一、引言：
在看《收获，不止Oracle》的神奇，走进逻辑体系世界一章时，需要新建一张表查看Extents的情况，由于该书的环境是ORACLE10G的，因此新建空表以后立刻就分配Segment，而我使用的是Oracle11gR2，新建空表后没有立即分配Segment。这就是11GR2的新特性，延迟段创建，就是说从11GR2开始默认创建的表不会立即分配segment，不会占用磁盘空间，当第一条数据insert时才会分配空间。
二、实验模拟：
SQL> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production PL/SQL Release 11.2.0.1.0 - Production CORE 11.2.0.1.0 Production TNS for Linux: Version 11.2.0.1.0 - Production NLSRTL Version 11.2.0.1.0 - Production SQL> create table testnew(id int primary key,name varchar2(10)); Table created SQL> create table testnew_IME(id int primary key,name varchar2(10)) segment creation immediate; Table created SQL> create table testnew_def(id int primary key,name varchar2(10)) segment creation deferred; Table created SQL> select segment_name from user_segments where segment_name like ‘TESTNEW%‘; SEGMENT_NAME -------------------------------------------------------------------------------- TESTNEW_IME SQL> select INDEX_NAME,TABLE_OWNER from USER_indexes where table_name=‘TESTNEW‘; INDEX_NAME TABLE_OWNER ------------------------------ ------------------------------ SYS_C0011192 JACK SQL> select INDEX_NAME,TABLE_OWNER from USER_indexes where table_name=‘TESTNEW_IME‘; INDEX_NAME TABLE_OWNER ------------------------------ ------------------------------ SYS_C0011193 JACK SQL> select INDEX_NAME,TABLE_OWNER from USER_indexes where table_name=‘TESTNEW_DEF‘; INDEX_NAME TABLE_OWNER ------------------------------ ------------------------------ SYS_C0011194 JACK SQL> select segment_name from user_segments where segment_name=‘SYS_C0011192‘; SEGMENT_NAME -------------------------------------------------------------------------------- SQL> select segment_name from user_segments where segment_name=‘SYS_C0011193‘; SEGMENT_NAME -------------------------------------------------------------------------------- SYS_C0011193 SQL> select segment_name from user_segments where segment_name=‘SYS_C0011194‘; SEGMENT_NAME -------------------------------------------------------------------------------- SQL> insert into testnew values(1,‘anbob.com‘); 1 row inserted SQL> commit; Commit complete SQL> select segment_name from user_segments where segment_name like ‘TESTNEW%‘; SEGMENT_NAME -------------------------------------------------------------------------------- TESTNEW TESTNEW_IME SQL> select INDEX_NAME,TABLE_OWNER from USER_indexes where table_name=‘TESTNEW‘; INDEX_NAME TABLE_OWNER ------------------------------ ------------------------------ SYS_C0011192 JACK SQL> select segment_name from user_segments where segment_name=‘SYS_C0011192‘; SEGMENT_NAME -------------------------------------------------------------------------------- SYS_C0011192 SQL> truncate table testnew; Table truncated SQL> select segment_name from user_segments where segment_name like ‘TESTNEW%‘; SEGMENT_NAME -------------------------------------------------------------------------------- TESTNEW TESTNEW_IME SQL> conn /as sysdba 已连接。 SQL> create table testnew_def(id int primary key,name varchar2(10)) segment creation deferred; create table testnew_def(id int primary key,name varchar2(10)) segment creation deferred * 第 1 行出现错误: ORA-14223: 此表不支持延迟创建段
注意：
11gR2默认是使用segment creation
deferred建立，新建的无记录表不分配segment，当insert第一条记录时分配段空间，不会因truncate而回收，并且在sys
schema里不支持，exp也不会导出。
关于这个主要还是跟deferred_segment_creation参数有关，在11gR2中该参数的值为true，说明当创建对象(如表)，初始没有数据，不会立即创建segment。
如果该参数设置为false，表明之后的创建的表，初始没有数据，会立即创建segment。
下面看一下它的效果：
SQL> alter system set deferred_segment_creation=false; System altered SQL> create table jack(x int); Table created SQL> select segment_name,segment_type,extents,blocks from user_segments where segment_name=‘JACK‘; SEGMENT_NAME SEGMENT_TYPE EXTENTS BLOCKS -------------------------------------------------------------------------------- ------------------ ---------- ---------- JACK TABLE 1 8