presto-main/src/main/java/io/prestosql/sql/planner/DistributedExecutionPlanner.java/Visitor/processSources
class Visitor:        private Map<PlanNodeId, SplitSource> processSources(List<PlanNode> sources, Void context)
        {
            ImmutableMap.Builder<PlanNodeId, SplitSource> result = ImmutableMap.builder();
            for (PlanNode child : sources) {
                result.putAll(child.accept(this, context));
            }

            return result.build();
        }