presto-main/src/main/java/io/prestosql/operator/unnest/UnnestBlockBuilder.java/UnnestBlockBuilder/appendElement
class UnnestBlockBuilder:    public void appendElement(int index)
    {
        checkState(source != null, "source is null");
        checkElementIndex(index, source.getPositionCount());

        if (usingCopiedBlock) {
            type.appendTo(source, index, outputBlockBuilder);
        }
        else {
            appendId(index);
        }
    }