presto-main/src/main/java/io/prestosql/sql/planner/optimizations/QueryCardinalityUtil.java/QueryCardinalityUtil/visitExchange
class QueryCardinalityUtil:        @Override
        public Range<Long> visitExchange(ExchangeNode node, Void context)
        {
            if (node.getSources().size() == 1) {
                return getOnlyElement(node.getSources()).accept(this, null);
            }
            return Range.atLeast(0L);
        }