これは非常に簡単です：
09:38:54 [email protected]_xe> l
1 select sys_connect_by_path(dp.dir_name, '/') path
2 ,(select count(file_id)
3 from dirs dc
4 ,files f
5 where f.dir_id(+) = dc.dir_id
6 connect by prior dc.dir_id = dc.parent_dir_id
7 start with dc.dir_id = dp.dir_id
8 ) count
9 from dirs dp
10 connect by prior dp.dir_id = dp.parent_dir_id
11* start with dp.parent_dir_id is null
09:38:55 [email protected]_xe> /
PATH COUNT
------------------------------ ----------
/ROOT 6
/ROOT/DIR1_1 4
/ROOT/DIR1_1/DIR2_1 1
/ROOT/DIR1_1/DIR2_2 3
/ROOT/DIR1_2 2
5 rows selected.
Elapsed: 00:00:00.02