presto-main/src/main/java/io/prestosql/sql/planner/DistributedExecutionPlanner.java/DistributedExecutionPlanner/visitSemiJoin
class DistributedExecutionPlanner:        @Override
        public Map<PlanNodeId, SplitSource> visitSemiJoin(SemiJoinNode node, Void context)
        {
            Map<PlanNodeId, SplitSource> sourceSplits = node.getSource().accept(this, context);
            Map<PlanNodeId, SplitSource> filteringSourceSplits = node.getFilteringSource().accept(this, context);
            return ImmutableMap.<PlanNodeId, SplitSource>builder()
                    .putAll(sourceSplits)
                    .putAll(filteringSourceSplits)
                    .build();
        }