(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Batches=1,则说明所有...
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. 3. 进行...
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 ...
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...
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 ...
(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:...
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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 ...