innodb 通过undo构造出老版本记录流程

trx_undo_prev_version_build：roll_ptr = row_get_rec_roll_ptr(rec, index, offsets);//rec为新记录rec_trx_id = row_get_rec_trx_id(rec, index, offsets);//获取记录里面的事务idtrx_undo_get_undo_rec(roll_ptr, rec_trx_id, &undo_rec, heap);//根据roll_ptr获取undo recptr = trx_undo_rec_get_pars(undo_rec, &type, &cmpl_info,//解析出type、cmpl_info等    &dummy_extern, &undo_no, &table_id);//解析出老记录的trx_id和roll_ptr和info_bitsptr = trx_undo_update_rec_get_sys_cols(ptr, &trx_id, &roll_ptr,&info_bits);//update的结构：{info_bits = 0, n_fields = 3, fields = 0xa368bb8}// {field_no = 3, orig_len = 0, exp = 0x0, new_val = {data = 0xa368ba2, ext = 0, len = 4, type = // {prtype = 1027, mtype = 6, len = 4, mbminmaxlen = 0}}}ptr = trx_undo_rec_skip_row_ref(ptr, index);//跳过主键列//构建update vector，即更新的老记录列值.将trx_id和roll_ptr放到update最后ptr = trx_undo_update_rec_get_update(ptr, index, type, trx_id,     roll_ptr, info_bits, NULL, heap, &update);buf = static_cast<byte*>(mem_heap_alloc(heap, rec_offs_size(offsets)));//将新记录拷贝到新申请的buf中*old_vers = rec_copy(buf, rec, offsets);rec_offs_make_valid(*old_vers, index, offsets);//替换新记录临时记录的update vectorrow_upd_rec_in_place(*old_vers, index, offsets, update, NULL);

问题：

1、通过undo log和新版本的记录就可以构造出老版本记录了，而且mvcc中也是通过这个方法构造出的老版本记录，并没有直接访问打上delete标签的老版本，那么为什么页内打上delete标签的记录并不直接删除呢？为了回滚？