https://ru.wikipedia.org/wiki/%D0%92%D0%BB%D0%BE%D0%B6%D0%B5%D0%BD%D0%BD%D0%BE%D0%B5_%D0%BC%D0%BD%D0%BE%D0%B6%D0%B5%D1%81%D1%82%D0%B2%D0%BE_(%D0%BC%D0%BE%D0%B4%D0%B5%D0%BB%D1%8C)
https://postgres.men/database/postgresql/nested-sets-introduction/
https://postgres.men/database/trees/nested-sets-rules/
https://postgres.men/database/trees/nested-sets-manage/
https://postgres.men/database/postgresql/nested-sets-postgresql-triggers/