presto-main/src/main/java/io/prestosql/sql/planner/InputExtractor.java/InputExtractor/visitPlan
class InputExtractor:        @Override
        protected Void visitPlan(PlanNode node, Void context)
        {
            for (PlanNode child : node.getSources()) {
                child.accept(this, context);
            }
            return null;
        }