Here is a C program that sorts elements using the quicksort technique. It takes user input for array elements and displays the sorted order − #include<stdio.h>voidquicksort(intnumber[25],intfirst,intlast){inti,j,pivot,temp;if(first<last){pivot=first;i=first;j=last;while(i<j){while...
为了更好的理解EXPLAIN,rows参数也是一个很好的参考,以下来自官网对rows的解释: Therowscolumn indicates the number of rows MySQL believes it must examine to execute the query. ForInnoDBtables, this number is an estimate, and may not always be exact. 这个字段表示MySQL认为执行查询必须检查的行数。 对...
这个filesort是说, MySQL要多做一次额外的排序, 确切的说是快速排序(Quicksort). 先初步了解下Quicksort排序的概念(From Wikipedia). Quicksort is a divide and conquer algorithm. Quicksort first divides a large array into two smaller sub-arrays: the low elements and the high elements. Quicksort can ...
这个filesort是说, MySQL要多做一次额外的排序, 确切的说是快速排序(Quicksort). 先初步了解下Quicksort排序的概念(From Wikipedia). Quicksort is a divide and conquer algorithm. Quicksort first divides a large array into two smaller sub-arrays: the low elements and the high elements. Quicksort can ...
}else{// Check if we have statistic about the distributionif(keyinfo->has_records_per_key(cur_used_keyparts -1)) { cur_fanout= keyinfo->records_per_key(cur_used_keyparts -1);if(!table_deps && table->quick_keys.is_set(key) &&// (1)table->quick_key_parts[key] > cur_used_key...
filesort使用的算法是QuickSort,即对需要排序的记录生成元数据进行分块排序,然后再使用mergesort方法合并块。其中filesort可以使用的内存空间大小为参数sort_buffer_size的值,默认为2M。当排序记录太多sort_buffer_size不够用时,mysql会使用临时文件来存放各个分块,然后各个分块排序后再多次合并分块最终全局完成排序。 Us...
The Original filesort Algorithm 1. 扫描或根据WHERE条件, 获取所有记录. 2. 把每条记录的sort key和row ID, 即<sort_key, rowid>, 放入sort buffer中. 若sort buffer满了, 就在内存中进行一次quicksort, 然后将<sort_key, rowid>写入临时文件, 并记录指向指针. 重复该过程, 直到读取了所有记录. ...
Filtered measures - filtered measures are visual level calculations with a specific filter applied (for example, Total Sales for France) and are used on some of the visuals created by the insights feature Categorical columns on X-axis unless it defines a sort by column that's scalar. If using...
table->quick_key_parts[key] == cur_used_keyparts && //(C2) table->quick_n_ranges[key] == 1+MY_TEST(ref_or_null_part)) //(C3) { tmp_fanout= cur_fanout= (double) table->quick_rows[key]; } else { // Check if we have statistic about the distribution ...
(actual time=0.143..0.145 rows=10 loops=1) Output: students.studentid, students.firstname, students.lastname, enrollments.coursename, enrollments.grade Sort Key: students.studentid, enrollments.coursename Presorted Key: students.studentid Full-sort Groups: 1 Sort Method: quicksort Average Memory:...