deallocate unused :仅适用于释放HWM高水位以上的空间，而无法释放高水位以下的空间；比如对表预分配的空间
使用说明和方法，官方文档有说明，如下：
Use the deallocate_unused_clause to explicitly deallocate unused space at the end of
a database object segment and make the space available for other segments in the
tablespace.
You can deallocate unused space using the following statements:
■ ALTER CLUSTER (see ALTER CLUSTER on page 10-5)
■ ALTER INDEX: to deallocate unused space from the index, an index partition, or an
index subpartition (see ALTER INDEX on page 10-78)
■ ALTER MATERIALIZED VIEW: to deallocate unused space from the overflow segment
of an index-organized materialized view (see ALTER MATERIALIZED VIEW on
page 11-3)
■ ALTER TABLE: to deallocate unused space from the table, a table partition, a table
subpartition, the mapping table of an index-organized table, the overflow segment
of an index-organized table, or a LOB storage segment (see ALTER TABLE on
page 12-2)
一、测试环境：
Oracle11.2.0.4
使用admin用户在test表空间新建一张测试分区表，并提前对子分区预分配一些空间，然后插入少量的数据，最后使用：alter table .....deallocate unused;来释放未使用的空间
二、开始测试
①：查看测试环境表空间使用情况：
SQL> set linesize 2500
SQL> set pagesize 300
SQL> select a.tablespace_name "表空间名称",100 - round((nvl(b.bytes_free, 0) / a.bytes_alloc) * 100, 2) "占用率(%)",round(a.bytes_alloc / 1024 / 1024, 2) "容量(M)",round(nvl(b.bytes_free, 0) / 1024 / 1024, 2) "空闲(M)",round((a.bytes_alloc - nvl(b.bytes_free, 0)) / 1024 / 1024, 2) "使用(M)",to_char(sysdate, 'yyyy-mm-dd hh24:mi:ss') "采样时间" from (select f.tablespace_name,sum(f.bytes) bytes_alloc,sum(decode(f.autoextensible, 'YES', f.maxbytes, 'NO', f.bytes)) maxbytes from dba_data_files f group by tablespace_name) a,(select f.tablespace_name, sum(f.bytes) bytes_free from dba_free_space f group by tablespace_name) b where a.tablespace_name = b.tablespace_name order by 2 desc;
??????????????? ?????????(%) ??????(M) ??????(M) ??????(M) ????????????
------------------------------ ------------ ---------- ---------- ---------- -------------------
SYSAUX 94.41 510 28.5 481.5 2017-08-07 17:01:20
SYSTEM 93.24 800 54.06 745.94 2017-08-07 17:01:20
USERS 26.25 5 3.69 1.31 2017-08-07 17:01:20
UNDOTBS1 17.97 470 385.56 84.44 2017-08-07 17:01:20
TEST .05 20480 20470 10 2017-08-07 17:01:20
SQL> show user
USER is "ADMIN" ----测试用的用户
②：创建测试分区表：test_emp
SQL> create table test_emp (TMP_UPSTATE_CASEKEY char(14),TMP_NUM_STATUS_ID number(38),updated_date date)
2 partition by range(TMP_NUM_STATUS_ID)
3 (partition pt_1 values less than(1000000) ) nologging;
Table created.
③：给pt_1分区表预分配10G的空间：
SQL> alter table test_EMP modify PARTITION pt_1 ALLOCATE EXTENT (size 10240m);
Table altered.
④：收集该分区表的统计信息，并查看该表的block块占用情况
SQL> exec dbms_stats.gather_table_stats(ownname =>'ADMIN',tabname =>'TEST_EMP',cascade=>TRUE);
PL/SQL procedure successfully completed.
SQL> select B.SEGMENT_NAME, B.blocks,B.blocks * 8096 / 1024 / 1024, A.BLOCKS,A.blocks * 8096 / 1024 / 1024, A.EMPTY_BLOCKS from user_tables a, USER_SEGMENTS B WHERE TABLE_NAME = 'TEST_EMP' AND A.TABLE_NAME = B.SEGMENT_NAME;
SEGMENT_NAME BLOCKS B.BLOCKS*8096/1024/1024 BLOCKS A.BLOCKS*8096/1024/1024 EMPTY_BLOCKS
--------------------------------------------------------------------------------- ---------- ----------------------- ---------- ----------------------- ------------
TEST_EMP 1315840 10159.5313 0 0 0
SQL> SELECT segment_name, SUM(bytes) / 1024 / 1024 Mbytes FROM dba_segments WHERE PARTITION_NAME = 'PT_1' GROUP BY segment_name;
SEGMENT_NAME MBYTES
--------------------------------------------------------------------------------- ----------
TEST_EMP 10288
BIN$VibHVCPfDL/gU8gCqMDDfw==$0 8
---从上面可以看出，test_emp分区表的大小是10G，而占用的blocks有1315840个；
⑤：向分区表插入一些数据，并查看该表的大小