Paging is often used in our programming. This article will lead everyone to discuss the performance of mysql paging, hoping to help everyone.
Several common paging methods:
1. Escalator method
The escalator method usually only provides two modes of previous page/next page in navigation. Some products do not even provide the previous page function, only provide a "more/more" method, and there are also drop-down automatic loading. More methods can technically be summarized as escalator methods.
The escalator method is relatively simple and efficient in terms of technical implementation. Just get one page further based on the offset of the last item on the current page. Written as SQL it may be similar to
SELECT*FROMLIST_TABLEWHEREid> offset_id LIMIT n;
1. Elevator method
Another data acquisition method is reflected in the product as a precise page turning method, such as 1,2,3...n. At the same time, the user can also input directly to n pages in the navigation. Most scenes in China use elevators, but the technical implementation cost of elevators is relatively high.
In MySQL, the b-tree usually mentioned is usually b+tree in terms of storage engine implementation.
When using the elevator method, when the user specifies to turn to the nth page, there is no direct method to address the location. Instead, it needs to count one by one from the first floor and scan to count*page to obtain the data. It has just started, so the efficiency is not high.
Traditional paging technology (elevator method)
First, the front end needs to pass you the paging entity and query conditions
//分页实体 structFinanceDcPage{ 1:i32 pageSize,//页容量 2:i32 pageIndex,//当前页索引 }
Then you need to return the total number of queries to the front end;
SELECTCOUNT(*)FROMmy_tableWHEREx= y ORDERBYid;
Then return the number of items on the specified page To the front-end:
SELECT*FROMmy_tableWHEREx= y ORDERBYdate_colLIMIT (pageIndex - 1)* pageSize, pageSize;
The results queried from the above two sql statements need to be returned to the front-end paging entity and the single-page result set
//分页实体 structFinanceDcPage{ 1:i32 pageSize,//页容量 2:i32 pageIndex,//当前页索引 3:i32 pageTotal,//总页数 4:i32 totalRecod,//总条数 }
In the traditional query method, only the pageIndex value changes with each request, which is the offset of limit offset, num
For example, limit 0,10; limit 10, 10; …. limit10000,10;
The above changes will cause a deviation in the execution time of each query. The larger the offset value, the longer it takes. For example, limit10000,10 needs to read 10010 pieces of data. Only then can you get the 10 pieces of data you want.
Optimization method
In the traditional method, we learned that the key to efficiency is that the program traverses a lot of unnecessary data and finds the key Click So start here.
If there is no need to use the elevator, we can use the escalator to improve performance.
But in most cases, the elevator form can better meet the needs of users, so we need to find another way to optimize the elevator form.
Optimization based on traditional methods
The optimization methods mentioned above are either difficult to meet the needs of users or too complicated to implement, so If the amount of data is not particularly large, such as millions of pieces of data, there is actually no need to use the above optimization method.
The traditional method is sufficient, but the traditional method may also need optimization. For example:
orderby optimization
SELECT*FROMpa_dc_flowORDERBYsubject_codeDESCLIMIT100000,5
The ORDERBY keyword is used in this statement , then what to sort is very important. If you are sorting auto-incrementing IDs, then this statement does not need to be optimized. If it is an index or even a non-index, then it needs to be optimized.
First of all, you have to make sure it is an index, otherwise it will be really slow. Then if it is an index, but it is not as ordered as an auto-incrementing ID, then it must be rewritten as the following statement.
SELECT*FROMpa_dc_flowINNERJOIN(SELECTidFROMpa_dc_flowORDERBYsubject_codeDESCLIMIT100000,5)ASpa_dc_flow_idUSING(id);
The following is the EXPLAIN for two sql
We can see from the picture that the second sql can scan many fewer pages.
In fact, this involves the optimization issue of order by. The subject_code index is not used in the first sql. If you select subject_code instead... the index is used. The following is the optimization of order by.
order by后的字段,如果要走索引,须与where 条件里的某字段建立复合索引!!或者说orcerby后的字段如果要走索引排序,它要么与where条件里的字段建立复合索引【这里建立复合索引的时候,需要注意复合索引的列顺序为(where字段,order by字段),这样才能满足最左列原则,原因可能是order by字段并能算在where 查询条件中!】,要么它自身要在where条件里被引用到!
表asubject_code为普通字段,上面建有索引,id是自增主键
select*fromaorderbysubject_code//用不上索引 selectidfromaorderbysubject_code//能用上索引 selectsubject_codefromaorderbysubject_code//能用上索引 select*fromawheresubject_code= XX orderbysubject_code//能用上索引
意思是说order by 要避免使用文件系统排序,要么把order by的字段出现在select后,要么使用order by字段出现在where 条件里,要么把order by字段与where条件字段建立复合索引!
第二条sql就是巧妙的利用第二种方式利用上了索引。 select id from a order bysubject_code,这种方式
count优化
当数据量非常大时,其实可以输出总数的大概数据,利用explain语句,他并没有真正去执行sql,而是进行的估算。
相关推荐:
The above is the detailed content of Mysql paging performance exploration. For more information, please follow other related articles on the PHP Chinese website!

本篇文章给大家带来了关于mysql的相关知识,其中主要介绍了关于架构原理的相关内容,MySQL Server架构自顶向下大致可以分网络连接层、服务层、存储引擎层和系统文件层,下面一起来看一下,希望对大家有帮助。

在mysql中,可以利用char()和REPLACE()函数来替换换行符;REPLACE()函数可以用新字符串替换列中的换行符,而换行符可使用“char(13)”来表示,语法为“replace(字段名,char(13),'新字符串') ”。

方法:1、利用right函数,语法为“update 表名 set 指定字段 = right(指定字段, length(指定字段)-1)...”;2、利用substring函数,语法为“select substring(指定字段,2)..”。

mysql的msi与zip版本的区别:1、zip包含的安装程序是一种主动安装,而msi包含的是被installer所用的安装文件以提交请求的方式安装;2、zip是一种数据压缩和文档存储的文件格式,msi是微软格式的安装包。

转换方法:1、利用cast函数,语法“select * from 表名 order by cast(字段名 as SIGNED)”;2、利用“select * from 表名 order by CONVERT(字段名,SIGNED)”语句。

本篇文章给大家带来了关于mysql的相关知识,其中主要介绍了关于MySQL复制技术的相关问题,包括了异步复制、半同步复制等等内容,下面一起来看一下,希望对大家有帮助。

本篇文章给大家带来了关于mysql的相关知识,其中主要介绍了mysql高级篇的一些问题,包括了索引是什么、索引底层实现等等问题,下面一起来看一下,希望对大家有帮助。

在mysql中,可以利用REGEXP运算符判断数据是否是数字类型,语法为“String REGEXP '[^0-9.]'”;该运算符是正则表达式的缩写,若数据字符中含有数字时,返回的结果是true,反之返回的结果是false。


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.

EditPlus Chinese cracked version
Small size, syntax highlighting, does not support code prompt function

Dreamweaver Mac version
Visual web development tools

Notepad++7.3.1
Easy-to-use and free code editor

VSCode Windows 64-bit Download
A free and powerful IDE editor launched by Microsoft
