presto-spi/src/main/java/io/prestosql/spi/block/SingleRowBlockWriter.java/SingleRowBlockWriter/appendStructure
class SingleRowBlockWriter:    @Override
    public BlockBuilder appendStructure(Block block)
    {
        checkFieldIndexToWrite();
        fieldBlockBuilders[currentFieldIndexToWrite].appendStructure(block);
        entryAdded();
        return this;
    }