presto-main/src/main/java/io/prestosql/sql/planner/optimizations/StreamPreferredProperties.java/StreamPreferredProperties/defaultParallelism
class StreamPreferredProperties:    public static StreamPreferredProperties defaultParallelism(Session session)
    {
        if (getTaskConcurrency(session) > 1 && !preferStreamingOperators(session)) {
            return new StreamPreferredProperties(Optional.of(MULTIPLE), Optional.empty(), false);
        }
        return any();
    }