此处仅为自己的一些简单测试
SQL> select segment_name,extents,bytes/1024/1024 mb from user_segments;

SEGMENT_NAME                 EXTENTS         MB
-------------------------                   ---------- ----------
TT2                                               206       1472
TT1                                               206       1472
TT                                                 206        1472

SQL> truncate table tt1 reuse storage;

Table truncated.

Elapsed: 00:00:58.48

SQL> truncate table tt;

Table truncated.

Elapsed: 00:01:19.65

SQL> truncate table tt2 drop storage;

Table truncated.

Elapsed: 00:06:12.85
SQL> alter table tt1 deallocate unused;

Table altered.

Elapsed: 00:07:59.81

SQL> select segment_name,extents,bytes/1024/1024 mb from user_segments;

SEGMENT_NAME                 EXTENTS         MB
------------------------- ---------- ----------
TT1                              206       1472
TT                                 1      .0625

Elapsed: 00:00:00.03

SQL> select segment_name,extents from user_segments;

SEGMENT_NAME                 EXTENTS
------------------------- ----------
TT1                              203
TT                                 1
TT2                              206

Elapsed: 00:00:00.02

delete ,truncate ,truncate xxx reuse storage
truncate xxx drop storage;
delete table_name;   对于大表不建议使用
truncate table xxx;   清空表，并回收extent，速度比reuse storage慢
truncate table xxx reuse storage; 清空表，但不回收extent，速度快
truncate table xxx drop storage; 速度慢
alter table xxx deallocate unused; 速度慢

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