presto-main/src/main/java/io/prestosql/execution/SqlTaskExecution.java/SqlTaskExecution/scheduleUnpartitionedSource
class SqlTaskExecution:    private synchronized void scheduleUnpartitionedSource(TaskSource sourceUpdate, Map<PlanNodeId, TaskSource> updatedUnpartitionedSources)
    {
        // create new source
        TaskSource newSource;
        TaskSource currentSource = unpartitionedSources.get(sourceUpdate.getPlanNodeId());
        if (currentSource == null) {
            newSource = sourceUpdate;
        }
        else {
            newSource = currentSource.update(sourceUpdate);
        }

        // only record new source if something changed
        if (newSource != currentSource) {
            unpartitionedSources.put(sourceUpdate.getPlanNodeId(), newSource);
            updatedUnpartitionedSources.put(sourceUpdate.getPlanNodeId(), newSource);
        }
    }