presto-main/src/main/java/io/prestosql/operator/exchange/LocalMergeSourceOperator.java/LocalMergeSourceOperator/getOutput
class LocalMergeSourceOperator:    @Override
    public Page getOutput()
    {
        if (!mergedPages.process() || mergedPages.isFinished()) {
            return null;
        }

        Page page = mergedPages.getResult();
        operatorContext.recordProcessedInput(page.getSizeInBytes(), page.getPositionCount());
        return page;
    }