oracle --(三)数据段（segment）

基本关系：数据库---表空间---数据段---分区---数据块

数据段（segment）
段（segment)由一系列的extent组成。通常一张表是一个segment。

Oracle中的段可以分成4种类型：数据段、索引段、回滚段、临时段。
数据段用来存储用户的数据，每个表都有一个对应的回滚段，其名称和数据表的名字相同。索引段用来存储系统、用户的索引信息。回滚段用来存储用户数据修改前的值，回退段与事务是一对多的关系，一个事务只能使用一个回退段，而一个回退段可存放一个或多个事务的回退数据。临时段用于order by语句的排序以及一些汇总。

SQL>  desc dba_segments;
Name                 Type          Nullable Default Comments
-------------------- ------------- -------- ------- --------------------------------------------------------------------------------------------------------------------------------------
OWNER                VARCHAR2() Y                Username of the segment owner
SEGMENT_NAME         VARCHAR2() Y         　　   Name, if any, of the segment
PARTITION_NAME       VARCHAR2() Y          　　  Partition/Subpartition Name, if any, of the segment
SEGMENT_TYPE         VARCHAR2()  Y          　　  Type of segment:  "TABLE", "CLUSTER", "INDEX", "ROLLBACK","DEFERRED ROLLBACK", "TEMPORARY","SPACE HEADER", "TYPE2 UNDO" or "CACHE" 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　                   "TEMPORARY","SPACE HEADER", "TYPE2 UNDO" or "CACHE"
SEGMENT_SUBTYPE      VARCHAR2()  Y      　　　　  SubType of Lob segment:  "SECUREFILE", "ASSM", "MSSM", NULL
TABLESPACE_NAME      VARCHAR2()  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, in bytes, of the initial extent of the segment
NEXT_EXTENT          NUMBER        Y                Size, in bytes, 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
MAX_SIZE          　 NUMBER        Y                Maximum number of blocks allowed in the segment
RETENTION        　  VARCHAR2()   Y                Retention option for SECUREFILE segment
MINRETENTION         NUMBER        Y                Minimum Retention Duration for SECUREFILE 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()   Y                The default buffer pool to be used for segments blocks
FLASH_CACHE       　 VARCHAR2()   Y
CELL_FLASH_CACHE     VARCHAR2()   Y
INMEMORY             VARCHAR2()   Y               Whether in-memory is enabled or not
INMEMORY_PRIORITY    VARCHAR2()   Y               User defined priority in which in-memory column store object is loaded
INMEMORY_DISTRIBUTE  VARCHAR2()  Y                How the in-memory columnar store object is distributed
INMEMORY_DUPLICATE   VARCHAR2()  Y                How the in-memory column store object is duplicated
INMEMORY_COMPRESSION VARCHAR2()  Y        　　　　  Compression level for the in-memory column store option
CELLMEMORY           VARCHAR2()  Y                Cell columnar cache