presto-main/src/main/java/io/prestosql/sql/planner/DistributedExecutionPlanner.java/DistributedExecutionPlanner/visitJoin
class DistributedExecutionPlanner:        @Override
        public Map<PlanNodeId, SplitSource> visitJoin(JoinNode node, Void context)
        {
            Map<PlanNodeId, SplitSource> leftSplits = node.getLeft().accept(this, context);
            Map<PlanNodeId, SplitSource> rightSplits = node.getRight().accept(this, context);
            return ImmutableMap.<PlanNodeId, SplitSource>builder()
                    .putAll(leftSplits)
                    .putAll(rightSplits)
                    .build();
        }