ORACLE数据库逻辑结构包括：数据库块（block)，扩展（extent 区），段(segment)，表空间(tablespace)。高水位线就存在于段（segment)中，它用于标识段中已使用过的数据块与未使用地的数据块二者间交界，扫描表数据的时候，高水位线以下的所有数据块都必须被扫描。

首先需要提醒大家，高水位线存在于段，且高水位线的位置记录在段头，也就是段的第一个数据块中。因此，我们可以转存储段头信息来看高水位信息。

段又有很多种，数据段，索引段，临时段，回滚段等。当创建段的时候会分配区，区是由若干个物理连续的数据块组成。区的分配是需要初始化数据块的，默认初始化（format）单位为1M。记住，高水位线并不是初始化的交界片，被初始化过的数据库并不一定使用过。

以下通过实验来进一步阐述高水位线：

创建测试表

SQL> create table hwm as select * from dba_segments;

Table created.

SQL> BEGIN
DBMS_STATS.gather_table_stats('SYS','HWM');
END;
2 3 4
5 /

PL/SQL procedure successfully completed.

SQL> select HEADER_FILE,HEADER_BLOCK from DBA_SEGMENTS where SEGMENT_NAME='HWM'
;

HEADER_FILE HEADER_BLOCK
----------- ------------
1 89168

SQL> ALTER SYSTEM DUMP DATAFILE 1 BLOCK 89168;

Extent Header:: spare1: 0 spare2: 0 #extents: 13 #blocks: 103
last map 0x00000000 #maps: 0 offset: 4128
Highwater:: 0x00415cb5 ext#: 12 blk#: 5 ext size: 8
#blocks in seg. hdr's freelists: 0
#blocks below: 100
mapblk 0x00000000 offset: 12

删除3000行数据提交修改,再看看高水位线有没有变化:

SQL> delete from hwm where rownum < 3000;

2999 rows deleted.

SQL> commit;

Commit complete.

SQL> alter system checkpoint;

System altered.

SQL> ALTER SYSTEM DUMP DATAFILE 1 BLOCK 89168;

Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 13 #blocks: 103
last map 0x00000000 #maps: 0 offset: 4128
Highwater:: 0x00415cb5 ext#: 12 blk#: 5 ext size: 8
#blocks in seg. hdr's freelists: 0
#blocks below: 100
mapblk 0x00000000 offset: 12

并没有发现高水位线有变化,那么插入2000行继续观察

SQL> insert into hwm select * from dba_segments where rownum < 2001;

2000 rows created.

SQL> commit;

Commit complete.

SQL> alter system checkpoint;

System altered.

SQL> ALTER SYSTEM DUMP DATAFILE 1 BLOCK 89168;

System altered.

Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 13 #blocks: 103
last map 0x00000000 #maps: 0 offset: 4128
Highwater:: 0x00415cb5 ext#: 12 blk#: 5 ext size: 8
#blocks in seg. hdr's freelists: 0
#blocks below: 100
mapblk 0x00000000 offset: 12
Unlocked
Map Header:: next 0x00000000 #extents: 13 obj#: 84253 flag: 0x40000000

高水位线并没有发生变化,说明高水位线以下的delete过的数据块是可以被复用的.

此时如果再插入1000行,高水位线应该是会变化的,求证一下:

SQL> insert into hwm select * from dba_segments where rownum < 1001;

1000 rows created.

SQL> commit;

Commit complete.

SQL> alter system checkpoint;

System altered.

SQL> ALTER SYSTEM DUMP DATAFILE 1 BLOCK 89168;

System altered.

Extent Header:: spare1: 0 spare2: 0 #extents: 13 #blocks: 103
last map 0x00000000 #maps: 0 offset: 4128
Highwater:: 0x00415cb5 ext#: 12 blk#: 5 ext size: 8
#blocks in seg. hdr's freelists: 3
#blocks below: 100
mapblk 0x00000000 offset: 12
Unlocked
Map Header:: next 0x00000000 #extents: 13 obj#: 84254 flag: 0x40000000

发现高水位线依旧没有改变,继续插入新数据并观察:

SQL> insert into hwm select * from dba_segments where rownum < 1001;

1000 rows created.

SQL> commit;

Commit complete.

SQL> alter system checkpoint;

System altered.

SQL> ALTER SYSTEM DUMP DATAFILE 1 BLOCK 89168;

System altered.

Extent Header:: spare1: 0 spare2: 0 #extents: 15 #blocks: 119
last map 0x00000000 #maps: 0 offset: 4128
Highwater:: 0x00415cc5 ext#: 14 blk#: 5 ext size: 8
#blocks in seg. hdr's freelists: 1
#blocks below: 116
mapblk 0x00000000 offset: 14
Unlocked
Map Header:: next 0x00000000 #extents: 15 obj#: 84254 flag: 0x40000000