Oracle块，区，段

SEGMENT_NAME?? VARCHAR2(81) Y?????????????? Name,of the segment?????????????????????????????????????????????????????????????????????????????????????????????????????????

PARTITION_NAME VARCHAR2(30) Y??????? Partition/Subpartition Name,of the segment??????????????????????????????????????????????????????????????????????????????????

SEGMENT_TYPE?? VARCHAR2(18) Y??? Type of segment: "TABLE","CLUSTER","INDEX","ROLLBACK",

"DEFERRED ROLLBACK","TEMPORARY","SPACE HEADER","TYPE2 UNDO" or "CACHE"

TABLESPACE_NAME VARCHAR2(30) Y?????????????? Name of the tablespace containing the segment????????????????????????????????????????????????????????????????????????????????????????

HEADER_FILE??? NUMBER????? Y?????????????? ID of the file containing the segment header?????????????????????????????????????????????????????????????????????????????????????????

HEADER_BLOCK?? NUMBER????? Y?????????????? ID of the block containing the segment header????????????????????????????????????????????????????????????????????????????????????????

BYTES????????? NUMBER????? Y?????????????? Size,in bytes,of the segment???????????????????????????????????????????????????????????????????????????????????????????????????????

BLOCKS???????? NUMBER????? Y?????????????? Size,in Oracle blocks,of the segment???????????????????????????????????????????????????????????????????????????????????????????????

EXTENTS??????? NUMBER????? Y?????????????? Number of extents allocated to the segment

INITIAL_EXTENT NUMBER????? Y??????? Size,of the initial extent of the segment?????????????????????????????????????????????????????????????????????????????????

NEXT_EXTENT??? NUMBER????? Y? Size,of the next extent to be allocated to the segment????????????????????????????????????????????????????????????????????

MIN_EXTENTS??? NUMBER????? Y?????????????? Minimum number of extents allowed in the segment?????????????????????????????????????????????????????????????????????????????????????

MAX_EXTENTS??? NUMBER????? Y?????????????? Maximum number of extents allowed in the segment?????????????????????????????????????????????????????????????????????????????????????

PCT_INCREASE?? NUMBER????? Y? Percent by which to increase the size of the next extent to be allocated?????????????????????????????????????????????????????????????

FREELISTS????? NUMBER????? Y? Number of process freelists allocated in this segment????????????????????????????????????????????????????????????????????????????????

FREELIST_GROUPS NUMBER????? Y? Number of freelist groups allocated in this segment??????????????????????????????????????????????????????????????????????????????????

RELATIVE_FNO?? NUMBER????? Y???? Relative number of the file containing the segment header????????????????????????????????????????????????????????????????????????????

BUFFER_POOL??? VARCHAR2(7) Y???? The default buffer pool to be used for segments blocks??????????

从segment_type列的comment信息中，可以看出数据段的类型是多样的。任何种类的数据库对象，本质上都是一种数据段。数据表、索引、回滚、聚集这些都是数据段的一种表现形式。同时，数据段是在数据对象创建的时候就已经创建出来，随着对象体积的增大，而不断分配多个extents进行管理。

另一部分信息可以从dba_segments中读出的，就是该数据对象分配的空间大小和数据块、分区个数。使用这个视图，可以方便的获取到指定schema的所有对象大小。

SQL> select owner,sum(bytes)/1024/1024 as vol,sum(blocks) as totalblocks,sum(extents) as totalextents from dba_segments group by owner having wner=‘SYS‘;

OWNER??????????? VOL TOTALBLOCKS TOTALEXTENTS

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

SYS??????????????? 585.5????? 74944??????? 3248

上面查询，说明SYS的schema，所占用空间585.5MB，包括74944个数据块和3248个分区。

一个对象创建出来之后，在segment层次上是分配一个分区extent和八个数据块block。

有一个问题需要注意，通常我们的数据段是与数据对象相关。一个数据对象对应一个segment。但是，分区表的时候，一个分区要对应一个segment对象。还有就是，segment对象是可以指定存储在那个表空间里，实现存储划分的基础也就在于此。不同类型的segment划分建立在不同的表空间里，才有可能存放在不同的文件中，最后分布在不同的物理存储。

分区实际上就是存在分开存储的可能。一般一个对象是不会跨物理存储进行存放的，分区表是对应的多个segment。所以，分区表分开存储空间是可能的。

表空间tablespace

TableSpace是存储结构中的最高层结构。建立一个表空间的时候，是需要指定存储的文件。一个表空间可以指定多个数据文件，多个文件可以在不同的物理存储上。也就是说，表空间是可以跨物理存储的。但是有一点就是，表空间下一级对象数据段的存储，是不能指定存储在那个文件里的。所以，要想让数据对象访问IO负载均衡，需要指定不同的数据对象在不同的表空间里。这也就是为什么将数据表和索引建立在不同的表空间的原因。

表空间通过v$tablespace进行访问。

SQL> desc v$tablespace;

Name?????????????????????? Type??????? Nullable Default Comments

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

TS#??????????????????????? NUMBER????? Y???????????????????????

NAME?????????????????????? VARCHAR2(30) Y???????????????????????

INCLUDED_IN_DATABASE_BACKUP VARCHAR2(3) Y???????????????????????

BIGFILE??????????????????? VARCHAR2(3) Y???????????????????????

FLASHBACK_ON?????????????? VARCHAR2(3) Y???????????????????????

ENCRYPT_IN_BACKUP????????? VARCHAR2(3) Y?????????????????????

相对于前面的结构视图，表空间视图的结构要简单的多，只是一些描述信息。其中两个参数需要注意一下。

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