presto-main/src/main/java/io/prestosql/operator/OrderByOperator.java/OrderByOperator/mergeSpilledAndMemoryPages
class OrderByOperator:    private WorkProcessor<Page> mergeSpilledAndMemoryPages(List<WorkProcessor<Page>> spilledPages, Iterator<Page> sortedPagesIndex)
    {
        List<WorkProcessor<Page>> sortedStreams = ImmutableList.<WorkProcessor<Page>>builder()
                .addAll(spilledPages)
                .add(WorkProcessor.fromIterator(sortedPagesIndex))
                .build();

        return mergeSortedPages(
                sortedStreams,
                orderingCompiler.compilePageWithPositionComparator(sourceTypes, sortChannels, sortOrder),
                sourceTypes,
                operatorContext.aggregateUserMemoryContext(),
                operatorContext.getDriverContext().getYieldSignal());
    }