一. DB(Data block)
From: http://www.orafaq.com/wiki/Data_block
A data block is the smallest unit of storage in an Oracle database. Every database has a default block size (specified when the database is created), although blocks in different tablespaces may have different block sizes.
An extent consist of one or more contiguous Oracle data blocks. A block determines the finest level of granularity of where data can be stored. One data block corresponds to a specific number of bytes of physical space on disk.
Information about data blocks can be retrieved from the data dictionary views USER_SEGMENTS and USER_EXTENTS. These views show how many blocks are allocated for database object and how many blocks are available(free) in a segment/extent.
数据块是oracle中的最小存储单位，每个数据库创建的时候都有一个默认的block size，同时不同的表空间可以有不同的数据块大小。
一个extent是由多个连续的block组成，一个oracle的数据块由操作系统中磁盘中的块来组成。
关于data blocks的信息，可以从user_segments和user_extents视图来取得，这些视图可以显示数据库对象分配了多少数据库，并且在segment和extents中有多少是空闲的。
1.1 Dumping data blocks（导出数据块）
SQL> SELECT
2 dbms_rowid.rowid_relative_fno(rowid) REL_FNO,
3 dbms_rowid.rowid_block_number(rowid) BLOCKNO,
4 dbms_rowid.rowid_row_number(rowid) ROWNO,
5 empno, ename
6 FROM scott.emp WHERE empno = 7369;
REL_FNO BLOCKNO ROWNO EMPNO ENAME
---------- ---------- ---------- ---------- ----------
4 32 0 7369 SMITH
如果想导出多个块，可用
ALTER SYSTEM dump datafile <file_id> block min <block_id> block max <block_id+blocks-1>;
分析导出数据：
SQL> show user
USER is "SYS"
SQL>
SQL> oradebug setmypid
Statement processed.
SQL> alter system dump datafile 4 block 32;
System altered.
SQL> oradebug tracefile_name
/u01/admin/kxone/udump/kxone_ora_2425.trc
查看文件：/u01/admin/kxone/udump/kxone_ora_2425.trc
114 tab 0, row 0, @0x1f72
115 tl: 38 fb: --H-FL-- lb: 0x1 cc: 8
116 col 0: [ 3] c2 4a 46
117 col 1: [ 5] 53 4d 49 54 48
118 col 2: [ 5] 43 4c 45 52 4b
119 col 3: [ 3] c2 50 03
120 col 4: [ 7] 77 b4 0c 11 01 01 01
121 col 5: [ 2] c2 09
122 col 6: *NULL*
123 col 7: [ 2] c1 15
转化成表中数据：
SQL> SELECT utl_raw.cast_to_number(replace(\'c2 4a 46\',\' \')) value FROM dual;
VALUE
----------
7369
SQL> SELECT utl_raw.cast_to_varchar2(replace(\'53 4d 49 54 48\',\' \')) value FROM dual;
VALUE
--------------------------------------------------------------------------------
SMITH
二. DBA（Data Block Address）数据块地址
From：http://www.orafaq.com/wiki/Data_block_address
A Data Block Address (DBA) is the address of an Oracle data block for access purposes.
DBA一般指绝对数据块地址. rowid用来表示一行的物理地址，一行唯一确定一个rowid，并且在使用中一般不会改变，除非rowid之后在行的物理位置发生改变的情况下才会发生变化。在rowid 中，就有一段是来表示DBA的。有关rowid的内容，参考我的Blog：
Oracle Rowid 介绍
http://blog.csdn.net/tianlesoftware/archive/2009/12/16/5020718.aspx
2.1 Find the DBA for a given row，用给定的行取得dba
SQL> SELECT
2 dbms_rowid.rowid_relative_fno(rowid) REL_FNO,
3 dbms_rowid.rowid_block_number(rowid) BLOCKNO,
4 empno, ename
5 FROM scott.emp WHERE empno = 7521;
REL_FNO BLOCKNO EMPNO ENAME
---------- ---------- ---------- ----------
4 32 7521 WARD
SQL> variable dba varchar2(30)
SQL> exec :dba := dbms_utility.make_data_block_address(4, 32);
PL/SQL procedure successfully completed.
SQL>
SQL> print dba
DBA
--------------------------------------------------------------------------------
16777248
2.2 将dba地址转换成数据文件和行号
SQL> SELECT dbms_utility.data_block_address_block(16777248) "BLOCK",
2 dbms_utility.data_block_address_file(16777248) "FILE"
3 FROM dual;
BLOCK FILE
---------- ----------
32 4
三. RDBA（Tablespace relative database block address）
在讲RDBA 之前，要先了解下rowid的组成。 关于rowid的内容，参考我的blog：
Oracle Rowid 介绍
http://blog.csdn.net/tianlesoftware/archive/2009/12/16/5020718.aspx
RDBA是相对数据块地址，是数据字典(表空间及一些对象定义)所在块的地址。
oracle 8以后，rowid的存储空间扩大到了10个字节(32bit object#+10bit rfile#+22bit block#+16bit row#)。rdba就是rowid中的rfile#+block#
SQL> SELECT
2 rowid,
3 dbms_rowid.rowid_relative_fno(rowid) REL_FNO,
4 dbms_rowid.rowid_block_number(rowid) BLOCKNO,
5 dbms_rowid.rowid_row_number(rowid) ROWNO,
6 empno, ename
7 FROM scott.emp WHERE empno = 7521;
ROWID REL_FNO BLOCKNO ROWNO EMPNO ENAME
------------------ ---------- ---------- ---------- ---------- ----------
AAAMl3AAEAAAAAgAAC 4 32 2 7521 WARD
把这个block dump到trace:
SYS@anqing1(rac1)> alter system dump datafile 4 block 32;
System altered.
查看当前的trace 文件位置：
SQL> oradebug setmypid;
Statement processed.
SQL> oradebug tracefile_name
/u01/admin/kxone/udump/kxone_ora_2425.trc
[oracle@oraclenode01 udump]$ nl /u01/admin/kxone/udump/kxone_ora_2425.trc
14 *** 2013-04-23 15:00:17.485
15 *** ACTION NAME:() 2013-04-23 15:00:17.485
16 *** MODULE NAME:(sqlplus@oraclenode01 (TNS V1-V3)) 2013-04-23 15:00:17.485
17 *** SERVICE NAME:(SYS$USERS) 2013-04-23 15:00:17.485
18 *** SESSION ID:(235.8187) 2013-04-23 15:00:17.485
19 Start dump data blocks tsn: 4 file#: 4 minblk 32 maxblk 32
20 buffer tsn: 4 rdba: 0x01000020 (4/32) -- rdba 的值
21 scn: 0x0000.0005deed seq: 0x01 flg: 0x06 tail: 0xdeed0601
22 frmt: 0x02 chkval: 0x7cd7 type: 0x06=trans data
23 Hex dump of block: st=0, typ_found=1
24 Dump of memory from 0x000000001EC92800 to 0x000000001EC94800
244 tab 0, row 13, @0x1d61
245 tl: 39 fb: --H-FL-- lb: 0x1 cc: 8
246 col 0: [ 3] c2 50 23
247 col 1: [ 6] 4d 49 4c 4c 45 52
248 col 2: [ 5] 43 4c 45 52 4b
249 col 3: [ 3] c2 4e 53
250 col 4: [ 7] 77 b6 01 17 01 01 01
251 col 5: [ 2] c2 0e
252 col 6: *NULL*
253 col 7: [ 2] c1 0b
254 end_of_block_dump
255 End dump data blocks tsn: 4 file#: 4 minblk 32 maxblk 32
其中：
rdba: 0x01000020 (4/32)
SQL> /* Formatted on 2011/6/7 11:27:10 (QP5 v5.163.1008.3004) */
SQL> SELECT DBMS_UTILITY.data_block_address_file (
2 TO_NUMBER (LTRIM (\'0x01000020\', \'0x\'), \'xxxxxxxx\'))
3 AS file_no,
4 DBMS_UTILITY.data_block_address_block (
5 TO_NUMBER (LTRIM (\'0x01000020\', \'0x\'), \'xxxxxxxx\'))
6 AS block_no
7 FROM DUAL;
FILE_NO BLOCK_NO
---------- ----------
4 32
刚才说了，在32这个块里保存了14条row记录，我们继续查询一下我们where=7521 那条：
SQL> select DBMS_UTILITY.data_block_address_block (TO_NUMBER (LTRIM (\'0x1d61\', \'0x\'),\'xxxxxxxx\')) as block_no from dual;
BLOCK_NO
----------
7521
SQL>
最后这个语句想证明什么？ 7521是记录中empno字段的值，这个值怎么凑巧和block_no一样了
dump结果中“tab 0, row 13, @0x1d61” 的@0x1d61表示什么？
现在暂时还没有弄清楚。