清理某个表空间，所有segment全部清理了，查dba_tables视图，该表空间中还有表存在，奇了怪了，google下。

ORACLE 11g 新增了一个参数：deferred_segment_creation，默认是true。具体是如果deferred_segment_creation为true，当新建空表时不会立即分配extent，即不占数据空间，只有insert数据后才分配空间，这样做可以节省少量的空间。

开始测试

1. 先在11g中测试

SQL> select * from v$version where rownum<2;

BANNER

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

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

SQL> select a.name, a.value, a.DESCRIPTION FROM v$parameter a WHERE a.name like '%deferred_segment_creation%';

NAME VALUE DESCRIPTION

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

deferred_segment_creation TRUE defer segment creation to first insert

新建空表，不插入任何行：

SQL> create table t0821 as select * from dba_objects where 1 = 2;

Table created.

--查询，该表没有分配任何extent

SQL> select * from dba_extents a where a.segment_name = 'T0821';

no rows selected

需要分配空间的话， alter TABLE tb_name allocate EXTENT (SIZE size_num);

(2) 修改 deferred_segment_creation 为false

SQL> alter session set deferred_segment_creation=false;

Session altered.

SQL> create table t0822 as select * from dba_objects where 1=2;

Table created.

之后查询 dba_extents，表T0822已经分配extent

2. 10g中测试，10.2.0.1.0,

10g中没有参数 deferred_segment_creation

select * FROM v$parameter a WHERE lower(a.name) like '%deferred_segment_creation%'; --0

SQL> CREATE TABLE t0821 STORAGE(INITIAL 0k) AS SELECT * from dba_objects WHERE 1 = 2;

Table created.

SQL> select count(*) from dba_extents where segment_name = 'T0821';

COUNT(*)

----------

2

11g extent延迟创建