presto-main/src/main/java/io/prestosql/operator/aggregation/builder/SpillableHashAggregationBuilder.java/SpillableHashAggregationBuilder/mergeSortedPages
class SpillableHashAggregationBuilder:    private WorkProcessor<Page> mergeSortedPages(WorkProcessor<Page> sortedPages, long memoryLimitForMerge)
    {
        merger = Optional.of(new MergingHashAggregationBuilder(
                accumulatorFactories,
                step,
                expectedGroups,
                groupByTypes,
                hashChannel,
                operatorContext,
                sortedPages,
                operatorContext.aggregateSystemMemoryContext(),
                memoryLimitForMerge,
                hashAggregationBuilder.getKeyChannels(),
                joinCompiler));

        return merger.get().buildResult();
    }