with da as (
 SELECT owner, segment_name, SUM(bytes)/1024/1024 size_mb
   FROM dba_extents
   group by rollup(owner, segment_name)
) select owner, segment_name, size_mb, round(size_mb/total_mb*100)
  from da 
    cross join (
      select size_mb as total_mb 
      from da t where owner is null and segment_name is null
    )
order by size_mb desc