Hacker News Reader

Doesn’t feel natural in Postgres. I’d check an auxiliary table nodes_nodes (integer, integer) having an index of all ancestor-descendants.

Agree its a bit unnatural, but it seems workable. My problem with the auxiliary table is, I wouldn't know how to implement the unique constraint.

In your current implementation you either have no unique constraint or it is a more particular case of UNIQUE(ancestor, descendant).