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

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