How to understand MySQL indexes?

One 、 What is index

The index is defined as ： Indexes （index） Help MySQL Efficient access to data data structure （ Orderly ）. Out of data , The database system also maintains a data structure that satisfies a specific search algorithm , These data structures are referenced in some way （ Point to ） data , In this way, advanced search algorithms can be implemented on these data structures , This data structure is the index .

The process of sorting data is called Indexes

Let's check according to the index , Increase of efficiency

MySQL The establishment of index for MySQL It is very important to run efficiently , The index can be greatly improved MySQL Retrieval speed of .

If the index is properly designed and used MySQL It's a Lamborghini , So there's no index designed and used MySQL It's a human powered tricycle .

Two 、MySQL Index classification

markdown * Primary key （ constraint ） Indexes Primary key constraint + Improve query efficiency

• only （ constraint ） Indexes Unique constraint + Improve query efficiency
• General index Only improve query efficiency
• Combine （ union ） Indexes Multiple fields form an index name age
• hash Indexes according to key-value Very efficient
• Full-text index By looking for keywords in the text , Similar to search engines

explain ： When we create a table, we specify gradual and unique constraints , Then it is equivalent to adding primary keys and unique indexes to the fields of the table .

3、 ... and 、MySQL Index Syntax

3.1 Create index

① Directly create... On the fields of the existing table 【 understand 】

--  Create a normal index 
create index  Index name  on  Table name ( Field );

--  Create unique index 
create unique index  Index name  on  Table name ( Field );

--  Create a normal Composite Index 
create index  Index name  on  Table name ( Field 1, Field 2,..);

--  Create a unique composite index 
create unique index  Index name  on  Table name ( Field 1, Field 2,..);

explain ：

1. If you create multiple indexes in the same table , Make sure that the index name cannot be repeated .
2. The above method of creating index is troublesome , You also need to specify the index name .
3. You cannot add a primary key index in the above way .

【 Ready to create table SQL sentence 】

create database day04;

use day04;
--  Create student table 
CREATE TABLE student(
 id INT,
 name VARCHAR(32),
 telephone VARCHAR(11)
);

【 Index the fields of the above table 】

1. to name Field to set the general index

CREATE INDEX name_idx ON student(name);

2. to telephone Field sets a unique index

CREATE UNIQUE INDEX telephone_uni_idx ON student(telephone);

After setting, you can view the set index through graphical tools ：

② Specify when modifying a table on a field of an existing table 【 understand 】

--  Add a primary key , This means that the index value must be unique , And cannot be NULL
alter table  Table name  add primary key( Field );  -- Default index name ：primary

--  Add unique index （ except NULL Outside ,NULL There may be many times ）
alter table  Table name  add unique( Field ); --  Default index name ： Field name 

--  Add a normal index , Index values can appear multiple times .
alter table  Table name  add index( Field ); --  Default index name ： Field name 

【 Prepared to create a table SQL sentence 】

--  Create student table 
CREATE TABLE student2(
 id INT,
 name VARCHAR(32),
 telephone VARCHAR(11)
);

【 Index the fields of the above table 】

1. Appoint id Index for primary key

ALTER TABLE student2 ADD PRIMARY KEY(id);

2. Appoint name For general index

ALTER TABLE student2 ADD INDEX(name);

3. Appoint telephone For unique index

ALTER TABLE student2 ADD UNIQUE(telephone);

After setting, you can view the set index through graphical tools ：

③ Specify... When creating a table 【 master 】

--  Create student table 
CREATE TABLE student3(
 id INT PRIMARY KEY AUTO_INCREMENT, --  primary key 
 name VARCHAR(32),
 telephone VARCHAR(11) UNIQUE, --  unique index 
 sex VARCHAR(5),
 birthday DATE,
 INDEX(name) --  General index 
);

3.2 Look at the index

show index from  Table name ; 

【 see student3 Table index information 】

show index from student3; 

result ：

3.3 Delete index

• grammar

【 grammar 1】 Delete directly

--  Delete directly 
drop index  Index name  on  Table name ;

【 grammar 2】 Delete when modifying table

--  Delete when modifying table 
alter table  Table name  drop index  Index name ;

• practice

（1） Delete student Tabular name General index

DROP INDEX name_idx ON student;

（2） Delete student Tabular telephone unique index

ALTER TABLE student DROP INDEX telephone_uni_idx;

Four 、 Demonstration of index effect of tens of millions of table records

Use the previously created user Test tens of millions of data in the data sheet . Be careful user There is no index in the table .

【1. Let's test the query without index 】

-- 1. Appoint id Inquire about 
select * from user where id = 8888888;
-- 2. Appoint username Accurate query 
select * from user  where username = 'jack1234567';

【2. To give the 2 Add an index to a field 】

explain ： When adding an index to a field of a table , The bottom layer carries out association and combination through sorting . So it takes some time , And the index will also take up hard disk space . So we need to be careful when using the index .

Before adding an index , The amount of data in hard disk space ：

Add indexes to the following fields as follows ：

--  Appoint id Index for primary key 
ALTER TABLE USER ADD PRIMARY KEY(id);
--  Appoint username For general index 
ALTER TABLE USER ADD INDEX(username);

After adding the index , The amount of data in hard disk space ：

If you add an index to more than one , Then the amount of hard disk space will increase . If the table is complex , If you add more indexes , There may be several more than before G It's also possible .

【3. Then test the query with index 】

-- 1. Appoint id Inquire about 
select * from user where id = 8888888;
-- 2. Appoint username Accurate query 
select * from user  where username = 'jack1234567';

explain ： From the above results, we can see that with the index , The query speed is dozens of times faster than before . Fly fast .

5、 ... and 、 Advantages and disadvantages of index

advantage

1） It's like a catalog index of books , Improve the efficiency of data retrieval , Reduce the IO cost .

2） At the bottom of the index is sorting , Sort data through index columns , Reduce the cost of sorting data , Reduce CPU Consumption of .

Inferiority

- In the process of database establishment , It takes more time to build and maintain the index , Especially with the increase of the total amount of data , The time spent will continue to increase .

- The index created in the database needs to occupy a certain amount of physical storage space , This includes the data space occupied by the data table and the physical space occupied by each index created .

- When modifying the data in the table , For example, add it 、 When deleting or modifying , Indexes also need to be dynamically maintained , This brings some trouble to the maintenance speed of the database .

6、 ... and 、 Follow up learning

What about indexing , also Learning data structures > Clustered index and non clustered index >hash Indexes > Overlay index > The principle of index creation > Avoid index invalidation > Optimization scheme for batch data paging query, etc Content .

There are also some high-frequency interview questions
【1】Hash Index and B+ What's the difference or advantage of tree ownership ?
【2】 What happens when an index is created for the column but not used in the query ( Index failure )？
【3】 You know, mysql What are the indexes ？
【4】 Is the index created used in ? Or how can we know why this statement runs slowly ?
【5】 What is a federated index ? Why should we pay attention to the order in the union index ?
【6】 Will non clustered index return to table query ?