oracle datafile 生成查询树

SELECT DECODE(file_name, 'M', tablespace_name, LPAD(' --->',5) || file_name) tablespace_name FROM (SELECT * FROM (SELECT tablespace_name, 'M' file_nameFROM dba_tablespaces UNION SELECT dba_tablespaces.tablespace_name, dba_data_files.file_name || '  ' || '(Allocated:' || ROUND(bytes/(1024*1024),2) || 'MB)' FROM dba_tablespaces, dba_data_files WHERE dba_tablespaces.tablespace_name = dba_data_files.tablespace_name) ORDER BY tablespace_name, file_name DESC);