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...
DFEApply – Executes the function specified by functor in the arguments section, on the value stored in the specified variable DFEAlgoWriteProperty – Explain operator for the property-writing portion of mutate algorithm invocations. DFEBFSAlgo – Explain operator for invocations of the Breadth First...
SORT:Operators and steps used when queries have to sort or merge result sets. SortsortSort performs the sorting specified by the ORDER BY clause as well as other operations such as UNIONs and joins. Can operate from disk. MergemergeProduces final sorted results of a query based on intermediate...
unionM,N: The row refers to the union of the rows with id values of M and N. derivedN: The row refers to the derived table result for the row with an id value of N. A derived table may result, for example, from a subquery in the FROM clause. subqueryN: The row refers to the...
In this case the leaves of the tree are implemented using a full table scans of the PRODUCTS and the SALES tables. The rows produced by these table scans will be consumed by the join operator. Here the join operator is a hash-join (other alternatives include nested-loop or sort-merge ...
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 to forecast the weather or help Google search the web, but they can also be ...
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...
When an SQL statement is delivered, the QO compares the costs of redistribution and broadcast, and then generates an execution plan based on the algorithm. Optimization suggestion: If the table that you want to query is small and the instance contains a small number of shards, such as 5 ...
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...
An algorithm can fall into an infinite loop when ___. a) the input operations were missing. b) the algorithm uses more than one loop. c) the output operations were missing. d) the continuation condition of the loop never becomes false. What is the advantage...