一、引言：
在看《收獲，不止Oracle》的神奇，走進邏輯體系世界一章時，需要新建一張表查看Extents的情況，由於該書的環境是ORACLE10G的，因此新建空表以后立刻就分配Segment，而我使用的是Oracle11gR2，新建空表后沒有立即分配Segment。這就是11GR2的新特性，延遲段創建，就是說從11GR2開始默認創建的表不會立即分配segment，不會占用磁盤空間，當第一條數據insert時才會分配空間。
二、實驗模擬：
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 已連接。 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 行出現錯誤: ORA-14223: 此表不支持延遲創建段
注意：
11gR2默認是使用segment creation deferred建立，新建的無記錄表不分配segment，當insert第一條記錄時分配段空間，不會因truncate而回收，並且在sys schema里不支持，exp也不會導出。
關於這個主要還是跟deferred_segment_creation參數有關，在11gR2中該參數的值為true，說明當創建對象(如表)，初始沒有數據，不會立即創建segment。
如果該參數設置為false，表明之后的創建的表，初始沒有數據，會立即創建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