presto-main/src/main/java/io/prestosql/sql/planner/optimizations/StreamPreferredProperties.java/StreamPreferredProperties/withDefaultParallelism
class StreamPreferredProperties:    public StreamPreferredProperties withDefaultParallelism(Session session)
    {
        if (getTaskConcurrency(session) > 1 && !preferStreamingOperators(session)) {
            return withParallelism();
        }
        return this;
    }