Oracle DataBlock是如何存放数据的

问题：
1、Oracle DataBlock是如何存放数据的
2、当发生Update导致记录变大时，Oracle是如何保证Rowid不变的
3、发生行迁移和行连接的条件是什么
4、Update后留下的空间是如何被再次利用的

构造测试环境  

create table MILIATEST
(
 ID     NUMBER(10),
 NAME   VARCHAR2(2000),
 ADRESS VARCHAR2(2000)
) tablespace test;


SQL> select b.tablespace_name,b.block_size,b.segment_space_management from dba_tablespaces b where tablespace_name='TEST';

TABLESPACE_NAME                BLOCK_SIZE SEGMENT_SPACE_MANAGEMENT
------------------------------ ---------- ------------------------
TEST                           8192       MANUAL                  手动管理表空间


SQL> select a.pct_free,a.pct_used from all_tables a where a.table_name='MILIATEST';

 PCT_FREE   PCT_USED
---------- ----------
        10         40

SQL> set serverout on;
SQL> exec show_space('MILIATEST');

Free Blocks.............................0
Total Blocks............................8
Total Bytes.............................65536
Unused Blocks...........................7
Unused Bytes............................57344
Last Used Ext FileId....................6
Last Used Ext BlockId...................33
Last Used Block.........................1


SQL> alter system dump datafile 6 block 33;
System altered 
可以看到MILIATEST段，占用的8块block ,第33块block存放了段的一些信息

Start dump data blocks tsn: 7 file#: 6 minblk 33 maxblk 33
buffer tsn: 7 rdba: 0x01800021 (6/33)
scn: 0x0000.002169b6 seq: 0x01 flg: 0x04 tail: 0x69b61001
frmt: 0x02 chkval: 0x289a type: 0x10=DATA SEGMENT HEADER - UNLIMITED
Hex dump of block: st=0, typ_found=1
Dump of memory from 0x090C8400 to 0x090CA400
90C8400 0000A210 01800021002169B6 04010000 [....!....i!.....]
90C8410 0000289A000000000000000000000000 [.(..............]
90C8420 00000000000000010000000700001020 [............ ...]
90C8430 00000000000000000000000701800022 [............"...]
90C8440 00000000 00000000 00000000 00000000 [................]
90C8450 00000000 00000000 00000000 00000001 [................]
90C8460 00000000 0000CF2C 40000000 01800022 [....,......@"...]
90C8470 00000007000000000000000000000000 [................]
90C8480 00000000000000000000000000000000 [................]
        Repeat 250 times
90C9430 00000000000100000001000100000000 [................]
90C9440 00000000000000000000000000000000 [................]
        Repeat 250 times
90CA3F0 00000000000000000000000069B61001 [...............i]
 Extent Control Header
 -----------------------------------------------------------------
 Extent Header:: spare1: 0      spare2: 0      #extents: 1      #blocks: 7    
                  last map 0x00000000 #maps: 0      offset: 4128 
      Highwater:: 0x01800022 ext#: 0      blk#: 0      ext size: 7    
 #blocks in seg. hdr's freelists: 0    
 #blocks below: 0    
 mapblk 0x00000000 offset: 0    
                   Unlocked
     Map Header:: next 0x00000000 #extents: 1    obj#: 53036 flag: 0x40000000
 Extent Map
 -----------------------------------------------------------------
   0x01800022 length: 7    
 
 nfl = 1, nfb = 1 typ = 1 nxf = 0 ccnt = 0
 SEG LST:: flg: UNUSED lhd: 0x00000000 ltl: 0x00000000

 rdba: 0x01800021 表示此时的datablock address 为0x01800021

 Highwater:: 0x01800022 表示此时的HWM地址为0x01800022

 

向表里插入3条数据：

declare
m_name varchar2(2000):='';
begin
for i in 1..10 loop
m_name:=m_name||'a';
end loop;
for j in 1..3 loop
insert into MILIATEST(ID,name) values(j,m_name);
commit;
end loop;
end;

 

 

SQL> exec show_space('MILIATEST');

Free Blocks.............................1

Total Blocks............................8

Total Bytes.............................65536

Unused Blocks...........................6

Unused Bytes............................49152

Last Used Ext FileId....................6

Last Used Ext BlockId...................33

Last Used Block.........................2

 

此时段MILIATEST用了2块block，33放了段head信息，34放数据信息。

SQL> alter system dump datafile 6 block min 33 block max 34;

System altered

 

Start dump data blocks tsn: 7 file#: 6 minblk 33 maxblk 34

buffer tsn: 7 rdba: 0x01800021 (6/33)

scn: 0x0000.00216f8b seq: 0x01 flg: 0x04 tail: 0x6f8b1001

frmt: 0x02 chkval: 0x289a type: 0x10=DATA SEGMENT HEADER - UNLIMITED

Hex dump of block: st=0, typ_found=1

Dump of memory from 0x090C8400 to 0x090CA400

90C8400 0000A210 01800021 00216F8B 04010000 [....!....o!.....]

90C8410 0000289A 00000000 00000000 00000000 [.(..............]

90C8420 00000000 00000001 00000007 00001020 [............ ...]

90C8430 00000000 00000001 00000007 01800023 [............#...]

90C8440 00000000 00000000 00000001 00000001 [................]

90C8450 00000000 00000000 00000000 00000001 [................]

90C8460 00000000 0000CF2C 40000000 01800022 [....,......@"...]

90C8470 00000007 00000000 00000000 00000000 [................]

90C8480 00000000 00000000 00000000 00000000 [................]

        Repeat 250 times

90C9430 00000000 00010000 00010001 00000001 [................]

90C9440 00000000 00000001 01800022 01800022 [........"..."...]

90C9450 00000000 00000000 00000000 00000000 [................]

        Repeat 249 times

90CA3F0 00000000 00000000 00000000 6F8B1001 [...............o]

 Extent Control Header

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

 Extent Header:: spare1: 0      spare2: 0      #extents: 1      #blocks: 7    

                  last map 0x00000000 #maps: 0      offset: 4128 

      Highwater:: 0x01800023 ext#: 0      blk#: 1      ext size: 7  --此时HWM为：0x01800023

 #blocks in seg. hdr's freelists: 1    

 #blocks below: 1    

 mapblk 0x00000000 offset: 0    

                   Unlocked

     Map Header:: next 0x00000000 #extents: 1    obj#: 53036 flag: 0x40000000

 Extent Map

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

   0x01800022 length: 7    

 

 nfl = 1, nfb = 1 typ = 1 nxf = 0 ccnt = 1

 SEG LST:: flg: USED   lhd: 0x01800022 ltl: 0x01800022

buffer tsn: 7 rdba: 0x01800022 (6/34)

scn: 0x0000.00217094 seq: 0x02 flg: 0x06 tail: 0x70940602

frmt: 0x02 chkval: 0x22ca type: 0x06=trans data

Hex dump of block: st=0, typ_found=1

Dump of memory from 0x090C8400 to 0x090CA400

90C8400 0000A206 01800022 00217094 06020000 [...."....p!.....]

90C8410 000022CA 00000001 0000CF2C 00217093 [."......,....p!.]

90C8420 00000000 00030002 00000000 000B0002 [................]

90C8430 00000200 00800345 00190179 00002001 [....E...y.... ..]

90C8440 00217094 002A0003 0000020D 008003DB [.p!...*.........]

90C8450 00260161 00008000 00217045 00030100 [a.&.....Ep!.....]

90C8460 0018FFFF 1F551F6D 00001F55 1F8F0003 [....m.U.U.......]

90C8470 1F6D1F7E 00000000 00000000 00000000 [~.m.............]

90C8480 00000000 00000000 00000000 00000000 [................]

        Repeat 499 times

90CA3C0 00000000 00000000 02012C00 0A04C102 [.........,......]

90CA3D0 61616161 61616161 002C6161 03C10202 [aaaaaaaaaa,.....]

90CA3E0 6161610A 61616161 2C616161 C1020200 [.aaaaaaaaaa,....]

90CA3F0 61610A02 61616161 61616161 70940602 [..aaaaaaaaaa...p]

Block header dump: 0x01800022

 Object id on Block? Y

 seg/obj: 0xcf2c csc: 0x00.217093 itc: 2 flg: O typ: 1 - DATA

     fsl: 0 fnx: 0x0 ver: 0x01

 

 Itl           Xid                  Uba         Flag Lck        Scn/Fsc

0x01   0x0002.00b.00000200 0x00800345.0179.19 --U-    1 fsc 0x0000.00217094

0x02   0x0003.02a.0000020d 0x008003db.0161.26 C---    0 scn 0x0000.00217045

 

data_block_dump,data header at 0x90c845c

===============

tsiz: 0x1fa0   --Total data area size： 8k的block: 8192-20(block head)-24(Transaction Header)-24*2(一个事务条)-4(block tail)=8096(0x1fa0)

hsiz: 0x18     --data head size   

pbl: 0x090c845c

bdba: 0x01800022

     76543210

flag=--------

ntab=1                      --表示存放一张表的数据，当存放cluster时，可能出现ntab〉1

nrow=3                      --表示现在该block 内有三行数据

frre=-1

fsbo=0x18                  --表示可以放数据的空间的起始位置（发现该值和hsiz保持一至）

fseo=0x1f6d                --表示可以存放数据的end位置: 8062

avsp=0x1f55

tosp=0x1f55

0xe:pti[0] nrow=3 offs=0

0x12:pri[0] offs=0x1f8f   --0x1f8f+tl:17=8079+17=8096

0x14:pri[1] offs=0x1f7e  --0x1f7e+t1:17=8062+17=8079

0x16:pri[2] offs=0x1f6d   --0x1f6d+t1:17=8045+17=8062

block_row_dump:

tab 0, row 0, @0x1f8f

tl: 17 fb: --H-FL-- lb: 0x0 cc: 2

col 0: [ 2] c1 02

col 1: [10] 61 61 61 61 61 61 61 61 61 61

tab 0, row 1, @0x1f7e

tl: 17 fb: --H-FL-- lb: 0x0 cc: 2

col 0: [ 2] c1 03

col 1: [10] 61 61 61 61 61 61 61 61 61 61

tab 0, row 2, @0x1f6d

tl: 17 fb: --H-FL-- lb: 0x1 cc: 2

col 0: [ 2] c1 04

col 1: [10] 61 61 61 61 61 61 61 61 61 61

end_of_block_dump

End dump data blocks tsn: 7 file#: 6 minblk 33 maxblk 34

 

分析上面的灰色部分，翻译如下：

 

90CA3C0 00000000 00000000 02012C00 0A04C102 [.........,......]

翻译：                         002C0102 02C1040A  

          . . . . . . . . . , . . . . . .                    

90CA3D0 61616161 61616161 002C6161 03C10202 [aaaaaaaaaa,.....]

          61616161 61616161 61612C00 0202C103

          a a a a a a a a a a, . . . . .

90CA3E0 6161610A 61616161 2C616161 C1020200 [.aaaaaaaaaa,....]

          0A616161 61616161 6161612C 000202C1

          . a a a a a a a a a a,   . . . .

90CA3F0 61610A02 61616161 61616161 70940602 [..aaaaaaaaaa...p]

          020A6161 61616161 61616161 02069470 –最后四位为block tail

          . . a a a a a a a a a a   . . . P

0x12:pri[0] offs=0x1f8f 对应得物理地址为90CA3EB-90CA3FB共17个字节

0x14:pri[1] offs=0x1f7e  对应得物理地址为90CA3DA-90CA3EA共17个字节

0x16:pri[2] offs=0x1f6d  对应得物理地址为90CA3C9-90CA3D9共17个字节

0,1,2对应rowid的行号，如下：

 

SQL> select a.id,a.name,dbms_rowid.rowid_block_number(rowid) blockid,dbms_rowid.rowid_row_number(rowid) row# from miliatest a;

         ID NAME             BLOCKID       ROW#
----------- --------------- ---------- ----------
          1 aaaaaaaaaa        34          0
          2 aaaaaaaaaa        34          1
          3 aaaaaaaaaa        34          2

 

注：ROWID组成 OOOOOOFFFBBBBBBRRR 其中，O是对象ID，F是文件ID，B是块ID，R是行ID

   通过分析导出的文件，我们看到rowid里存放了偏移量数组下标，即所说的行ID，

   而没有直接在rowid里存放偏移量，那oracle为何要如此设计呢，猜想，一个重要原因

   是可以在做更新操作的时候（非分区表），而不需要修改索引（更新列不为索引列）

在做更新操作的时候，偏移量会改变吗？

SQL> select rowid

 2 from miliatest a

 3 where id=2;

 

ROWID

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

AAAM8sAAGAAAAAiAAB

 

更新第二条记录：

declare
m_address varchar2(2000):='';
begin
for i in 1..2000 loop
m_address:=m_address||'b';
end loop;
update MILIATEST
   set ADRESS=m_address
 where id=2;
commit;
end;

 

 

SQL> select dbms_rowid.rowid_block_number(rowid) blockid,dbms_rowid.rowid_row_number(rowid) row#

 2 from miliatest a

 3 where id=2;

 

   BLOCKID       ROW#

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

34                          1

 

SQL> select rowid

 2 from miliatest a

 3 where id=2;

 

ROWID

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

AAAM8sAAGAAAAAiAAB

 

可以看出更新后rowid没有变化。

alter system dump datafile 6 block 34;

 

Dump of memory from 0x090C8400 to 0x090CA400
90C8400 0000A206 0180002200217CB3 02020000 [...."....|!.....]
90C8410 00000000 00000001 0000CF2C 00217093 [........,....p!.]
90C8420 00000000 00030002 00000000 000B0002 [................]
90C8430 00000200 00800345 00190179 00002001 [....E...y.... ..]
90C8440 00217094 002F000A 000001C5 0080014B [.p!.../.....K...]
90C8450 0016016D 00002001 00217CB3 00030100 [m.... ...|!.....]
90C8460 0018FFFF 17821789 00001782 1F8F0003 [................]
90C8470 1F6D1789 00000000 00000000 00000000 [..m.............]
90C8480 00000000 00000000 00000000 00000000 [................]
        Repeat 373 times
90C9BE0 00000000 03022C00 0A03C102 61616161 [.....,......aaaa]
90C9BF0 61616161 D0FE6161 62626207 62626262 [aaaaaa...bbbbbbb]
90C9C00 62626262 62626262 62626262 62626262 [bbbbbbbbbbbbbbbb]
        Repeat 123 times
90CA3C0 62626262 62626262 02012C62 0A04C102 [bbbbbbbbb,......]-更新后第1行
90CA3D0 61616161 61616161 002C6161 03C10202 [aaaaaaaaaa,.....]-第2行
90CA3E0 6161610A 61616161 2C616161 C1020200 [.aaaaaaaaaa,....]-更新前第1行（这里的数据已经无效了，因为第1行偏移量地址已经改变）
90CA3F0 61610A02 61616161 61616161 7CB30602 [..aaaaaaaaaa...|]—第0行
Block header dump: 0x01800022
 Object id on Block? Y
 seg/obj: 0xcf2c csc: 0x00.217093 itc: 2 flg: O typ: 1 - DATA
     fsl: 0 fnx: 0x0 ver: 0x01
 
 Itl           Xid                  Uba         Flag Lck        Scn/Fsc
0x01   0x0002.00b.00000200 0x00800345.0179.19 --U-    1 fsc 0x0000.00217094
0x02   0x000a.02f.000001c5 0x0080014b.016d.16 --U-    1 fsc 0x0000.00217cb3
 
data_block_dump,data header at 0x90c845c
===============
tsiz: 0x1fa0
hsiz: 0x18
pbl: 0x090c845c
bdba: 0x01800022
     76543210
flag=--------
ntab=1
nrow=3
frre=-1
fsbo=0x18
fseo=0x1789
avsp=0x1782
tosp=0x1782
0xe:pti[0] nrow=3 offs=0
0x12:pri[0] offs=0x1f8f
0x14:pri[1] offs=0x1789 –更新后第1行偏移量地址改变了，而第0行和第2行没有影响

-0x1789+ tl: 2020=8062，此时fseo=0x1789
0x16:pri[2] offs=0x1f6d
block_row_dump:
tab 0, row 0, @0x1f8f
tl: 17 fb: --H-FL-- lb: 0x0 cc: 2
col 0: [ 2] c1 02
col 1: [10] 61 61 61 61 61 61 61 61 61 61
tab 0, row 1, @0x1789
tl: 2020 fb: --H-FL-- lb: 0x2 cc: 3
col 0: [ 2] c1 03
col 1: [10] 61 61 61 61 61 61 61 61 61 61
col 2: [2000]
 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62
 62 62 62 62 62 62........................................................
 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62 62
tab 0, row 2, @0x1f6d
tl: 17 fb: --H-FL-- lb: 0x1 cc: 2
col 0: [ 2] c1 04
col 1: [10] 61 61 61 61 61 61 61 61 61 61
end_of_block_dump

 

 

设计修改数据导致产生行迁移：

declare
m_address varchar2(2000):='';
m_name varchar2(2000):='';
begin
for i in 1..2000 loop
m_address:=m_address||'b';
end loop;
for i in 1..1000 loop
m_name:=m_name||'c';
end loop;
for j in 1..3 loop
update MILIATEST
   set ADRESS=m_address
      ,name=m_name
 where id=j;
commit;
end loop;
end;

 

SQL> exec show_space('MILIATEST');

 

Free Blocks.............................1

Total Blocks............................8

Total Bytes.............................65536

Unused Blocks...........................5

Unused Bytes............................40960

Last Used Ext FileId....................6

Last Used Ext BlockId...................33

Last Used Block.........................3 

 

Start dump data blocks tsn: 7 file#: 6 minblk 34 maxblk 35
buffer tsn: 7 rdba: 0x01800022 (6/34)
scn: 0x0000.0021803d seq: 0x04 flg: 0x02 tail: 0x803d0604
frmt: 0x02 chkval: 0x0000 type: 0x06=trans data
Hex dump of block: st=0, typ_found=1

tsiz: 0x1fa0
hsiz: 0x18
pbl: 0x090c845c
bdba: 0x01800022
     76543210
flag=--------
ntab=1
nrow=3
frre=-1
fsbo=0x18
fseo=0x80f
avsp=0x7f7
tosp=0x7f7
0xe:pti[0] nrow=3 offs=0
0x12:pri[0] offs=0x13dc =5084+3012byte=8096
0x14:pri[1] offs=0x13d3 =5075 +9byte=5084
0x16:pri[2] offs=0x80f   +3012byte=5075
block_row_dump:
tab 0, row 0, @0x13dc
tl: 3012 fb: --H-FL-- lb: 0x0 cc: 3
col 0: [ 2] c1 02
col 1: [1000]
 63636363636363636363636363636363636363636363636363
 ..........................................................................
 63636363636363636363636363636363636363636363636363
col 2: [2000]
 63636363636363636363636363636363636363636363636363
 .........................................................................
 63636363636363636363636363636363636363636363636363
tab 0, row 1, @0x13d3
tl: 9 fb: --H----- lb: 0x0 cc: 0列的个数为0，表示没有存放数据
nrid: 0x01800023.0       表示迁移的地址为： rdba为0x01800023，行号为0的地方     
tab 0, row 2, @0x80f
tl: 3012 fb: --H-FL-- lb: 0x2 cc: 3
col 0: [ 2] c1 04
col 1: [1000]
 63636363636363636363636363636363636363636363636363
 .........................................................................
 63636363636363636363636363636363636363636363636363
col 2: [2000]
 62626262626262626262626262626262626262626262626262
 .........................................................................
 62626262626262626262626262626262626262626262626262
end_of_block_dump

 

我们再看看block35的dump文件：

buffer tsn: 7 rdba: 0x01800023 (6/35)
scn: 0x0000.0021803b seq: 0x05 flg: 0x02 tail: 0x803b0605
frmt: 0x02 chkval: 0x0000 type: 0x06=trans data
Hex dump of block: st=0, typ_found=1
Dump of memory from 0x090C8400 to 0x090CA400

Block header dump: 0x01800023
 Object id on Block? Y
 seg/obj: 0xcf2c csc: 0x00.218039 itc: 3 flg: O typ: 1 - DATA
     fsl: 0 fnx: 0x0 ver: 0x01
 
 Itl           Xid                  Uba         Flag Lck        Scn/Fsc
0x01   0x0009.00b.00000216 0x00800058.0198.1c --U-    1 fsc 0x0000.0021803b
0x02   0x0000.000.00000000 0x00000000.0000.00 ----    0 fsc 0x0000.00000000
0x03   0x0000.000.00000000 0x00000000.0000.00 C---    0 scn 0x0000.00000000
 
data_block_dump,data header at 0x90c8474
===============
tsiz: 0x1f88
hsiz: 0x14
pbl: 0x090c8474
bdba: 0x01800023
     76543210
flag=--------
ntab=1
nrow=1
frre=-1
fsbo=0x14
fseo=0x13be
avsp=0x13aa
tosp=0x13aa
0xe:pti[0] nrow=1 offs=0
0x12:pri[0] offs=0x13be
block_row_dump:
tab 0, row 0, @0x13be
tl: 3018 fb: ----FL-- lb: 0x1 cc: 3
hrid: 0x01800022.1 －－表示这行是从rdba为0x01800022，行号为1的地方迁移来的
col 0: [ 2] c1 03
col 1: [1000]
 62626262626262626262626262626262626262626262626262
 .........................................................................
 62626262626262626262626262626262626262626262626262
col 2: [2000]
 62626262626262626262626262626262626262626262626262
 .........................................................................
 62626262626262626262626262626262626262626262626262
end_of_block_dump

 

为什么说上面发生的是行迁移而不是行链接呢？

要解答这个问题，就要明白行链接和行迁移的区别了，发生行迁移后，在dump文件里该行的cc: 0  列的个数为0

而发生行链接，cc的个数是不为零的，在原来的地址仍然存放着数据的。

 

从上面地实验可以看出oracle 是这样存储和访问数据的：

结论：
1、当我们往非分区表里插入一条数据后：在block内记录下它的行id（安先后插入顺序，从0开始），以及它在block内的偏移量地址（offs=XXXX）
oracle数据是从block底向上存放的，第一次Insert时offs从大到小

2、当我们修改数据时，行id是不变的，也就是它的rowid不变，而只是修改偏移量数组中对应的偏移量
Oracle在通过rowid读取数据时，先定位block，在通过行id，得到offs，最后得到具体值
当全表扫描时，在同一个block内数据顺序是偏移量数组的顺序，不是实际存放数据的顺序，发生行迁移后遵循block顺序

3、行迁移时，它的rowid也是不变的，只是在原来存放数据的地方放一个nrid，指示数据被存放的block地址及行号。