presto-spi/src/main/java/io/prestosql/spi/block/AbstractSingleMapBlock.java/AbstractSingleMapBlock/getSingleValueBlock
class AbstractSingleMapBlock:    @Override
    public Block getSingleValueBlock(int position)
    {
        position = getAbsolutePosition(position);
        if (position % 2 == 0) {
            return getRawKeyBlock().getSingleValueBlock(position / 2);
        }
        else {
            return getRawValueBlock().getSingleValueBlock(position / 2);
        }
    }