM empx where 1=0;
Table created.

--//網上找到的腳本,參考鏈接:http://www.xifenfei.com/3898.html
set serveroutput on
set concat off
DECLARE
nrows number;
rid rowid;
dobj number;
ROWSPERBLOCK number;
BEGIN
ROWSPERBLOCK:=736; --估算最大的一個塊中記錄條數,8K最多736條記錄1塊.
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;
/

--//執行後:
SCOTT@book> 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.你可以將這個恢復不實際,我僅僅恢復一個塊.而且是1條1條的恢復,應該效率很低.多個數據塊就比較麻煩了,我僅僅提供1個解決方法.
--//有時間寫一個大一點的表的恢復.總之通過修改對於數據塊的段號恢復是可行的.

[20180627]truncate table的另類恢復.txt