-- Total de espaço utilizados apenas pelos datafiles - Espaço alocado e desalocado
select round((sum(bytes) / (1024*1024))) from dba_data_files

-- Total de segmentos do banco de dados - Espaço alocado pelos objetos
select round((sum(bytes) / (1024*1024))) from Dba_Segments 

-- Total de datafile e tempfiles
select a.datafile + b.tempfile from 
(select round((sum(bytes) / (1024*1024*1024))) as datafile from dba_data_files ) a,
(select round((sum(user_bytes) / (1024*1024*1024))) as tempfile from dba_temp_files) b