presto-main/src/main/java/io/prestosql/operator/unnest/UnnestBlockBuilder.java/UnnestBlockBuilder/appendRange
class UnnestBlockBuilder:    public void appendRange(int startPosition, int length)
    {
        // check range validity
        checkState(source != null, "source is null");
        checkPositionIndexes(startPosition, startPosition + length, source.getPositionCount());

        if (usingCopiedBlock) {
            for (int i = 0; i < length; i++) {
                type.appendTo(source, startPosition + i, outputBlockBuilder);
            }
        }
        else {
            for (int i = 0; i < length; i++) {
                appendId(startPosition + i);
            }
        }
    }