presto-main/src/main/java/io/prestosql/sql/planner/optimizations/StreamPreferredProperties.java/StreamPreferredProperties/withParallelism
class StreamPreferredProperties:    public StreamPreferredProperties withParallelism()
    {
        // do not override an existing parallel preference
        if (isParallelPreferred()) {
            return this;
        }
        return new StreamPreferredProperties(Optional.of(MULTIPLE), Optional.empty(), orderSensitive);
    }