let vtables_using_column_part = fun c ->
let db = c.col_table.ta_db
and vtable_use_column = fun vtable ->
List.exists
(fun (_, columns) ->
List.exists (fun (c1, c2) -> c1 == c || c2 == c) columns)
vtable.vt_join
in
List.partition vtable_use_column db.db_vtables