如何收回索引的空闲空间

10号数据文件只有一个段，是一个索引

select e.segment_name ,max(block_id + blocks - 1) HWM

  from dba_extents e

 where file_id = 10

 group by e.segment_name ;

SEGMENT_NAME HWM
--------------------- ----------
ASC_MSG_PARA_MID_IDX 16498704

查询可释放的空间

select file# ,df.bytes/1024/1024 file_M,

       free_M ,HWM ,

       (df.bytes - HWM * df.BLOCK_SIZE)/1024/1024 release_M 

  from v$datafile df

  left join (select file_id ,sum(bytes)/1024/1024 free_M

          from dba_free_space 

         group by file_id) fs 

    on df.file# = fs.file_id

  left join (select file_id ,max(block_id + blocks - 1) HWM

          from dba_extents 

         group by file_id) e

    on df.file# = e.file_id

 where file# = 10

 order by 1;

FILE# FILE_M FREE_M HWM RELEASE_M
---------- ---------- ---------- ---------- ----------
10 128924 27.69 16498704 27.88

对索引重建

alter index ASC_MSG_PARA_MID_IDX rebuild online;

但是重建后再次执行上面的查询，水位升高了，空闲空间也增加了
FILE# FILE_M FREE_M HWM RELEASE_M
---------- ---------- ---------- ---------- ----------
10 139924 128873 17907984 17.87

请问这是什么原因呢？我要怎样释放这个数据文件的空间呢？