首页 \ 问答 \ 基于对的第二个元素，如何对对的向量进行排序？(How do I sort a vector of pairs based on the second element of the pair?)

基于对的第二个元素，如何对对的向量进行排序？(How do I sort a vector of pairs based on the second element of the pair?)

 如果我有一对矢量：  
std::vector<std::pair<int, int> > vec;
 
 是否有简单的方法来根据对的第二个元素以增加的顺序对列表进行排序？  
 我知道我可以编写一些功能对象来做这些工作，但是有没有办法使用STL和std::less现有部分来直接进行工作？  
 编辑：我明白我可以写一个单独的函数或类来传递给第三个参数进行排序。 问题是我是否可以用标准的东西来构建它。 我真的会看起来像：  
std::sort(vec.begin(), vec.end(), std::something_magic<int, int, std::less>());

If I have a vector of pairs: 
std::vector<std::pair<int, int> > vec;
 
Is there and easy way to sort the list in increasing order based on the second element of the pair? 
I know I can write a little function object that will do the work, but is there a way to use existing parts of the STL and std::less to do the work directly? 
EDIT: I understand that I can write a separate function or class to pass to the third argument to sort. The question is whether or not I can build it out of standard stuff. I'd really something that looks like: 
std::sort(vec.begin(), vec.end(), std::something_magic<int, int, std::less>());

原文：https://stackoverflow.com/questions/279854

更新时间：2023-04-19 09:04

最满意答案

 如前所述，“使用filesort”和“使用临时”并不总是意味着糟糕的性能。  
 以下是提高ORDER BY语句性能的一些基本指导原则。 亮点：  
 
  如果你想增加ORDER BY的速度，检查你是否可以让MySQL使用索引而不是额外的排序阶段。 如果这不可行，您可以尝试以下策略：  
  增加sort_buffer_size变量的大小。  
  增加read_rnd_buffer_size变量的大小。  
  通过声明列的大小，每列使用更少的RAM，因为它们需要保存存储在其中的值。 例如，如果值不超过16个字符，则CHAR（16）优于CHAR（200）。  
 
 首先尝试使用索引（确保您按排序的字段有索引）。 请注意，增加系统变量sort_buffer_size和read_rnd_buffer_size也会对其他查询产生负面影响 - 请考虑将它们专门设置为您需要的会话，并将其保留为所有其他会话的默认设置。 

As said already, "using filesort" and "using temporary" do not always imply bad performance. 
Here are some basic guidelines for improving performance of ORDER BY statements. The highlights: 
 
 If you want to increase ORDER BY speed, check whether you can get MySQL to use indexes rather than an extra sorting phase. If this is not possible, you can try the following strategies: 
 Increase the size of the sort_buffer_size variable. 
 Increase the size of the read_rnd_buffer_size variable. 
 Use less RAM per row by declaring columns only as large as they need to be to hold the values stored in them. For example, CHAR(16) is better than CHAR(200) if values never exceed 16 characters. 
 
First try to use indices (make sure the fields you are sorting by have indices). Note that increasing the system variables sort_buffer_size and read_rnd_buffer_size can also have a negative effect on other queries - consider setting them specifically for the session you need them for and leave them at default for all other sessions.

基于对的第二个元素，如何对对的向量进行排序？(How do I sort a vector of pairs based on the second element of the pair?)

最满意答案

相关问答

摆脱“临时使用;(Getting rid of “Using temporary; Using filesort”)[2023-07-03]

为什么MySQL在没有ORDER BY的情况下使用filesort？(Why is MySQL using filesort when there is no ORDER BY?)[2022-03-29]

使用LEFT JOIN + ORDER BY时如何避免FileSort？(How to avoid FileSort when using LEFT JOIN + ORDER BY?)[2022-07-12]

如何在不使用临时文件排序的情况下按sum（：field）进行排序(How to sort by sum(:field) without using temp filesort)[2021-10-28]

mysql索引和优化，使用where;(mysql indexes and optimizations, Using where; using temporary; using filesort)[2023-07-07]

带有“order by”的派生表使用临时表和filesort，即使我只选择主键(Derived table with “order by” uses temporary table and filesort, even though I'm only selecting primary key)[2023-08-31]

为什么MySql不使用我的索引以及如何避免“使用临时;(Why MySql doesn't use my index and how to avoid the “Using temporary; Using filesort”?)[2021-12-19]

分组使用临时;(Group by causing using temporary; using filesort)[2022-04-15]

避免UNION RESULT上的“filesort”(Avoid “filesort” on UNION RESULT)[2021-12-24]

MySQL查询优化。(MySQL query optimization. Avoiding temporary & filesort)[2023-09-18]

相关文章

最新问答