When you write to a postgres jsonb field it updates the entire JSONB content, because that's how postgres's engine works. Mongo allows you to $set two fields on the same document at the same time, for example, and have both writes win, which is very useful and removes distributed locks etc. This is just like updating specific table columns on postgres, but postgres doesn't allow that within columns, you'd have to lock the row for updating to do this safely which is a PITA.