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Several ways of MySQL database optimization (pen interview must ask)

2022-07-06 09:35:00 【Jiang Xia】

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This article altogether 3929 word , Estimated reading time 10 minute

Preface

I've changed jobs recently , I have been interviewing some time ago, and some questions are asked more frequently , Here is some sorting and summary , I hope I can help students who are also preparing to find jobs .

Among them, questions about databases are asked frequently , Then the frequency of asking about database, especially about database optimization, is basically that every company will ask . So here is a summary of the content about database optimization , Some of them are their own experience in peacetime development , Some of them are self-study contents . So if there is anything wrong, please discuss and correct it ！

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In fact, the optimization of database will not only try to ask more on the pen , It's really because in actual work , Database optimization can affect the performance of the system , It's really important , Therefore, it is often necessary to write reasonable sql Script , And optimize the database . The optimization of database can be mainly considered from the following aspects ：

1、 Select the most appropriate field properties

MySQL It can support large amount of data access , But generally speaking , The smaller the table in the database , The faster queries are executed on it . therefore , When creating tables , For better performance , We can set the size of the fields in the table as reasonable as possible , It is not necessary to choose the largest .

for example , stay user If username Set this field to varchar(255), Obviously, it will add unnecessary space to the database , Because we know that names can't reach such a long length anyway . alike .

Another way to improve efficiency is to , You should try to set the field to NOTNULL, In this way, when the query is executed in the future , Databases don't have to be compared NULL value .

For some text fields , Such as “ Gender ”, We can define them as ENUM type . Because in MySQL in ,ENUM Type is treated as numeric data , Numerical data can be processed much faster than text types . such , We can improve the performance of the database .

2、 Rational design table

about mysql It is best to follow the following principles in the design of the table ：

a、 Data volume of single table

All tables need to be annotated , It is suggested that the data quantity of single table should be controlled within 3000 All within ;

Try not to save large field data , For example, do not store pictures in the database 、 Big data such as files ;

b、 Table usage specification

You need to split large fields and fields with low access frequency , Separate hot and cold data ;

When you create a data table , Try to limit the number of fields in a single table to 20 Within fields ;

c、 The index specification

Creating an index can greatly improve the efficiency of data access , But the index should be created reasonably , such as ：

1、 The number of indexes in a single table should not exceed 5 individual , And the number of fields in a single index does not exceed 5 individual ;

2、INNODB Auto increment columns are recommended for primary keys , The primary key should not be modified , Strings should not be primary keys , If you don't specify a primary key ,INNODB Instead of using a unique and non null index ;

3、 If it's a composite index , Distinguish the largest field before the index ;

4、 Avoid redundant or duplicate indexes ： Reasonably create a federated index ( Avoid redundancy );

5、 Don't index low base sequences , for example ‘ Gender ’ Such fields ;

6、 Mathematical and functional operations are not performed on index columns

Some incorrect index usage will lead to index invalidation , Refer to previous articles ： A few things you must master SQL Optimization techniques ( 7、 ... and )： The best way to use an index

3、 Use connections （JOIN） Instead of subquery (Sub-Queries)

We often need to pass some sub queries , Use the result of the query as a condition in another query .MySQL from 4.1 Start supporting SQL Subquery of . for example , We need to find out the basic information of the students who fail the exam , You need to check the score of the failed students from the score table first id, Then query the basic information data from the students' basic information table ：

select cname,age,gender from student where id in (select stu_id from score where score<60)

Although subqueries can be used to complete many tasks at one time, which logically require multiple steps SQL operation , At the same time, transaction or table lock can be avoided , And it's easy to write . however , The sentence written like this is very verbose , Query efficiency is also relatively slow , Therefore, under certain circumstances , Subqueries can be used more efficiently join replace . For example, the above query statement can be written like this

select cname,age,gender from student s left join score c on s.id=c.stu_id where c.score<60

Use join Why it's more efficient , Because MySQL You don't need to create a temporary table in memory to complete this logical two-step query .

4、 Business

We know that in many cases , For some database operations, you can only use one or a few SQL Statement can be completed . More often than not, you need to use a series of statements to complete a certain task . But in this case , When one of the statements in this block runs wrong , The operation of the whole statement block will become uncertain . A classic example , Bank transfer requires an account deduction , Deposit into an account , If one a The account has been deducted successfully , At this time, something unexpected happened suddenly , Or there is an error in the execution of the following statement ,b The account has not been deposited , At this time, the consistency of data is destroyed , Nor does it conform to atomicity . So you need to use transactions at this time . You can refer to the previous article about transactions ： The isolation level of the transaction .

Things with BEGIN Keyword start ,COMMIT Keyword end . In the middle of this SQL operation failed , that ,ROLLBACK Command to restore the database to BEGIN The state before the beginning . such as

BEGIN;
  INSERT INTO student SET cname=' Jiangxia ';
  UPDATE score set score=99 where id=20
COMMIT;

Another important role of transactions is when multiple users use the same data source at the same time , It can use the method of locking database to provide users with a safe access way , This can ensure that the user's operation is not interfered by other users .

5、 Using the foreign key

Sometimes foreign keys can be used to ensure the relevance of data .

for example , Using foreign keys in the score table can ensure that each score record points to the corresponding student . ad locum , Foreign keys can put score In the table sty_id Mapping to student In the table id, This will ensure that no one is legal stu_id None of the records will be updated or inserted into score in .

The foreign key syntax is as follows ：

 The specific syntax format is as follows ：
[CONSTRAINT < Foreign key name >] FOREIGN KEY  Field name  [, Field name 2,…]
REFERENCES < Main table name >  Primary key column 1 [, Primary key column 2,…]

For two tables that have an association , The table where the primary key in the associated field is located is the primary table （ Parent table ）, The table where the foreign key is located is the slave table （ Sub table ）.

When defining a foreign key , The following rules need to be followed ：

1、 The primary table must already exist in the database , Or the table that is currently being created . In the latter case , Then the master table and the slave table are the same table , Such a table is called a self reference table , This structure is called self referential integrity . The primary key must be defined for the primary table .

2、 Primary key cannot contain null value , But null values are allowed in foreign keys . in other words , As long as each non null value of the foreign key appears in the specified primary key , The content of this foreign key is correct .

3、 Specify the column name or combination of column names after the table name of the main table . This column or combination of columns must be the primary key or candidate key of the main table .

4、 The number of columns in the foreign key must be the same as the number of columns in the primary key of the primary table .

5、 The data type of the column in the foreign key must be the same as that of the corresponding column in the primary key of the primary table .

6、 Use index

Index is a common method to improve database performance , It allows the database server to retrieve specific rows much faster than without an index , Especially in the query statements include MAX(),MIN() and ORDERBY When these orders come , The performance improvement is more obvious .

Which fields should be indexed ？

As a general rule , Indexes should be built on those that will be used for JOIN,WHERE Judgement and ORDERBY Sort the fields on . Try not to index a field with a lot of duplicate values in the database . For one ENUM For fields of type , It is possible that a large number of repetitions will occur

for example user In the table “ Gender ” Indexing fields will not help , It's also possible to degrade the performance of the database . When creating tables, you can create appropriate indexes at the same time , You can also use ALTER TABLE or CREATE INDEX Create indexes later . Besides ,MySQL From version 3.23.23 Start supporting full text indexing and search . Full text index in MySQL There is a FULLTEXT Type index , But only for MyISAM Type of watch . For a large database , Loading data into a system without FULLTEXT In the indexed table , And then use ALTER TABLE or CREATE INDEX Create index , It will be very fast . But if you load data into an existing FULLTEXT In the indexed table , The execution will be very slow .

For some use of index, please refer to the previous article ： A few things you must master SQL Optimization techniques ( 7、 ... and )： The best way to use an index ; Don't you know what a database index is .

7、 Lock the table

Because during the execution of a transaction , The database will be locked , Therefore, other user requests can only wait temporarily until the transaction ends . In some cases, we can get better performance by locking the magnification of the table .

A lock is a flag associated with a table .MySQL Allow client sessions to explicitly acquire table locks , To prevent other sessions from accessing the same table for a specific period of time . A client session can only acquire or release table locks for itself . It cannot acquire or release table locks for other sessions .

The following statement explicitly obtains the table lock ：

LOCK TABLES table_name [READ | WRITE]

To lock a table , Can be in LOCK TABLES Keyword followed by its name . Besides , You can also specify the type of lock , It can be READ or WRITE.

To release the lock on the table , Please use the following sentence ：

UNLOCK TABLES;

summary

That's about mysql Some points in database optimization , Database optimization is not only a knowledge point often asked in pen interviews , It is also often used in practical development and Application , If there is anything wrong, welcome to discuss and correct ！ And if there are other better methods or places not mentioned in this article , Also welcome to exchange and discuss ！