The nested set tree class does not seem to have a function to allow one to retrieve all entries in a table using trees. It only seems to allow access to sub-trees. In other words, can it get the entire tree, all roots/parents and all children, just as a general function (not based on any particular supplied item_id)?
Using $this->get_subtree_data(0, $order_asc, $include_item); as suggested, does not work, because 0 returns nothing if it is used as the item_id, and if you use an overwrite, i.e.: $this->column_item_id = 'parent_id' to make 0 refer to the parent_id in the SQL generated by get_set_of_nodes_data(), it still fails, because that function returns an array $rows[(int) $row[$this->column_item_id]] = $row; in which each $row has the same parent_id key, so you only end up with one parent + one child in the end.
To get the entire tree of data, should it have a basic way to get everything, similar to:
More specifically, a function something like this does what we (naderman, EXreaction and myself) were sort of intending: