ORACLE 如何知道一个对象占用空间情况？

select segment_name,segment_type,bytes/1024/1024 from dba_segments where owner='SYSTEM' order by bytes desc

select segment_type, sum(bytes)/1024/1024 from dba_segments where tablespace_name='AA' group by segment_type;

select bytes/1024/1024 sizeM, owner, segment_name
from dba_segments
where tablespace_name = 'DD'
and segment_type = 'TABLE' order by sizeM desc;