oracle 11g 压缩数据文件

通过以下语句直接分析出每个数据库文件可压缩量

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;'||
       'alter database datafile ''' || a.name || ''' autoextend on next 100M;'

        ResizeCMD
  from v$datafile a,
       (select file_id, max(block_id + blocks - 1) HWM
          from dba_extents
         group by file_id) b
 where a.file# = b.file_id(+)
   and (a.bytes - HWM * block_size) > 0
 order by 5 desc