presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/PruneProjectColumns.java/PruneProjectColumns/pushDownProjectOff
class PruneProjectColumns:    @Override
    protected Optional<PlanNode> pushDownProjectOff(
            PlanNodeIdAllocator idAllocator,
            ProjectNode childProjectNode,
            Set<Symbol> referencedOutputs)
    {
        return Optional.of(
                new ProjectNode(
                        childProjectNode.getId(),
                        childProjectNode.getSource(),
                        childProjectNode.getAssignments().filter(referencedOutputs)));
    }