从老牛那里, 学了一招：

查询表空间的使用量、剩余量：

select * from sm$ts_used;
 
select * from sm$ts_free;

比起用其它的视图写长长的一段脚本， 简便多了。

两个视图的定义：

SQL>select text from dba_views where  view_name =   'SM$TS_FREE';
 
TEXT

--------------------------------------------------------------------------------

select tablespace_name, sum(bytes) bytes from dba_free_space

    group by tablespace_name
 
SQL>select text from dba_views where  view_name =   'SM$TS_USED';
TEXT

--------------------------------------------------------------------------------

select tablespace_name, sum(bytes) bytes from dba_segments

    group by tablespace_name

据说从7.x到11g都可以用。