SQL> select sum(bytes)/1024/1024/1024 “SIZE_GB_TOTAL” from dba_data_files;
SIZE_GB_TOTAL
2671.33588
SQL> select sum(bytes)/1024/1024/1024 “SIZE_GB_ACTUAL” from dba_segments;
2084.94
SQL> select sum(bytes)/1024/1024/1024 “SIZE_GB_FREE” from dba_free_space;
SIZE_GB_FREE
585.972672
Note :-
dba_free_space:- describes the free extents in all tablespaces in the database.
dba_segments :- describes the storage allocated for all segments in the database.
dba_data_files :- describes d