Flawed script for finding free tablespace space

Question

Currently i am using the below mentioned script to see the freespace in tablespace. But the Freespace shown in this report is mistakenly shown smaller than the actual freespace in the datafiles.
For example i ran the below mentioned script and found that the tablespace CUSTDATA has only 4634 MB free. So i added another datafile with 2GB space using the following syntax.
ALTER TABLESPACE CUSTDATA
ADD DATAFILE '/u04/oradata/ora145/ora145_CUSTDATA20.dbf' SIZE 500M AUTOEXTEND ON NEXT 20M MAXSIZE 2000M;
After the addition of this datafile, the freespace should be 6634 MB. But my script's resut is showing only 5134.75 MB.
Why is this happening?
My tablespace free space report script
select ddf.tablespace_name,total ttlSize,
(total-nvl(freespace,0))/1024 used, freespace
from (select tablespace_name,sum(bytes/1024/1024)
total from dba_data_files group by tablespace_name) ddf,
(select tablespace_name,sum(bytes/1024/1024)
freespace from dba_free_space group by tablespace_name) dfs where ddf.tablespace_name=dfs.tablespace_name(+); Message was edited by:
for_good_reason

Answer 1

The MAXSIZE (2000M) is the actual size of the
datafile and not SIZE 500M . Right?No, at least when you create the datafile. Actual size is identified by BYTES column, which you use in your query. Example :
SYS@db102 > select file_name,bytes, maxbytes
  2  from dba_data_files
  3  where tablespace_name = 'USERS';
FILE_NAME                                                     BYTES         MAXBYTES
/home/oracle/base/oradata/db102/users01.dbf               104857600      34359721984
SYS@db102 > select tablespace_name,sum(bytes) from dba_free_space
  2  where tablespace_name = 'USERS'
  3  group by tablespace_name;
TABLESPACE_NAME                      SUM(BYTES)
USERS                                  61341696
SYS@db102 > alter tablespace users add datafile
  2  '/home/oracle/base/oradata/db102/users02.dbf'
  3  size 100M autoextend on next 10M maxsize 500M;
Tablespace altered.
SYS@db102 > select file_name,bytes, maxbytes
  2  from dba_data_files
  3  where tablespace_name = 'USERS';
FILE_NAME                                                     BYTES         MAXBYTES
/home/oracle/base/oradata/db102/users01.dbf               104857600      34359721984
/home/oracle/base/oradata/db102/users02.dbf               104857600        524288000
SYS@db102 > select tablespace_name,sum(bytes) from dba_free_space
  2  where tablespace_name = 'USERS'
  3  group by tablespace_name;
TABLESPACE_NAME                      SUM(BYTES)
USERS                                 166133760
SYS@db102 >