presto-spi/src/main/java/io/prestosql/spi/block/SingleMapBlockWriter.java/SingleMapBlockWriter/beginBlockEntry
class SingleMapBlockWriter:    @Override
    public BlockBuilder beginBlockEntry()
    {
        BlockBuilder result;
        if (writeToValueNext) {
            result = valueBlockBuilder.beginBlockEntry();
        }
        else {
            result = keyBlockBuilder.beginBlockEntry();
        }
        return result;
    }