oracle -- 数据段（segment）
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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(128) Y Username of the segment owner
SEGMENT_NAME VARCHAR2(128) Y Name, if any, of the segment
PARTITION_NAME VARCHAR2(128) Y Partition/Subpartition Name, if any, of the segment
SEGMENT_TYPE VARCHAR2(18) Y Type of segment: "TABLE", "CLUSTER", "INDEX", "ROLLBACK","DEFERRED ROLLBACK", "TEMPORARY","SPACE HEADER", "TYPE2 UNDO" or "CACHE"
SEGMENT_SUBTYPE VARCHAR2(10) Y SubType of Lob segment: "SECUREFILE", "ASSM", "MSSM", NULL
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, 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(7) 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(7) Y The default buffer pool to be used for segments blocks
FLASH_CACHE VARCHAR2(7) Y
CELL_FLASH_CACHE VARCHAR2(7) Y
INMEMORY VARCHAR2(8) Y Whether in-memory is enabled or not
INMEMORY_PRIORITY VARCHAR2(8) Y User defined priority in which in-memory column store object is loaded
INMEMORY_DISTRIBUTE VARCHAR2(15) Y How the in-memory columnar store object is distributed
INMEMORY_DUPLICATE VARCHAR2(13) Y How the in-memory column store object is duplicated
INMEMORY_COMPRESSION VARCHAR2(17) Y Compression level for the in-memory column store option
CELLMEMORY VARCHAR2(24) Y Cell columnar cache
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