presto-main/src/main/java/io/prestosql/sql/planner/FragmentTableScanCounter.java/FragmentTableScanCounter/visitPlan
class FragmentTableScanCounter:        @Override
        protected Integer visitPlan(PlanNode node, Void context)
        {
            int count = 0;
            for (PlanNode source : node.getSources()) {
                count += source.accept(this, context);
            }
            return count;
        }