select username,default_tablespace from dba_users order by username
需要有dba的权限 
1、查看用户使用的缺省表空间名称
你一定知道你登陆的用户名是吧，
以sysdba登陆。
sqlplus / as sysdba
select username,default_tablespace from dba_users;
2、查看表空间总大小，及其已使用大小
select a.tablespace_name,a.bytes/1024/1024 "Sum MB",(a.bytes-b.bytes)/1024/1024 "used MB",b.bytes/1024/1024 "free MB",
round(((a.bytes-b.bytes)/a.bytes)*100,2) "percent_used"
from
(select tablespace_name,sum(bytes) bytes from dba_data_files group by tablespace_name) a,
(select tablespace_name,sum(bytes) bytes,max(bytes) largest from dba_free_space group by tablespace_name) b
where a.tablespace_name=b.tablespace_name
order by ((a.bytes-b.bytes)/a.bytes) desc;