presto-main/src/main/java/io/prestosql/operator/aggregation/builder/SpillableHashAggregationBuilder.java/SpillableHashAggregationBuilder/mergeFromDisk
class SpillableHashAggregationBuilder:    private WorkProcessor<Page> mergeFromDisk()
    {
        checkState(spiller.isPresent());

        mergeHashSort = Optional.of(new MergeHashSort(operatorContext.newAggregateSystemMemoryContext()));

        WorkProcessor<Page> mergedSpilledPages = mergeHashSort.get().merge(
                groupByTypes,
                hashAggregationBuilder.buildIntermediateTypes(),
                spiller.get().getSpills().stream()
                        .map(WorkProcessor::fromIterator)
                        .collect(toImmutableList()),
                operatorContext.getDriverContext().getYieldSignal());

        return mergeSortedPages(mergedSpilledPages, memoryLimitForMerge);
    }