oracle 11gR2 创建无数据对象临时不分配segment

做实验发现oracle11gR2，创建无数据的table、index等对象，oracle为了节省空间，开始并不分配segment，而是等有dml操作时才会分配segment，即使dml回滚，分配的segment并不回收。简单实验如下：
SQL> create table test(a number);

Table created.

SQL> create index ind_test on test(a);

Index created.

QL> select segment_name,bytes from dba_segments wheresegment_name='TEST' and owner='WXLUN';

no rows selected

SQL> insert into test values(1);

1 row created.

SQL>
SQL> col SEGMENT_NAME for a22
SQL> select segment_name,bytes from dba_segments wheresegment_name in ('TEST','IND_TEST') and owner='WXLUN';

SEGMENT_NAME 

              BYTES
---------------------- ----------
IND_TEST               &nbs