truncate 空间不释放问题

SQL> set linesize 200

SQL> select segment_name, sum(bytes / 1024 / 1024/1024)

from dba_segments

where segment_name='T_PM_DEBIT_CARD'

group by segment_name;  2    3    4

SEGMENT_NAME                                                                 SUM(BYTES/1024/1024/1024)

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

T_PM_DEBIT_CARD                                                                  16.1968994

truncate table T_PM_DEBIT_CARD;

SQL> select segment_name, sum(bytes / 1024 / 1024 / 1024)

from dba_segments

where segment_name = 'T_PM_DEBIT_CARD'

group by segment_name;  2    3    4

SEGMENT_NAME                                 
  SUM(BYTES/1024/1024/1024)

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

T_PM_DEBIT_CARD                                       
 .836181641

为什么还有800M呢？

SQL> alter table T_PM_DEBIT_CARD deallocate unused keep 0;

Table altered.

SQL> select segment_name, sum(bytes / 1024 / 1024 / 1024)

from dba_segments

where segment_name = 'T_PM_DEBIT_CARD'

group by segment_name;  2    3    4

SEGMENT_NAME                                    SUM(BYTES/1024/1024/1024)

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

T_PM_DEBIT_CARD                                       
 .051513672

是因为initial
分配的缘故.