presto-main/src/main/java/io/prestosql/sql/planner/optimizations/StreamPropertyDerivations.java/StreamPropertyDerivations/withUnspecifiedPartitioning
class StreamPropertyDerivations:        private StreamProperties withUnspecifiedPartitioning()
        {
            // a single stream has no symbols
            if (isSingleStream()) {
                return this;
            }
            // otherwise we are distributed on some symbols, but since we are trying to remove all symbols,
            // just say we have multiple partitions with an unknown scheme
            return new StreamProperties(distribution, Optional.empty(), ordered);
        }