presto-main/src/main/java/io/prestosql/operator/DistinctLimitOperator.java/DistinctLimitOperator/maskToDistinctOutputPositions
class DistinctLimitOperator:    private Page maskToDistinctOutputPositions(int distinctCount, int[] distinctPositions)
    {
        Page result = null;
        if (distinctCount > 0) {
            Block[] blocks = outputChannels.stream()
                    .map(inputPage::getBlock)
                    .map(block -> block.getPositions(distinctPositions, 0, distinctCount))
                    .toArray(Block[]::new);
            result = new Page(distinctCount, blocks);
        }
        return result;
    }