code: private ImmutableMultimap<RelationDefinition, ExtensionalDataNode> extractDataNodeMap(IntermediateQuery query,
                                                                                            InnerJoinNode joinNode) {
          return query.getChildren(joinNode).stream()
                  .filter(c -> c instanceof ExtensionalDataNode)
                  .map(c -> (ExtensionalDataNode) c)
                  .map(c -> Maps.immutableEntry(c.getProjectionAtom().getPredicate().getRelationDefinition(), c))
                  .collect(ImmutableCollectors.toMultimap());
      }
docs: Predicates not having a DatabaseRelationDefinition are ignored