public Set<Attribute> references() {
        return expressions().stream().flatMap(e -> e.references().stream()).collect(Collectors.toSet());
    }
All Attributes that appear in expressions from this operator.  Note that this set does not
include attributes that are implicitly referenced by being passed through to the output tuple.
indexr-query-opt/src/main/java/io/indexr/query/plan/QueryPlan.java