[paulmd]

Can you define what "nested sets" means more specifically?

Postgres supports hierarchical/nested structures using the "ltree" column type. There is nothing stopping you from defining a primary key of (eg) "set1.set10.set100". There is also support for recursive views/etc to operate on these kinds of sets.

Again, if you have some kind of "sparse" column, it can make sense to put that into a JSONB column. This is effectively the same thing as attaching an unstructured document to a record for this use-case.

[porker]

Sure: http://mikehillyer.com/articles/managing-hierarchical-data-i...

Joe Celko popularised them. I've been unable to find when they were first introduced; but a search of my source code archive points to having written one ~14 years ago.