理解方式一:就是select的数据列只用从索引中就能够取得,不必读取数据行,MySQL可以利用索引返回select列表中的字段,而不必根据索引再次读取数据文件,换句话说查询列要被所建的索引覆盖。 理解方式二:索引是高效找到行的一个方法,但是一般数据库也能使用索引找到一个列的数据,因此它不必读取整个行。毕竟索引叶子节点存储...
index_merge unique_subquery index_subquery range index ALL 我们挑重点的说!为了便于理解,这里从性能最差到最佳排序一一说明。 all all便是所谓的全表扫描了,如果出现了all类型,通常意味着你的SQL处于一种最原始的状态,还有很大的优化空间!为什么这么说,因为all是一种非常原始的查找方式,低效且耗时,用all查找相当...
subtracting someone’s age from the current year to get the year they were born is an algorithm: regardless of how old someone is, if you follow those steps you’ll always get the year they were born. When you hear “algorithm” you probably think of some fancy equation...
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...
DFELabelPropagationAlgo – Explain operator for invocations of the Label Propagation algorithm, which is used for community detection. DFELoopSubQuery – This is a non-blocking operation that acts as a wrapper for a subquery, allowing it to be run repeatedly for use in loops. DFEMergeChunks –...
Merge JoinmjoinAlso used for inner joins and outer joins (for join tables that are both distributed and sorted on the joining columns). Typically the fastest Amazon Redshift join algorithm, not including other cost considerations. AGGREGATION:Operators and steps used for queries that involve aggrega...
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...
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 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...
It then scans the larger table, and performs the same hashing algorithm on the join column(s). It then probes the previously built hash table for each value and if they match, it returns a row. Nested Loops joins - Nested loops joins are useful when small subsets of data are being ...