presto-main/src/main/java/io/prestosql/operator/unnest/MapUnnester.java/MapUnnester/processCurrentPosition
class MapUnnester:    @Override
    protected void processCurrentPosition(int requiredOutputCount)
    {
        // Translate indices
        int mapLength = columnarMap.getEntryCount(getCurrentPosition());
        int startingOffset = columnarMap.getOffset(getCurrentPosition());

        // Append elements and nulls
        getBlockBuilder(0).appendRange(startingOffset, mapLength);
        getBlockBuilder(1).appendRange(startingOffset, mapLength);
        appendNulls(requiredOutputCount - mapLength);
    }