全表扫先读段头块,而后在段头块里面找到HWM
下面用实验由内而外来理解Oracle的HWM
--t表有一条数据
hr@ORCL> select * from t;
ID NAME
---------- ----------
1 AAAAA
--找t段的段头块
hr@ORCL> select header_file,header_block from dba_segments where segment_name='T' and owner='HR';
HEADER_FILE HEADER_BLOCK
----------- ------------
4 387
--另开一个session,dump段头块
sys@ORCL> alter session set tracefile_identifier='sys_dump_t_01';
Session altered.
sys@ORCL> alter system dump datafile 4 block 387;
System altered.
Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 1 #blocks: 8
last map 0x00000000 #maps: 0 offset: 2716
Highwater:: 0x01000189 ext#: 0 blk#: 8 ext size: 8 --Highwater就是高水位,0x01000189这个是HWM的地址
#blocks in seg. hdr's freelists: 0
#blocks below: 5
mapblk 0x00000000 offset: 0
Unlocked
--------------------------------------------------------
Low HighWater Mark :
Highwater:: 0x01000189 ext#: 0 blk#: 8 ext size: 8
#blocks in seg. hdr's freelists: 0
#blocks below: 5
mapblk 0x00000000 offset: 0
Level 1 BMB for High HWM block: 0x01000181
Level 1 BMB for Low HWM block: 0x01000181
--------------------------------------------------------
Segment Type: 1 nl2: 1 blksz: 8192 fbsz: 0
L2 Array start offset: 0x00001434
First Level 3 BMB: 0x00000000
L2 Hint for inserts: 0x01000182
Last Level 1 BMB: 0x01000181
Last Level II BMB: 0x01000182
Last Level III BMB: 0x00000000
Map Header:: next 0x00000000 #extents: 1 obj#: 52713 flag: 0x10000000
Inc # 0
Extent Map
-----------------------------------------------------------------
0x01000181 length: 8
Auxillary Map
--------------------------------------------------------
Extent 0 : L1 dba: 0x01000181 Data dba: 0x01000184
--------------------------------------------------------
Second Level Bitmap block DBAs
--------------------------------------------------------
DBA 1: 0x01000182
End dump data blocks tsn: 4 file#: 4 minblk 387 maxblk 387
--对t表做一次全表扫
hr@ORCL> set autot traceonly
hr@ORCL> select * from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 20 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T | 1 | 20 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
7 consistent gets --全表扫读了6个块
0 physical reads
0 redo size
469 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
hr@ORCL> select file_id,block_id,blocks from dba_extents where segment_name='T';
FILE_ID BLOCK_ID BLOCKS
---------- ---------- ----------
4 385 8
sys@ORCL> select to_number('01000189','xxxxxxxx') from dual;
TO_NUMBER('01000189','XXXXXXXX')
--------------------------------
16777609
sys@ORCL> select dbms_utility.data_block_address_file(16777609) from dual;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_FILE(16777609)
----------------------------------------------
4
sys@ORCL> select dbms_utility.data_block_address_block(16777609) from dual;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(16777609)
-----------------------------------------------
393
Low HighWater Mark :
Highwater:: 0x01000189 ext#: 0 blk#: 8 ext size: 8
先来理清dba_tables里面的字段blocks的含义
dba_tables.blocks记录的是分析得到的 formatted block 的总数
而 low hwm 和 high hwm之间可能同时存在 formatted block 和 unfomatted block
所以准确地说 blocks 不能代表 low hwm 或high hwm
如果 low hwm 和 high hwm之间正好没有formatted block时,dba_tables.blocks和low hwm下的blocks一致
那么什么是Oracle中未格式化的块呢?
未格式化,意思就是这个块,已经是属于这个段了,但是还保留着原来的样子没动
格式化就是把块中的数据清除掉,并把块头改为这个对象的
MSSM表空间中的段,只有一个高水位,高水位下的块都是格式化了的
但是ASSM表空间中的段,有两个高水位:低高水位和高高水位
即上文trc里的:Highwater:: 0x01000189和Low HighWater Mark Highwater:: 0x01000189
低高水位下的块全部是格式化了的
但是低高水位和高高水位之间的块,则可能是格式化了的,也可能是没有
现在的t的高高水位是file 4,block 393;其低高水位是file 4,block 393
我们现在再来看一下t现在data_object_id是多少:
hr@ORCL> select object_id,data_object_id from dba_objects where object_name='T';
OBJECT_ID DATA_OBJECT_ID
---------- --------------
52713 52714
sys@ORCL> select to_char('52714','XXXXXXXX') from dual;
TO_CHAR('
---------
CDEA
等等.......