MySQL - normal index

Create and view indexes

• Creating an index is to create an index on one or more columns of a table , In order to speed up the access to the table .
• Index creation has 3 Ways of planting , Namely Create indexes when creating tables 、 Create an index on an existing table and Use ALTER TABLE Statement to create an index . This section will explain this in detail according to the specific index classification 3 Ways to create .

General index

• The so-called general index , When creating an index , Without any restrictions （ only 、 Non empty limit ）. Indexes of this type can be created on fields of any data type .

• 1. Define indexes when creating tables

  CREATE TABLE tablename(
      propname1 type1,
      propname2 type2,
      ……
      propnamen type..n,
       INDEX | KEY
      [indexname] (propnamen [(length)] [ ASC | DESC ] ) );
  

  Parameters INDEX and KEY Is used to specify a field as an index , Just choose one of the two , It's the same thing ;
  Parameters indexname It's the index name , Omission ;
  Parameters propnamen Is the name of the field corresponding to the index , This field must be the one defined before ;
  Parameters length Is an optional parameter , It refers to the length of the index , Must be a string type to use ;
  Parameters ASC and DESC All optional parameters ,ASC Indicates ascending order ,DESC Represents a descending order , If you don't specify , In ascending order .

  example :

  mysql> create table class(id int, name varchar(64), teacher varchar(64), INDEX id_index (id DESC));	// establish class surface ,  Appoint id Column as index column ,  The sorting method is descending 
  mysql> show create table class;						// Query table structure 
  mysql> insert into class values(1, ' Class one ', ' Zhang San ');	// insert data 1
  mysql> insert into class values(2, ' Class two ', ' Li Si ');	// insert data 2
  mysql> select * from class where id>0;				// Query by column 
  

  
  

• 2. Create an index on an existing table
  Method 1 : perform create sentence

  CREATE INDEX indexname  
      ON tablename (propname [(length)] [ASC|DESC]);  
  

  Parameters INDEX Is used to specify a field as an index , It can't be KEY;
  Parameters indexname Is the name of the newly created index ;
  Parameters tablename Is the name of the table that needs to be indexed , The table must already exist , If it doesn't exist , You need to create ;
  Parameters propname Specify the name of the field corresponding to the index , This field must be the one defined before ;
  Parameters length Is an optional parameter , Indicates the length of the index , Must be a string type to use ;
  Parameters ASC and DESC All optional parameters ,ASC Indicates ascending order ,DESC Represents a descending order , Default ascending order .

  mysql> create table class(id int, name varchar(64), teacher varchar(64));	// establish class surface 
  mysql> create INDEX id_index ON class(id DESC);		// Appoint class In the table id Index column , Descending order 
  mysql> show create table class;						// Query table structure 
  mysql> insert into class values(1, ' Class one ', ' Zhang San ');	// insert data 1
  mysql> insert into class values(2, ' Class two ', ' Li Si ');	// insert data 2
  mysql> select * from class where id>0;				// Query by column 
  

  Method 2 : perform ALTER TABLE sentence

  ALTER TABLE tablename ADD INDEX | KEY indexname  
       (propname [(length)] [ASC|DESC]);  
  

  Parameters tablename Is the table that needs to be indexed ;
  keyword IDNEX or KEY Used to specify the creation of a normal index ;
  Parameters indexname Used to specify the name of the index created ;
  Parameters propname Used to specify the name of the field associated with the index ;
  Parameters length Used to specify the length of the index ;
  Parameters ASC Used to specify ascending sort ; Parameters DESC Used to specify descending sort .

  mysql> create table class(id int, name varchar(64), teacher varchar(64));	// establish class surface 
  mysql> alter table class INDEX id_index (id DESC);	// Appoint class In the table id Index column , Descending order 
  mysql> show create table class;						// Query table structure 
  mysql> insert into class values(1, ' Class one ', ' Zhang San ');	// insert data 1
  mysql> insert into class values(2, ' Class two ', ' Li Si ');	// insert data 2
  mysql> select * from class where id>0;				// Query by column 
  

Check the execution of the index

• EXPLAIN Query statement
• Output results ：
  key： Actual index used . If NULL, No index is used
  possible_keys： Show the indexes that may be applied to this table , One or more . If there is an index on the field involved in the query , Then the index will be listed , But it doesn't have to be actually used by the query
  key_len： Represents the number of bytes used in the index , You can use this column to calculate the length of the index used in the query . The shorter the value, the better ！