> I notice that they are by default turned off for mysql and wonder if this reflects a de facto judgment that logging slows performance more than is usually worthwhile.

I think it's more like your application is doing the logging already(probably; most of the frameworks do). If you really need it, turn it on yourself.

> Two, if the data were kept in a database, wouldn't something like this be prevented by a constraint preventing a comment from making itself an ancestor?

Copy pasting the table from another comment.

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

There isn't a simple check constraint you can place to ensure a parent's, or a grand-parent's, or a grand-grand-parent's parent_id isn't child.id You will have to write a trigger.

This isn't really a big problem to solve. pg simply overlooked this problem. Had he not, he would have checked child.created_at > parent.created_at in his mutator method. So, when you do a post.parent = some_post(assuming mutator is parent=; replace it with post.setParent or (send post set-parent some-post) or whatever), it checks if post.created_at > some_post.created_at, and then assigns post.parent_id = some_post.id