计算表空间可以收缩的大小

select a.file#,a.name,a.bytes/1024/1024 CurrentMB,
     ceil(HWM * a.block_size)/1024/1024 ResizeTo,
       (a.bytes - HWM * a.block_size)/1024/1024 ReleaseMB,
       'alter database datafile '''||a.name||''' resize '||
       ceil(HWM * a.block_size/1024/1024) || 'M;' ResizeCMD
from v$datafile a,
     (select file_id,max(block_id+blocks-1) HWM
       from dba_extents e 
       where exists(select 1 From dba_data_files
                             where tablespace_name='LMPS_USERS_201307' and file_id=e.file_id)
       group by file_id) b
where a.file# = b.file_id(+)
and (a.bytes - HWM *block_size)>0
order by 5