presto-main/src/main/java/io/prestosql/operator/GroupedTopNBuilder.java/GroupedTopNBuilder/getBufferedPages
class GroupedTopNBuilder:    @VisibleForTesting
    List<Page> getBufferedPages()
    {
        return IntStream.range(0, currentPageCount)
                .filter(i -> pageReferences.get(i) != null)
                .mapToObj(i -> pageReferences.get(i).getPage())
                .collect(toImmutableList());
    }