There are 2 steps to solve this one. Solution Share Step 1 ANSWER : The time complexity of the merge sort algorithm is mainly determined by the number of recursive cal...View the full answer Step 2 Unlock Answer UnlockPrevious question Ne...
Using sort_union(…), Using union(…), Using intersect(…) 这些指示索引扫描如何合并为index_merge连接类型。 (参考https://dev.mysql.com/doc/refman/5.7/en/index-merge-optimization.html) 索引合并交叉口访问算法(The Index Merge Intersection Access Algorithm): index intersect merge就是多个索引条件扫描...
index merge 的思路实际上就是 multiple range scans and merge result into one, 主要支持三种对结果集聚合的方式,intersections交集,unions并集,sort-union The difference between the sort-union algorithm and the union algorithm is that the sort-union algorithm must first fetch row IDs for all rows and ...
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...
The join order is determined by using an algorithm, and an optimal execution plan is generated. This may increase the QO overhead during multi-table join. query The execution plan is generated based on SQL statements. The QO does not make any changes. This value is applicable and helps de...
3. 进行若干次multi-merge操作, 将所有row ID写入结果文件. 4. 根据row ID再次获取记录. 很容易发现, 上面的步骤1和4, 一共读取了2遍记录, 所以也就有了下面的改进实现. The Modified filesort Algorithm 较Original改变的地方是, 在第2步记录的是sort key和涉及到的其它列, 即<sort_key, additional_field...
the library cache based upon a hashed representation of that query. When looking for a statement in the library cache, we first apply a hashing algorithm to the statement and then we look for this hash value in the library cache. This access path will be used until the query is reparsed...
7 index_merge:此类型表示使用了索引合并优化,表示一个查询里面用到了多个索引 8 unique_subquery:该类型和eq_ref类似,但是使用了IN查询,且子查询是主键或者唯一索引。例如: 代码语言:javascript 复制 valueIN(SELECTprimary_keyFROMsingle_tableWHEREsome_expr) ...
(Block Nested Loop) indicates use of the Block Nested-Loop algorithm and (Batched Key Access) indicates use of the Batched Key Access algorithm. That is, the keys from the table on the preceding line of the EXPLAIN output are buffered, and the matching rows are fetched in batches from ...
(Storage Rows Scanned: 10000). Also, the data had to be sorted (viaSort Method: quicksort) on value (viaSort Key: value). You can improve this execution by modifying the above created index into a range index, so that you can avoid the sorting operation, and reduce the number of ...