Mysql-- index

Indexes ： help MySQL Data structure for efficient data acquisition
Extract the sentence trunk , We can get the essence of index ： An index is a data structure

Classification of indexes

• primary key （primary key）： Unique identification , Do not repeat , There can only be One column A primary key
• unique index （unique key）： Avoid duplicate columns , The unique index can be repeated , Multiple Columns can be identified as unique indexes
• General index （key/index）： default ,index、key Keyword settings
• Full-text index （FullText）： Only under the specific database engine ,MyISAM

Use of index

• Add indexes to fields when creating tables
• After creation , Add index

-- Show all index information 
show index from student
-- Add a full text index 
alter table student add fulltext index 'studentName';

-- explain  analysis sql sentence 
explain select * from student; -- Non full text index 
-- Full text indexing is useless when there is a small amount of data 
explain select * from student where match(studentName) against(' Liu ');-- Look up a piece of data 

Test index

/* Insert a million pieces of data */
delimiter $$ /* Before you write a function, you have to write , sign */
create function mock_data()
returns int
begin
    declare num int default 1000;
    declare i int default 0;
    while i<num do
    /* Insert statement */
        set i = i+1;
    end while ;
    return i;
end;

– Common naming methods for indexes ：id_ Table name _ Field name

--create index  Index name  on  surface ( Field )
create index id_app_user_name on app_user('name');

explain select * from app_user where name='iii';

Index in a small amount of data , Not many users , But when it comes to big data , The difference is obvious

Indexing principles

• More indexes is not better
• Don't index frequently changing data
• A table with a small amount of data does not need to be indexed
• The index is usually added to the fields commonly used for query

The data structure of the index

hash Index of type
Btree：InnoDB Default data structure