Oracle ITL(Interested Transaction List)理解

ITL(Interested Transaction List)

ITL是位于数据块头部的事物槽列表，它是由一系列的ITS(Interested Transaction Slot,事物槽)组成,其初始的ITL Slot数量由INITRANS决定的，如果有足够的剩余空间，oracle也会根据需要动态的分配这些slot，直到受到空间限制或者达到MAXTRANS,注意10g以后MAXTRANS被废弃，默认为255。

ITL里的信息包括了xid,uba,flag,lck和scn/fsc,它是用来记录数据块上发生的事务相关信息，其中xid表示事物的id,uba表示对应到undo块上的地址，flag用来表示该条事务的状态，lck是块上的锁标记等。

我们使用dump命令来认识一下itl里的内容：

SQL> conn /as sysdba
Connected.
SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1. - Production
PL/SQL Release 11.2.0.1. - Production
CORE    11.2.0.1.      Production
TNS for Linux: Version 11.2.0.1. - Production
NLSRTL Version 11.2.0.1. - Production

SQL> conn scott/tiger
Connected.
SQL> select rowid,dbms_rowid.rowid_relative_fno(rowid) fno,
    dbms_rowid.rowid_block_number(rowid) block_id,
    dbms_rowid.rowid_row_number(rowid)
    from dept;

ROWID                     FNO   BLOCK_ID DBMS_ROWID.ROWID_ROW_NUMBER(ROWID)
------------------ ---------- ---------- ----------------------------------
AAAR3bAAEAAAACHAAA
AAAR3bAAEAAAACHAAB
AAAR3bAAEAAAACHAAC
AAAR3bAAEAAAACHAAD                                                    

SQL> update dept set dname='development' where deptno=;

 row updated.      --注意未提交

--另开一个会话

[oracle@std ~]$ sqlplus scott/tiger

SQL*Plus: Release 11.2.0.1. Production on Fri Oct  :: 

Copyright (c) , , Oracle.  All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1. - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> update dept set dname='AAAAA' where deptno=;

 row updated.

SQL> commit;

Commit complete.

SQL> conn /as sysdba
Connected.

SQL> alter system dump datafile  block ;

System altered.

SQL> oradebug setmypid
Statement processed.
SQL> oradebug tracefile_name
/u02/app/diag/rdbms/orcl/ORCL/trace/ORCL_ora_7978.trc

查看一下dump出来的内容:

more /u02/app/diag/rdbms/orcl/ORCL/trace/ORCL_ora_7978.trc
......
Block header dump:  0x01000087
 Object id on Block? Y
 seg/obj: 0x11ddb  csc: 0x00.  itc:   flg: E  typ:  - DATA
     brn:   bdba: 0x1000080 ver: 0x01 opc:
     inc:   exflg: 

 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0x0001.010.00000198  0x00c15aac.0061.36  C---      scn 0x0000.000b6f31
0x02   0x0000.000.00000000  0x00000000.0000.00  ----      fsc 0x0000.
bdba: 0x01000087
data_block_dump,data header at 0xcac664

可以看到dump出来的内容包括了事务id，对应的undo地址，事务的状态，锁信息等，通过这些信息就可以标识当前数据块的DML情况。

理解flg标识位含义：

---- = transaction is active, or committed pending cleanout
C--- = transaction has been committed and locks cleaned out
-B-- = this undo record contains the undo for this ITL entry
--U- = transaction committed (maybe long ago); SCN is an upper bound
---T = transaction was still active at block cleanout SCN

------ITL在ORACLE中的使用

当发出一条sql语句时，ORACLE会记录下这个时刻(SCN),然后在buffer cache中查找需要的BLOCK,或者从磁盘上读。当别的会话修改了数据，或者正在修改数据，就会在相应的block上记录ITL,此时ORACLE发现ITL中记录的SCN(Scn/Fsc)大于SELECT时刻的SCN，那么ORACLE就会根据ITL中的Uba找到UNDO信息获得该block的前镜像，然后在buffer cache 中构造出CR(consistent read)块，此时ORALCE也会检查构造出来的BLOCK中ITL记录的SCN(Scn/Fsc)，如果SCN(Scn/Fsc)还大于select时刻的SCN，那么一直重复构造前镜像，然后ORACLE找到前镜像BLOCK中的ITL的SCN是否小于select的SCN,同时检查这个事物有没有提交或者回滚，如果没有，那么继续构造前镜像，直到找到需要的BLOCK，如果在构造前镜像的过程中所需的UNDO信息被覆盖了，就会报ORA-01555快照过旧的错误。

参考:http://wenku.baidu.com/link?url=Pp9XTyF_4rYzhdNOeewFhmp_YAoVS1Pq8FJYvOy1_DS10DWIWreYDSLDpGwLhibjFO6SYGunPcvZ_aTP1P5fZzFCbw0m7qwXCT4EMYMSBpK