[csnweb]

While that might not fit your needs I map unions to Postgres with an enum encoding the type and then adding constraints on which additional columns must be set (and also must not be set) depending on the type. For your example the enum might be leaf or tree and then if it’s of type tree you have non nullable left_child and right_child columns and if it’s a leaf those must be null, and then you might have a value column that is non null in both cases or only for leaves.