presto-main/src/main/java/io/prestosql/operator/MarkDistinctOperator.java/MarkDistinctOperator/getOutput
class MarkDistinctOperator:    @Override
    public Page getOutput()
    {
        if (unfinishedWork == null) {
            return null;
        }

        if (!unfinishedWork.process()) {
            return null;
        }

        // add the new boolean column to the page
        Page outputPage = inputPage.appendColumn(unfinishedWork.getResult());

        unfinishedWork = null;
        inputPage = null;

        updateMemoryReservation();
        return outputPage;
    }