MySQL:非叶子节点指针

这个指针为8字节：4（唯一定位字段个数）+4（page no）。函数为dict_index_build_node_ptr。仅此记录，后续研究

/**********************************************************************//**
Builds a node pointer out of a physical record and a page number.
@return own: node pointer */
dtuple_t*
dict_index_build_node_ptr(
/*======================*/
    const dict_index_t* index,  /*!< in: index */
    const rec_t*        rec,    /*!< in: record for which to build node
                    pointer */
    ulint           page_no,/*!< in: page number to put in node
                    pointer */
    mem_heap_t*     heap,   /*!< in: memory heap where pointer
                    created */
    ulint           level)  /*!< in: level of rec in tree:
                    0 means leaf level */
{
    dtuple_t*   tuple;
    dfield_t*   field;
    byte*       buf;
    ulint       n_unique;

    if (dict_index_is_ibuf(index)) {
        /* In a universal index tree, we take the whole record as
        the node pointer if the record is on the leaf level,
        on non-leaf levels we remove the last field, which
        contains the page number of the child page */

        ut_a(!dict_table_is_comp(index->table));
        n_unique = rec_get_n_fields_old(rec);

        if (level > 0) {
            ut_a(n_unique > 1);
            n_unique--;
        }
    } else {
        n_unique = dict_index_get_n_unique_in_tree_nonleaf(index);// 唯一定位 到叶子节点的字段个数
    }

    tuple = dtuple_create(heap, n_unique + 1); //建立元组 

    /* When searching in the tree for the node pointer, we must not do
    comparison on the last field, the page number field, as on upper
    levels in the tree there may be identical node pointers with a
    different page number; therefore, we set the n_fields_cmp to one
    less: */

    dtuple_set_n_fields_cmp(tuple, n_unique); //比较字段为 数据域

    dict_index_copy_types(tuple, index, n_unique);  //进行类型复制

    buf = static_cast<byte*>(mem_heap_alloc(heap, 4)); //分配内存

    mach_write_to_4(buf, page_no);//写入 叶子节点page no

    field = dtuple_get_nth_field(tuple, n_unique);  //字段个数  能够唯一定位的字段个数
    dfield_set_data(field, buf, 4);//写入字段个数

    dtype_set(dfield_get_type(field), DATA_SYS_CHILD, DATA_NOT_NULL, 4);

    rec_copy_prefix_to_dtuple(tuple, rec, index, n_unique, heap); //复制数据
    dtuple_set_info_bits(tuple, dtuple_get_info_bits(tuple)
                 | REC_STATUS_NODE_PTR);

    ut_ad(dtuple_check_typed(tuple));

    return(tuple);
}