There are at least five ways to store a tree in PostgreSQL, for example: adjacency list, nested sets like MPTT Modified Preorder Tree Traversal, nested intervals, Materialized Paths, ltree, JSON

E.g. django-mptt : https://github.com/django-mptt/django-mptt/blob/main/mptt/mo... :

  indexed_attrs = (mptt_meta.tree_id_attr,)
  field_names = (
    mptt_meta.left_attr,
    mptt_meta.right_attr,
    mptt_meta.tree_id_attr,
    mptt_meta.level_attr, )

> The nested interval model stores the position of the nodes as rational numbers expressed as quotients (n/d).