presto-main/src/main/java/io/prestosql/operator/exchange/LocalMergeSourceOperator.java/LocalMergeSourceOperator/LocalMergeSourceOperator
class LocalMergeSourceOperator:    public LocalMergeSourceOperator(OperatorContext operatorContext, List<LocalExchangeSource> sources, List<Type> types, PageWithPositionComparator comparator)
    {
        this.operatorContext = requireNonNull(operatorContext, "operatorContext is null");
        this.sources = requireNonNull(sources, "sources is null");
        List<WorkProcessor<Page>> pageProducers = sources.stream()
                .map(LocalExchangeSource::pages)
                .collect(toImmutableList());
        mergedPages = mergeSortedPages(
                pageProducers,
                requireNonNull(comparator, "comparator is null"),
                types,
                operatorContext.aggregateUserMemoryContext(),
                operatorContext.getDriverContext().getYieldSignal());
    }