oracle回閃查詢的新特性

從oracle9i開始,oracle開始提供回閃查詢特性(flashback query),允許將回滾段中的資料進行回閃,透過下面的例子來看一下這個從oracle9i開始提供的新特性.

SQL> update emp set sal=4000 where empno=7788;

1 row updated.

SQL> update emp set sal=4000 where empno=7782;

1 row updated.

SQL> update emp set sal=4000 where empno=7698;

1 row updated.

先不提交這個事務,在另外視窗新開session,使用sys使用者查詢相關資訊,進行進一步的分析

獲得事務資訊

從事務表中可以獲得關於這個事務的資訊,該事務位於9號回滾段(XIDUSN),在9號回滾段

上,該事務位於第29號事務槽(XIDSLOT):

SQL> select xidusn,xidslot,xidsqn,ubablk,ubafil,ubarec from v$transaction;

XIDUSN XIDSLOT XIDSQN UBABLK UBAFIL UBAREC

---------- ---------- ---------- ---------- ---------- ----------

9 29 385 1350 2 22

從v$rollstat檢視中也可獲得事務資訊,xacts欄位代表的是活動事務的數量,同樣看到該事務

位於9號回滾段

SQL> select usn,writes,rssize,xacts,hwmsize,shrinks,wraps from v$rollstat;

USN WRITES RSSIZE XACTS HWMSIZE SHRINKS WRAPS

---------- ---------- ---------- ---------- ---------- ---------- ----------

0 7620 385024 0 385024 0 0

1 21390 29351936 0 29351936 0 0

2 22108 3268608 0 3268608 0 0

3 29954 450560 0 450560 0 0

4 23700 843776 0 843776 0 0

5 23334 450560 0 450560 0 0

6 21082 450560 0 450560 0 0

7 23146 2285568 0 2285568 0 0

8 28742 843776 0 843776 0 1

9 22648 2088960 1 2088960 0 0

10 24326 2220032 0 2220032 0 0

11 rows selected.

這是執行alter system dump datafile 2 block 1350

轉儲的回滾表空間中的資料塊的資訊的一部分

*-----------------------------

* Rec #0x1d slt: 0x24 objn: 517(0x00000205) objd: 517 tblspc: 0(0x00000000)

* Layer: 11 (Row) opc: 1 rci 0x00

Undo type: Regular undo Begin trans Last buffer split: No

Temp Object: No

Tablespace Undo: No

rdba: 0x00000000

*-----------------------------

uba: 0x00800546.0129.1b ctl max scn: 0x0000.000e4e9c prv tx scn: 0x0000.000e4ea6

txn start scn: scn: 0x0000.000e7526 logon user: 0

prev brb: 8389956 prev bcl: 0

KDO undo record:

KTB Redo

op: 0x04 ver: 0x01

op: L itl: xid: 0x0006.016.0000015d uba: 0x00800419.00fe.11

flg: C--- lkc: 0 scn: 0x0000.000e7524

KDO Op code: URP row dependencies Disabled

xtype: XAxtype KDO_KDOM2 flags: 0x00000080 bdba: 0x0040100f hdba: 0x00401001

itli: 2 ispac: 0 maxfr: 4863

tabn: 0 slot: 116(0x74) flag: 0x2c lock: 0 ckix: 191

ncol: 9 nnew: 7 size: 0

Vector content:

col 2: [ 2] c1 0a

col 3: [ 2] c1 0a

col 4: [ 1] 80

col 5: [ 1] 80

col 6: [ 1] 80

col 7: [ 1] 80

col 8: [ 7] 78 71 01 07 0b 07 34

先注意到這裡存在一個資訊ctl max scn: 0x0000.000e4e9c,這個轉換為scn值就是:

SQL> select (to_number('000','xxxx')*power(2,32)+to_number('e4e9c','xxxxxxxx')) scn

from dual;

SCN

----------

937628

查詢一下當前資料的scn:

SQL> select dbms_flashback.get_system_change_number scn from dual;

SCN

----------

949630

SQL>

透過特定的語法,可以將scn 937628的歷史狀態資料查詢出來:

SQL> select * from scott.emp as of scn 937628 where empno in(7788,7782,7698);

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

----- ---------- --------- ----- ----------- --------- --------- ------

7698 BLAKE MANAGER 7839 1981-5-1 2850.00 30

7782 CLARK MANAGER 7839 1981-6-9 2450.00 10

7788 SCOTT ANALYST 7566 1987-4-19 10.00 20

SQL>

在查詢結果中,注意到3名員工的薪水恢復到了之前的狀態.而在當前的查詢中,這個資料是變化

後的4000:

SQL> select * from scott.emp where empno in(7788,7782,7698);

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

----- ---------- --------- ----- ----------- --------- --------- ------

7698 BLAKE MANAGER 7839 1981-5-1 4000.00 30

7782 CLARK MANAGER 7839 1981-6-9 4000.00 10

7788 SCOTT ANALYST 7566 1987-4-19 4000.00 20

由於這個查詢需要從undo中獲限前映象資訊,如果undo中的資訊被覆蓋,則以上查詢將會失敗.

測試一下,當新建undo表空間,切換undo表空間,再將原表空間offline之後:

SQL> create undo tablespace undotbs2 datafile

2 '/u01/app/oracle/product/10.2.0/oradata/jingyong/undotbs02.dbf' size 10M;

Tablespace created

SQL> alter system set undo_tablespace=undotbs2;

System altered.

SQL> alter tablespace undotbs1 offline;

Tablespace altered.

SQL> alter session set events='immediate trace name flush_cache';

Session altered.

再來查詢,此時出現錯誤,記錄該檔案已經不可讀取:

SQL> select * from scott.emp as of scn 937628 where empno in(7788,7782,7698);

select * from scott.emp as of scn 937628 where empno in(7788,7782,7698)

ORA-00376: file 2 cannot be read at this time

ORA-01110: data file 2: '/u01/app/oracle/product/10.2.0/oradata/jingyong/undotbs01.dbf'

將undotbs1重新啟用:

SQL> alter tablespace undotbs1 online;

Tablespace altered.

SQL> alter system set undo_tablespace=undotbs1;

System altered.

此時前映象資訊再次可以查詢,

SQL> select * from scott.emp as of scn 937628 where empno in(7788,7782,7698);

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

----- ---------- --------- ----- ----------- --------- --------- ------

7698 BLAKE MANAGER 7839 1981-5-1 2850.00 30

7782 CLARK MANAGER 7839 1981-6-9 2450.00 10

7788 SCOTT ANALYST 7566 1987-4-19 10.00 20

如果在其他session中執行大量事務,使用前映象資訊被覆蓋:

在執行其它大量事務之前觀察回滾段的使用情況:

SQL> select usn,xacts,rssize,hwmsize from v$rollstat where usn=9;

USN XACTS RSSIZE HWMSIZE

---------- ---------- ---------- ----------

9 0 2088960 2088960

SQL> begin

2 for i in 1..2000 loop

3 update scott.emp set sal=4000;

4 rollback;

5 end loop;

6 end;

7 /

PL/SQL procedure successfully completed.

SQL> select usn,xacts,rssize,hwmsize from v$rollstat where usn=9;

USN XACTS RSSIZE HWMSIZE

---------- ---------- ---------- ----------

9 1 4186112 4186112

SQL>

那麼再次查詢就可能會收到如下錯誤:

SQL> select * from scott.emp as of scn 937628 where empno in(7788,7782,7698);

select * from scott.emp as of scn 937628 where empno in(7788,7782,7698)

ERROR at line 1:

ORA-01555: snapshot too old:rollback segment number 9 with name "_SYSSMU9$" too small

ORA-01555錯誤出現說明要查詢的前鏡偈資訊已經丟失了.

使用errorstack進行錯誤跟蹤

errorstack是oracle提供的介面用於診斷oracle的錯誤資訊.

診斷事件可以在session級設定,也可以在系統級設定,通常如果要診斷全域性錯誤,最好在

系統級設定.設定了errorstack事件之後,oracle會將出錯時的資訊記入跟蹤檔案中.

使用者就可以透過跟蹤檔案進行錯誤診斷和排查.

可以透過errorstack事件來跟蹤ora-01555錯誤:

OS Pid: 2928 executed alter session set events '1555 trace name errorstack level 4'

ORA-01555 caused by SQL statement below (SQL ID: bnaqc462nnjtb, Query Duration=0 sec, SCN: 0x0000.00068478):

Mon Jan 7 20:23:35 2013

select * from scott.emp as of scn 427128 where empno in(7788,7782,7698)

Mon Jan 7 20:23:35 2013

Errors in file /u01/app/oracle/admin/jingyong/udump/jingyong_ora_2928.trc:

ORA-01555: snapshot too old: rollback segment number 3 with name "???" too small

這裡注意到,觸發ora-01555錯誤的語句被記錄,出現錯誤的scn也被記錄,這個scn:0x0000.00068478

找到jingyong_ora_2928.trc跟蹤檔案,就可以獲得關於這次錯誤的相關資訊用於診斷.

錯誤資訊如下

ksedmp: internal or fatal error

ORA-01555: snapshot too old: rollback segment number 3 with name "???" too small

Current SQL statement for this session:

select * from scott.emp as of scn 427128 where empno in(7788,7782,7698)

資料塊資訊,這裡的塊頭就包含了ITL資訊,根據這個ITL資訊中的UBA,oracle可以定位回滾段.

查詢前映象資訊,如果不存在,就可能出現ora-01555錯誤

Block header dump: 0x01000024

Object id on Block? Y

seg/obj: 0xc7cd csc: 0x00.6bfcb itc: 2 flg: E typ: 2 - INDEX

brn: 0 bdba: 0x1000021 ver: 0x01 opc: 0

inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc

0x01 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000

0x02 0x0003.011.000000f2 0x00805794.00c8.4a --U- 14 fsc 0x0000.0006bfdb

oracle 10g回閃查詢特性的增強

oracle9i提供的閃回特性增強,為恢復帶來了極大的方便,但是oracle9i的回閃查詢只能提供某個

時間點的資料檢視,並不能告訴使用者這樣的資料經過幾個事務,怎樣被修改的(update,insert,delete)

,而這些資訊在回滾段中是存在的,在oracle10g中,oracle進一步加強了回閃查詢的特性,提供以下

兩種回閃查詢:

回閃版本查詢(flashback versions query)

回閃事務查詢(flashback transaction query)

回閃版本查詢允許使用一個新的versions子句查詢兩個時間點或者scn之間的資料版本.

這些版本可以按照事務進行區分,回閃版本查詢只返回提交資料.沒提交的資料不被顯示

先建立一個測試表,執行一些DML操作

SQL> create table test as select username from dba_users;

Table created.

SQL> select * from test;

USERNAME

------------------------------

MGMT_VIEW

SYS

SYSTEM

DBSNMP

SYSMAN

SCOTT

JYTEST1

JYTEST

OUTLN

MDSYS

ORDSYS

USERNAME

------------------------------

EXFSYS

DMSYS

WMSYS

CTXSYS

ANONYMOUS

XDB

ORDPLUGINS

SI_INFORMTN_SCHEMA

OLAPSYS

TSMSYS

BI

USERNAME

------------------------------

PM

MDDATA

IX

SH

DIP

OE

HR

29 rows selected.

SQL> delete from test where username='BI';

1 row deleted.

SQL> delete from test where username='JYTEST1';

1 row deleted.

SQL> commit;

Commit complete.

SQL> select * from test;

USERNAME

------------------------------

MGMT_VIEW

SYS

SYSTEM

DBSNMP

SYSMAN

SCOTT

JYTEST

OUTLN

MDSYS

ORDSYS

EXFSYS

USERNAME

------------------------------

DMSYS

WMSYS

CTXSYS

ANONYMOUS

XDB

ORDPLUGINS

SI_INFORMTN_SCHEMA

OLAPSYS

TSMSYS

PM

MDDATA

USERNAME

------------------------------

IX

SH

DIP

OE

HR

27 rows selected.

SQL>

再多執行一些DML操作

SQL> update test set username='JYTEST1' where username='JYTEST';

1 row updated.

SQL> delete from test where username='IX';

1 row deleted.

SQL> commit;

Commit complete.

SQL> select * from test;

USERNAME

------------------------------

MGMT_VIEW

SYS

SYSTEM

DBSNMP

SYSMAN

SCOTT

JYTEST1

OUTLN

MDSYS

ORDSYS

EXFSYS

USERNAME

------------------------------

DMSYS

WMSYS

CTXSYS

ANONYMOUS

XDB

ORDPLUGINS

SI_INFORMTN_SCHEMA

OLAPSYS

TSMSYS

PM

MDDATA

USERNAME

------------------------------

SH

DIP

OE

HR

JY

27 rows selected.

再來看看oracle10g的回閃版本查詢,透過使用versions子句,和對資料表引入了一系列的偽列(

version_startime等),可以獲得對資料表的所有事務操作,注意以下輸出中的versions_operation

代表了不同型別的操作(D-Delete,I-Insert,U-Update),versions_xid是一個重要資料,代表了不

同版本的事務ID:

SQL> select versions_starttime,versions_endtime,versions_xid,versions_operation,

2 username from test versions between timestamp minvalue and maxvalue;

VERSIONS_STARTTIME VERSIONS_ENDTIME VERSIONS_XID VERSIONS_OPERATION USERNAME

--------------------------- ---------------------------- ---------------- ------------------ ---------------------- --------

07-1月 -13 11.07.58 下午 0300150096010000 I JY

07-1月 -13 11.06.46 下午 06002B0093010000 D IX

07-1月 -13 11.06.46 下午 06002B0093010000 U JYTEST1

07-1月 -13 11.05.22 下午 05001E00A7010000 D JYTEST1

07-1月 -13 11.05.22 下午 05001E00A7010000 D BI

MGMT_VIEW

SYS

SYSTEM

DBSNMP

SYSMAN

SCOTT

07-1月 -13 11.05.22 下午 JYTEST1

07-1月 -13 11.06.46 下午 JYTEST

OUTLN

MDSYS

ORDSYS

EXFSYS

DMSYS

WMSYS

CTXSYS

VERSIONS_STARTTIME VERSIONS_ENDTIME VERSIONS_XID VERSIONS_OPERATION USERNAME

--------------------------- ---------------------------- ---------------- ------------------ ---------------------- --------

ANONYMOUS

XDB

ORDPLUGINS

SI_INFORMTN_SCHEMA

OLAPSYS

TSMSYS

07-1月 -13 11.05.22 下午 BI

PM

MDDATA

07-1月 -13 11.06.46 下午 IX

SH

DIP

OE

HR

34 rows selected

SQL>

透過以上輸出,根據versions_xid可以清晰地區分不同事務在不同時間對資料所作的更改

具備了flashback version query查詢的基礎,就可以進行基於flashback version query的事務級恢復了.

這就是flashback transaction query, flashback transaction query可以從flashback transaction query

檢視中獲得指定事務的歷史資訊以及undo_sql,透過undo_sql,就可以撤消特定的提交事務.flashback

transaction query需要用到flashback_transaction_query檢視,先看一下檢視:

SQL> desc flashback_transaction_query

Name Type Nullable Default Comments

---------------- -------------- -------- ------- -----------------------------------------

XID RAW(8) Y Transaction identifier

START_SCN NUMBER Y Transaction start SCN

START_TIMESTAMP DATE Y Transaction start timestamp

COMMIT_SCN NUMBER Y Transaction commit SCN

COMMIT_TIMESTAMP DATE Y Transaction commit timestamp

LOGON_USER VARCHAR2(30) Y Logon user for transaction

UNDO_CHANGE# NUMBER Y 1-based undo change number

OPERATION VARCHAR2(32) Y forward operation for this undo

TABLE_NAME VARCHAR2(256) Y table name to which this undo applies

TABLE_OWNER VARCHAR2(32) Y owner of table to which this undo applies

ROW_ID VARCHAR2(19) Y rowid to which this undo applies

UNDO_SQL VARCHAR2(4000) Y SQL corresponding to this undo

該檢視的定義為:

select xid, start_scn, start_timestamp,

decode(commit_scn, 0, commit_scn, 281474976710655, NULL, commit_scn)

commit_scn, commit_timestamp,

logon_user, undo_change#, operation, table_name, table_owner,

row_id, undo_sql

from sys.x$ktuqqry;

對於x$ktuqqry的查詢非常耗時

測試一下對於x$ktuqqry表的查詢

SQL> select count(addr) from x$ktuqqry;

COUNT(ADDR)

-----------

74786

如果需要撤消xid=0300150096010000的事務,可以透過如下步驟進行:

SQL> set autotrace on

SQL> select undo_sql from flashback_transaction_query where xid='0300150096010000';

UNDO_SQL

--------------------------------------------------------------------------------

delete from "SYS"."TEST" where ROWID = 'AAAM3VAABAAAO9KAAd';

Execution Plan

----------------------------------------------------------

Plan hash value: 1115820779

------------------------------------------------------------------------------

------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 2008 | 0 (0)| 00:00:01 |

|* 1 | FIXED TABLE FULL| X$KTUQQRY | 1 | 2008 | 0 (0)| 00:00:01 |

------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - filter(RAWTOHEX("XID")='0300150096010000')

Statistics

----------------------------------------------------------

90926 recursive calls

0 db block gets

238964 consistent gets

21783 physical reads

0 redo size

506 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

4115 sorts (memory)

0 sorts (disk)

2 rows processed

SQL>

透過執行相應的undo語句可以撤消該事務,透過這些新特性,oracle提供了一種"回滾"提交事務的

方法,極大地方便了使用者應對不同情況的資料庫恢復.

oracle的回閃查詢

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