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oracle core完成了oracle的核心功能，recovery，读一致性等。
深入的了解oracle的机制，就从一个最简单的更新开始。
对于oracle来说，最大的一个特性就是写了两次数据:
对于最常见的数据更新来说，oracle主要做了一下动作：
下面就以update为例：记录变更的过程。
实验环境：Oracle Database 10g Enterprise Edition Release
10.2.0.1.0
oracle对于update，会依次做以下变更：
这里可以看到，对于oracle来说，所有的数据块的变更都是日志先行。
1. 准备数据：
SYS/SYS@ORCL>create table test(id number, name varchar2(100)); Table created. SYS/SYS@ORCL>insert into test values(1,‘xpchild_1‘); 1 row created. SYS/SYS@ORCL>insert into test values(2,‘xpchild_2‘); 1 row created. SYS/SYS@ORCL>insert into test values(3,‘xpchild_3‘); 1 row created. SYS/SYS@ORCL>commit; Commit complete.
2.dump 当前数据块
SYS/SYS@ORCL>select owner, segment_name, tablespace_name, HEADER_FILE, HEADER_BLOCK , blocks from dba_segments where segment_name=‘TEST‘ AND OWNER=‘XPCHILD‘; OWNER SEGMENT_NAME TABLESPACE_NAME HEADER_FILE HEADER_BLOCK BLOCKS -------------------- -------------------- -------------------- ----------- ------------ ---------- XPCHILD TEST USERS 4 475 8 SYS/SYS@ORCL>select owner, segment_name, tablespace_name, file_id, block_id,blocks from dba_extents where owner=‘XPCHILD‘ and segment_name=‘TEST‘; OWNER SEGMENT_NAME TABLESPACE_NAME FILE_ID BLOCK_ID BLOCKS -------------------- -------------------- -------------------- ---------- ---------- ---------- XPCHILD TEST USERS 4 473 8
上面的两个数据字典的查询，可以得出，test表在users表空间中，file_id为4。一共分配了一个extents，包括8个block。
SYS/SYS@ORCL>alter system dump datafile 4 block 473,474,475,476; System altered.
block 473,474分别是first level bitmap block，second level bitmap
block。主要记录块的空闲情况。
如：
-------------------------------------------------------- DBA Ranges : -------------------------------------------------------- 0x010001d9 Length: 8 Offset: 0 0:Metadata 1:Metadata 2:Metadata 3:75-100% free 4:75-100% free 5:75-100% free 6:75-100% free 7:75-100% free
block 475为pagetable segment header。
block 480为存放真正数据的块，这也是堆管理的方式所决定，如下dump信息所示：
Itl Xid Uba Flag Lck Scn/Fsc 0x01 0x0001.01e.000000f4 0x008000aa.00f0.14 --U- 3 fsc 0x0000.000d61e9 0x02 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000 0xe:pti[0] nrow=3 offs=0 0x12:pri[0] offs=0x1f88 0x14:pri[1] offs=0x1f78 0x16:pri[2] offs=0x1f68 block_row_dump: tab 0, row 0, @0x1f88 tl: 16 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 2] c1 02 col 1: [ 9] 78 70 63 68 69 6c 64 5f 31 tab 0, row 1, @0x1f78 tl: 16 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 2] c1 03 col 1: [ 9] 78 70 63 68 69 6c 64 5f 32 tab 0, row 2, @0x1f68 tl: 16 fb: --H-FL-- lb: 0x1 cc: 2 col 0: [ 2] c1 04 col 1: [ 9] 78 70 63 68 69 6c 64 5f 33
2, 更新数据
SYS/SYS@ORCL>SYS/SYS@ORCL>update test set name=‘it is beyond the current length‘ where id=2; Session altered. SYS/SYS@ORCL>alter system dump datafile 4 block 480; System altered. ------------------------------------------------------- Itl Xid Uba Flag Lck Scn/Fsc 0x01 0x0001.01e.000000f4 0x008000aa.00f0.14 C--- 0 scn 0x0000.000d61e9 0x02 0x0001.018.000000f4 0x008000aa.00f0.26 --U- 1 fsc 0x0000.000d6475 0xe:pti[0] nrow=3 offs=0 0x12:pri[0] offs=0x1f88 0x14:pri[1] offs=0x1f41 0x16:pri[2] offs=0x1f68 block_row_dump: tab 0, row 0, @0x1f88 tl: 16 fb: --H-FL-- lb: 0x0 cc: 2 col 0: [ 2] c1 02 col 1: [ 9] 78 70 63 68 69 6c 64 5f 31 tab 0, row 1, @0x1f41 tl: 39 fb: --H-FL-- lb: 0x2 cc: 2 col 0: [ 2] c1 03 col 1: [32] 69 74 20 69 73 20 20 62 65 79 6f 6e 64 20 74 68 65 20 63 75 72 72 65 6e 74 20 6c 65 6e 67 74 68 tab 0, row 2, @0x1f68 tl: 16 fb: --H-FL-- lb: 0x0 cc: 2 col 0: [ 2] c1 04 col 1: [ 9] 78 70 63 68 69 6c 64 5f 33
这里主要说明两点：
接下来看下redo change vector的内容：
SYS/SYS@ORCL>alter system switch logfile; System altered. SYS/SYS@ORCL>update test set name=‘ssssssss‘ where id=2; 1 row updated. SYS/SYS@ORCL>commit; Commit complete. SYS/SYS@ORCL>alter system switch logfile; System altered. SYS/SYS@ORCL>select * from v$Log; GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARCHIV STATUS FIRST_CHANGE# FIRST_TIME ---------- ---------- ---------- ---------- ---------- ------ -------------------------------- ------------- ------------------- 1 1 5 52428800 1 NO CURRENT 879223 2012-01-11 17:11:00 2 1 4 52428800 1 YES ACTIVE 879196 2012-01-11 17:09:52 3 1 3 52428800 1 YES ACTIVE 879176 2012-01-11 17:08:57 SYS/SYS@ORCL>select SEQUENCE# , name from v$archived_log; SEQUENCE# NAME ---------- ----------------------------------------------------------------------------------- 1 /opt/oracle/flash_recovery_area/ORCL/archivelog/2012_01_11/o1_mf_1_1_7jvkvdfo_.arc 2 /opt/oracle/flash_recovery_area/ORCL/archivelog/2012_01_11/o1_mf_1_2_7jw23sfk_.arc 3 /opt/oracle/flash_recovery_area/ORCL/archivelog/2012_01_11/o1_mf_1_3_7jw25j8o_.arc 4 /opt/oracle/flash_recovery_area/ORCL/archivelog/2012_01_11/o1_mf_1_4_7jw27nhp_.arc 23 rows selected. SYS/SYS@ORCL>alter system dump logfile ‘/opt/oracle/flash_recovery_area/ORCL/archivelog/2012_01_11/o1_mf_1_4_7jw27nhp_.arc‘; System altered. KTB Redo op: 0x11 ver: 0x01 op: F xid: 0x0004.013.000000fb uba: 0x00800054.0110.05 Block cleanout record, scn: 0x0000.000d6a6c ver: 0x01 opt: 0x02, entries follow... itli: 1 flg: 2 scn: 0x0000.000d6a12 KDO Op code: URP row dependencies Disabled xtype: XA flags: 0x00000000 bdba: 0x010001e0 hdba: 0x010001db itli: 2 ispac: 0 maxfr: 4858 tabn: 0 slot: 1(0x1) flag: 0x2c lock: 2 ckix: 10 ncol: 2 nnew: 1 size: 1 col 1: [ 8] 73 73 73 73 73 73 73 73
这一部分代表的是更新数据库的redo。bdba代表更新的块，hdba代表segment header。
SYS/SYS@ORCL>select ubafil,ubablk,start_ubablk,used_ublk from v$transaction; UBAFIL UBABLK START_UBABLK USED_UBLK ---------- ---------- ------------ ---------- 2 155 155 1 1 row selected. SYS/SYS@ORCL>alter system dump datafile 2 block 155; System altered.
对于更新的事务，根据v$transaction可以查询到这个事务使用的undo块的信息，然后dump这个块的情况：
*----------------------------- * Rec #0x5 slt: 0x0f objn: 52634(0x0000cd9a) objd: 52634 tblspc: 4(0x00000004) * 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: 0x0080009b.00f2.03 ctl max scn: 0x0000.000d56ca prv tx scn: 0x0000.000d56e5 txn start scn: scn: 0x0000.000d6dec logon user: 0 prev brb: 8389342 prev bcl: 0 KDO undo record: KTB Redo op: 0x04 ver: 0x01 op: L itl: xid: 0x0004.013.000000fb uba: 0x00800054.0110.05 flg: C--- lkc: 0 scn: 0x0000.000d6a70 KDO Op code: URP row dependencies Disabled xtype: XA flags: 0x00000000 bdba: 0x010001e0 hdba: 0x010001db itli: 2 ispac: 0 maxfr: 4858 tabn: 0 slot: 1(0x1) flag: 0x2c lock: 0 ckix: 12 ncol: 2 nnew: 1 size: -1 col 1: [ 7] 78 70 63 68 69 6c 64
这便是undo块的dump信息，这里也可以看到，undo的信息也仅仅只保留了被改变的那一部分信息，而不是整个块的原始镜像。
txn start scn: 0x0000.000d6a12 logon user: 0 prev brb: 8388689 prev bcl: 0 KDO undo record: KTB Redo op: 0x04 ver: 0x01 op: L itl: xid: 0x0001.018.000000f4 uba: 0x008000aa.00f0.26 flg: C--- lkc: 0 scn: 0x0000.000d6475 KDO Op code: URP row dependencies Disabled xtype: XA flags: 0x00000000 bdba: 0x010001e0 hdba: 0x010001db itli: 2 ispac: 0 maxfr: 4858 tabn: 0 slot: 1(0x1) flag: 0x2c lock: 0 ckix: 10 ncol: 2 nnew: 1 size: -1 col 1: [ 7] 78 70 63 68 69 6c 64
这一部分代表undo的redo日志，因为oracle对所有对数据块的变更都写两份，一份是数据变更，一份是日志，这一部分就是
对undo 块变更的重做日志。
Oracle core02_数据块,码迷,mamicode.com
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原文地址：http://www.cnblogs.com/xpchild/p/3695131.html
