We have a cronjob at 8:00 and 16:00 to check tablespace free size , use below sql
***************************************************
select tablespace_name, sum(freesize) "FREE SIZE(M)", sum(extentleft) "EXTENTS LEFT" from
(
select file_id, a.tablespace_name tablespace_name, floor((maxbytes-bytes)/next_extent) extentleft
, floor((maxbytes-bytes)/1024/1024) freesize from dba_tablespaces a, dba_data_files b where
a.tablespace_name = b.tablespace_name and maxbytes > bytes
union all
select file_id, a.tablespace_name tablespace_name, floor(bytes/next_extent) extentleft
, floor(bytes/1024/1024) freesize from dba_tablespaces a, dba_free_space b where
a.tablespace_name = b.tablespace_name
union all
select 0 file_id,tablespace_name, 0 extentleft,0 freesize  from dba_tablespaces where tablespace_name
not in ( select tablespace_name from dba_free_space ) and contents = 'PERMANENT'
)
group by tablespace_name;
******************************************************************
in most time , the undo tablespace in the output result.
as below:
 tablespace_name     free size    extent left
SYSTEM                        637                          
UNDO                            13                          
UTIL_IDX_S                  334           682             

but sometimes at 8:00 the job output not include undo tablespace

that is really strange?

let find the root cause