[MySQL performance optimization] - optimize query

Preface

         Optimize MySQL Database is a necessary skill for database administrator , Through different Optimization method To improve MySQL The purpose of database performance .

        MySQL Database when the number of users and data is very small , It is difficult for us to judge MySQL Database performance . Only when the number of users and the amount of data increase ,MySQL How can the performance of . Inquire about Is the most frequent database operation , Improve Query speed Can effectively improve MySQL Database performance .

---

Optimized query

SHOW STATUS Statement query MySQL Database performance

Analyze query statements

The impact of index on query speed

LIKE Optimize index queries

Use multi column index

Use keywords or

---

SHOW STATUS Statement query MySQL Database performance

We can use SHOW STATUS Statement query MySQL Database performance .

Grammar format ：

SHOW STATUS LIKE 'VALUE'

VALUE The parameters are as follows

1. Connection： Connect MySQL The number of servers .
2. Uptime：MySQL The online time of the server .
3. Slow_queries： Slow query times .
4. Com_select： Number of query operations .
5. Com_insert： The number of insert operations .
6. Com_delete： The number of delete operations .

---

！ Through these parameters, we can analyze MySQL Database performance . Then according to the analysis results , Conduct Corresponding performance optimization .

---

---

Analyze query statements

have access to EXPLAIN Statement and DESCRIBE Statement to analyze the query statement .

Grammar format ：

#EXPLAIN  Grammar format 
EXPLAIN  SELECT  sentence ;

#DESCRIBE  Grammar format 
DESCRIBE SELECT  sentence ;

---

  Next, we're going to talk about EXPLAIN and DESCRIBE To introduce

Use EXPLAIN Statement analysis query statement

  The meaning of each field ：

id： Indicates that throughout the query select The location of .

table： Store the table name queried .

type： Connection type , Many values are stored in this column , Range from const To All.

possible_keys： Point out that in order to improve the search speed , stay MySQL Indexes that can be used in .

key： Indicate the keys actually used .

rows： Pointed out that MySQL You need to return the number of rows verified by the query result in the corresponding table , To get the total number of rows ,MySQL The entire query must be scanned , Multiply by the row value of each table .

Extra： Contains some additional information , Design MySQL How to handle queries .

---

Use DESCRIBE Statement analysis query statement

 DESCRIBE The use of statements is the same as EXPLAIN The grammar is the same , The results of these two analyses are also roughly the same .

---

The impact of index on query speed

We use the index in the query process , It can improve the efficiency of database query , Reduce the number of queries . Next, let's use an example to compare the difference in query speed between using an index and not using an index .

---

We're looking through student Table to analyze the use and non use of indexes .

 

---

Queries without index

explain select * from student where stuname = 'zhangsan';

  branch Analysis ： Without using an index , We found that in the original table Three pieces of data They were all checked . And then someone will say , What is the impact of only three pieces of data ？ Of course , When the amount of data is small , Query efficiency won't make much difference . If there is a large amount of data in the database , Like tens of millions of them , When we query, these tens of millions of data will be retrieved , Users traverse tens of millions of pieces of data in order to query one piece of data , It will consume a lot of time and resources .

---

Query using index

Let's give it first stuname Field plus index

CREATE INDEX index_stuname ON student(stuname);

  After adding the index, we are querying

explain select * from student where stuname = 'zhangsan';

  branch Analysis ： It can be seen from the above results that , The number of rows accessed due to the index created is determined by 3 Reduced to 1, therefore , In the query operation , Using indexes will not only Optimize query efficiency , Will Reduce server overhead .

---

LIKE Optimize index queries

When the first character is “%” when , Index will not be used , If “%” The position in the matching string is not the first position , Then the index will be Normal use .

explain select * from student where stuname like 'zh%';

 

---

Use multi column index

Multi column index creates an index on multiple fields of a table , Only when one of these fields is used in the query criteria , The index will be used normally .

Create multi column indexes

CREATE INDEX index_student ON student(stuname,stuage);

---

Be careful ： When we query, if we apply stuage Field , The index cannot be used normally , You have to use First field stuname when , The index takes effect normally . This must be remembered ！

---

---

Use keywords or

        ！ stay MySQL in , Query statements only contain keywords or when , It is required that the two fields of the query must Same as index , If the search criteria , One field is not indexed , The index will not be applied to the query , Indexes Will not enter into force , Query efficiency will not be improved .

---

If this article is helpful to my friends , I hope you can give me some praise and support ~      Thank you very much. ~

---