select TABLESPACE_NAME, BYTES_USED/1024/1024 "MB Used", BYTES_FREE/1024/1024 "MB Free" from V$TEMP_SPACE_HEADER;
List tempfiles:
select file_name, tablespace_name, bytes/1024/1024, autoextensible from dba_temp_files;
Add a tempfile:
ALTER TABLESPACE TempTablespaceName ADD TEMPFILE 'TempfilePathAndName' SIZE 1G;
Modify a tempfile:
alter database tempfile 'TempfilePathAndName' autoextend on next 1G maxsize 31G;