How are you modelling your data? I think this should be a self reference.

    create table post (id int primary_key, parent_id int references post(id), child_id int references post(id), created_at timestamp)

How will you place the check constraint? You only have parent_id and child_id, not parent and child entities. You will have to write a trigger.

I am not saying this can't or shouldn't be done. I am saying a db won't directly solve it.

However, you example will work perfectly for enforcing constraints in the code via the mutator which can compare child and parent timestamps, provided pg was doing it via a mutator, and not directly changing the ids.

Following http://stackoverflow.com/questions/3438066/check-constraint-..., and assuming that ID's get doled out in increasing order:

    create table post (
      id int primary_key,
      parent_id int references post(id),
      CONSTRAINT foo CHECK (id > parent_id)
    );