let hash_join table1 f1 table2 f2 =
let h = Hashtbl.create 42 in
(* hash phase *)
List.iter (fun s ->
Hashtbl.add h (f1 s) s) table1;
(* join phase *)
List.concat (List.map (fun r ->
List.map (fun s -> s, r) (Hashtbl.find_all h (f2 r))) table2)