oracle查看表空间使用情况

sql如下：

select 
a.a1 表空间名称,
c.c2 类型,
c.c3 区管理,
b.b2/1024/1024/1024 表空间大小G,
(b.b2-a.a2)/1024/1024/1024 已使用G,
substr((b.b2-a.a2)/b.b2*100,1,5) 已利用率,
round(100-(substr((b.b2-a.a2)/b.b2*100,1,5)), 3) 剩余利用率
from 
(select  tablespace_name a1, sum(nvl(bytes,0)) a2 from dba_free_space group by tablespace_name) a,
(select tablespace_name b1,sum(bytes) b2 from dba_data_files group by tablespace_name) b,
(select tablespace_name c1,contents c2,extent_management c3  from dba_tablespaces) c 
where a.a1=b.b1 and c.c1=b.b1;