presto-main/src/main/java/io/prestosql/sql/planner/optimizations/PreferredProperties.java/Global/mergeWithParent
class Global:        public Global mergeWithParent(Global parent)
        {
            if (distributed != parent.distributed) {
                return this;
            }
            if (!partitioningProperties.isPresent()) {
                return parent;
            }
            if (!parent.partitioningProperties.isPresent()) {
                return this;
            }
            return new Global(distributed, Optional.of(partitioningProperties.get().mergeWithParent(parent.partitioningProperties.get())));
        }