presto-main/src/main/java/io/prestosql/operator/GroupedTopNBuilder.java/GroupedTopNBuilder/ResultIterator
class GroupedTopNBuilder:        ResultIterator()
        {
            if (produceRowNumber) {
                pageBuilder = new PageBuilder(new ImmutableList.Builder<Type>().addAll(sourceTypes).add(BIGINT).build());
            }
            else {
                pageBuilder = new PageBuilder(sourceTypes);
            }
        }