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