presto-main/src/main/java/io/prestosql/sql/planner/optimizations/StreamPreferredProperties.java/StreamPreferredProperties/withFixedParallelism
class StreamPreferredProperties:    public StreamPreferredProperties withFixedParallelism()
    {
        if (distribution.isPresent() && distribution.get() == FIXED) {
            return this;
        }
        return fixedParallelism();
    }