presto-spi/src/main/java/io/prestosql/spi/type/MapType.java/MapType/writeObject
class MapType:    @Override
    public void writeObject(BlockBuilder blockBuilder, Object value)
    {
        if (!(value instanceof SingleMapBlock)) {
            throw new IllegalArgumentException("Maps must be represented with SingleMapBlock");
        }
        blockBuilder.appendStructure((Block) value);
    }