(1)sort 算子,会显示排序信息 Sort Method代表排序的方法,包括quicksort(快排)和disksort(外排)。快排即内存够用时,所有的排序操作均在内存中完成,外排说明当前可用内存不足,需要下盘。 (2)hashjoin算子 Buckets:代表hash表中实际使用的桶的个数 Batches:代表hashjoin中实际分块的数量。如果Batc
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...
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 ...
table_deps && //(C1) 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 ...
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. ...
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。 对于存储引擎是innodb的表,这个数值是个估计值,并不是精确值。
}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...
(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:...
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 ...
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...