presto-spi/src/main/java/io/prestosql/spi/block/SingleMapBlockWriter.java/SingleMapBlockWriter/writeBytes
class SingleMapBlockWriter:    @Override
    public BlockBuilder writeBytes(Slice source, int sourceIndex, int length)
    {
        if (writeToValueNext) {
            valueBlockBuilder.writeBytes(source, sourceIndex, length);
        }
        else {
            keyBlockBuilder.writeBytes(source, sourceIndex, length);
        }
        return this;
    }