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一. 准备知识：ORACLE 的逻辑存储管理.

ORACLE 在逻辑存储上分 4 个粒度: 表空间, 段, 区和块.

1.1 块: 是粒度最小的存储单位,现在标准的块大小是 8K,ORACLE 每一次 I/O

操作也是按块来操作的,也就是说当 ORACLE 从数据文件读数据时,是读取多少个

块,而不是多少行. 每一个 Block 里可以包含多个 row.

1.2 区: 由一系列相邻的块而组成,这也是 ORACLE 空间分配的基本单位,举个

例子来说,当我们创建一个表 Dave 时,首先 ORACLE 会分配一区的空间给这个表,

随着不断的 INSERT 数据到 Dave,原来的这个区容不下插入的数据时,ORACLE 是

以区为单位进行扩展的,也就是说再分配多少个区给 Dave,而不是多少个块.

1.3 段: 是由一系列的区所组成, 一般来说, 当创建一个对象时(表,索引),就

会分配一个段给这个对象. 所以从某种意义上来说,段就是某种特定的数据.如

CREATE TABLE Dave,这个段就是数据段,而 CREATE INDEX ON Dave(NAME), ORACLE

同样会分配一个段给这个索引,但这是一个索引段了.查询段的信息可以通过数

据字典: SELECT * FROM USER_SEGMENTS 来获得.

1.4 表空间: 包含段,区及块.表空间的数据物理上储存在其所在的数据文件

中.一个数据库至少要有一个表空间.

表空间(tableSpace) 段(segment) 盘区(extent) 块(block) 关系

http://blog.csdn.net/tianlesoftware/archive/2009/12/08/4962476.aspx

当我们创建了一个表，即使我没有插入任何一行记录, ORACLE 还是给它分配

了 8 个块. 当然这个跟建表语句的 INITIAL 参数及 MINEXTENTS 参数有关. 如：

STORAGE

(

INITIAL 64K

MINEXTENTS 1

MAXEXTENTS UNLIMITED

);

也就是说,在这个对象创建以后,ORACLE 至少给它分配一个区,初始大小是

64K,一个标准块的大小是 8K,刚好是 8 个 BLOCK.

Oracle Table 创建参数说明

http://blog.csdn.net/tianlesoftware/archive/2009/12/07/4954417.aspx

二. 高水线(High Water Mark)

2.1 官网说明如下

http://download.oracle.com/docs/cd/E11882_01/server.112/e16508/logica

l.htm#CNCPT89022

To manage space, Oracle Database tracks the state of blocks in the

segment. The high water mark (HWM) is the point in a segment beyond which

data blocks are unformatted and have never been used.

MSSM uses free lists to manage segment space. At table creation, no

blocks in the segment are formatted. When a session first inserts rows

into the table, the database searches the free list for usable blocks.

If the database finds no usable blocks, then it preformats a group of

blocks, places them on the free list, and begins inserting data into the

blocks. In MSSM, a full table scan reads all blocks below the HWM.

ASSM does not use free lists and so must manage space differently.

When a session first inserts data into a table, the database formats a

single bitmap block instead of preformatting a group of blocks as in MSSM.

The bitmap tracks the state of blocks in the segment, taking the place

of the free list. The database uses the bitmap to find free blocks and

then formats each block before filling it with data. ASSM spread out

inserts among blocks to avoid concurrency issues.

Oracle 自动段空间管理(ASSM:auto segment space management)

http://blog.csdn.net/tianlesoftware/archive/2009/12/07/4958989.aspx

Every data block in an ASSM segment is in one of the following states:

（1）Above the HWM

These blocks are unformatted and have never been used.

（2）Below the HWM

These blocks are in one of the following states:

（1）Allocated, but currently unformatted and unused

（2）Formatted and contain data

（3）Formatted and empty because the data was deleted

Figure 12-23 depicts an ASSM segment as a horizontal series of blocks.

At table creation, the HWM is at the beginning of the segment on the left.

Because no data has been inserted yet, all blocks in the segment are

unformatted and never used.

Figure 12-23 HWM at Table Creation

Suppose that a transaction inserts rows into the segment. The database

must allocate a group of blocks to hold the rows. The allocated blocks

fall below the HWM. The database formats a bitmap block in this group to

hold the metadata, but does not preformat the remaining blocks in the

group.

In Figure 12-24, the blocks below the HWM are allocated, whereas blocks

above the HWM are neither allocated or formatted. As inserts occur, the

database can write to any block with available space. The low high water

mark (low HWM) marks the point below which all blocks are known to be

formatted because they either contain data or formerly contained data.

Figure 12-24 HWM and Low HWM

In Figure 12-25, the database chooses a block between the HWM and low

HWM and writes to it. The database could have just as easily chosen any

other block between the HWM and low HWM, or any block below the low HWM

that had available space. In Figure 12-25, the blocks to either side of

the newly filled block are unformatted.

Figure 12-25 HWM and Low HWM

The low HWM is important in a full table scan. Because blocks below

the HWM are formatted only when used, some blocks could be unformatted,

as in Figure 12-25. For this reason, the database reads the bitmap block

to obtain the location of the low HWM. The database reads all blocks up

to the low HWM because they are known to be formatted, and then carefully

reads only the formatted blocks between the low HWM and the HWM.

Assume that a new transaction inserts rows into the table, but the

bitmap indicates that insufficient free space exists under the HWM. In

Figure 12-26, the database advances the HWM to the right, allocating a

new group of unformatted blocks.

Figure 12-26 Advancing HWM and Low HWM

When the blocks between the HWM and low HWM are full, the HWM advances

to the right and the low HWM advances to the location of the old HWM. As

the database inserts data over time, the HWM continues to advance to the

right, with the low HWM always trailing behind it. Unless you manually

rebuild, truncate, or shrink the object, the HWM never retreats.

Oracle 数据块 Block 说明

http://blog.csdn.net/tianlesoftware/archive/2011/05/12/6414765.aspx

2. 2 Oracle 表段中的高水位线 HWM

在 Oracle 数据的存储中，可以把存储空间想象为一个水库，数据想象为水

库中的水。水库中的水的位置有一条线叫做水位线，在 Oracle 中，这条线被称

为高水位线（High-warter mark, HWM）。在数据库表刚建立的时候，由于没有任

何数据，所以这个时候水位线是空的，也就是说 HWM 为最低值。当插入了数据以

后，高水位线就会上涨，但是这里也有一个特性，就是如果你采用 delete 语句

删除数据的话，数据虽然被删除了，但是高水位线却没有降低，还是你刚才删除

数据以前那么高的水位。也就是说，这条高水位线在日常的增删操作中只会上涨，

不会下跌。HWM 通常增长的幅度为一次 5 个数据块.

Select 语句会对表中的数据进行一次扫描，但是究竟扫描多少数据存储块呢，

这个并不是说数据库中有多少数据，Oracle 就扫描这么大的数据块，而是 Oracle

会扫描高水位线以下的数据块。现在来想象一下，如果刚才是一张刚刚建立的空

表，你进行了一次 Select 操作，那么由于高水位线 HWM 在最低的 0 位置上，所

以没有数据块需要被扫描，扫描时间会极短。而如果这个时候你首先插入了一千

万条数据，然后再用 delete 语句删除这一千万条数据。由于插入了一千万条数

据，所以这个时候的高水位线就在一千万条数据这里。后来删除这一千万条数据

的时候，由于 delete 语句不影响高水位线，所以高水位线依然在一千万条数据

这里。这个时候再一次用 select 语句进行扫描，虽然这个时候表中没有数据，

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