通常我们认为truncate和drop都是ddl语句,都会释放表占用的空间,且不可回退;而他们的之间的区别在于我们平时忽略的reuse/drop storage子句.
reuse storage不会立即释放表的extent,我们可以先使用truncate table tableName reuse storage,然后分批释放表的extent.这在删除大表时非常有用,避免大量的io操作,影响整体性能.如果使用默认的drop storage就会立即释放extent,删除的表如果非常大,这对系统有时候这可能是灾难性的.
版本：10.2.0.1
说明：在9i下如此操作为报错(后续说明)。
实验过程如下:
SQL> insert into yanga select * from yanga;
50474 rows created.
SQL> /
100948 rows created.
SQL> /
201896 rows created.
SQL> /
403792 rows created.
SQL> /
807584 rows created.
SQL> /
1615168 rows created.
SQL> select sum(bytes)/1024/1024 MB from dba_segments where segment_name=UPPER('yanga');
MB
----------
349
SQL> truncate table yanga reuse storage;
Table truncated.
SQL> select sum(bytes)/1024/1024 MB from dba_segments where segment_name=UPPER('yanga');
MB
----------
349
SQL> alter table yanga deallocate unused keep 50M;
Table altered.
SQL> select sum(bytes)/1024/1024 MB from dba_segments where segment_name=UPPER('yanga');
MB
----------
50.0625
SQL> alter table yanga deallocate unused keep 10M;
Table altered.
SQL> select sum(bytes)/1024/1024 MB from dba_segments where segment_name=UPPER('yanga');
MB
----------
10.0625
SQL> alter table yanga deallocate unused keep 0 M;
Table altered.
SQL> select sum(bytes)/1024/1024 MB from dba_segments where segment_name=UPPER('yanga');
MB
----------
.0625
SQL> drop table yanga purge;
Table dropped.
SQL> select sum(bytes)/1024/1024 MB from dba_segments where segment_name=UPPER('yanga');
MB
----------
以上是10g2的操作。
