[MySQL] - index principle and use

Preface  

I'm learning mysql In the process of , I believe most students are building tables and querying tables , I don't know mysql How to find these data through our given conditions , Index is an important feature of quickly finding data , Then let's introduce

Catalog

Why is there an index ? What is the index ?

The advantages and disadvantages of indexing

Classification of indexes

The leftmost prefix principle of Composite Index

The principle of index creation

The data structure of the index

  Clustered index and non clustered index

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Why is there an index ? What is the index ?

       mysql Stored data is Store by page Of , That is, the stored data page by page , If there is 1 Ten thousand data ( If points 10 page ), At this time, we go to find No 9000 Data , Then start with the first data on the first page and look back in turn , We can see that , This search is quite slow , But if there is a page number at this time , 9000 Article corresponds to article 9 page , We can start from 9 Start looking for , Is this a lot more efficient

        But the index we are talking about here is not like the index in the array , there An index is a data structure (InnoDB The engine uses B+ Trees ), Help us get data efficiently ,  So the database not only stores data , It also maintains such a data structure to help us get these data quickly

The advantages and disadvantages of indexing

  advantage

        The advantage of indexing is obvious , It speeds up our data acquisition , Increase of efficiency , Reduces database costs IO Cost of , You can sort the data by index columns , Reduce the cost of sorting data , To reduce the CPU Consumption of

Inferiority

        The index here is a data structure , That means we need to maintain such a data structure alone , So indexing increases the overhead , And if we write data ( Add, delete, modify ) When , Indexes also change , At this time, it is not only necessary to maintain data , Also maintain the index

        So we usually decide the use of the index according to the actual situation

Classification of indexes

         mysql Different index types are defined according to our different requirements

primary key

        When we set a field in the table as the primary key , The database will be automatically indexed , The primary key is the unique identifier of the table , It is suitable for indexing

ALTER TABLE  Table name  add PRIMARY KEY  Table name ( Name );
--  Delete the primary key index ： 
ALTER TABLE  Table name  drop PRIMARY KEY ;

Single value index

        An index contains only a single column , A table can have multiple single valued indexes ,  Of course, in addition to the primary key , We can also create indexes through other fields of the table

--  Create a single value index  
CREATE INDEX  Index name  ON  Table name ( Name ); 
--  Delete index ： 
DROP INDEX  Index name ;

unique index

        The unique index is also relative to the column , But it requires that the data in this column cannot be repeated , It can be for null

CREATE UNIQUE INDEX  Index name  ON  Table name ( Name ); 
--  Delete index  
DROP INDEX  Index name  ON  Table name ;

Composite index ( Composite index )

        An index contains multiple columns , Its creation cost is less , Relative to single valued index , Generally, composite indexes are used when there are many data in the table ( The number of rows in the table is much larger than the number of index columns )

--  Create composite index 
CREATE INDEX  Index name  ON  Table name ( Column  1, Column  2...); 
--  Delete index ：
DROP INDEX  Index name  ON  Table name ;

The leftmost prefix principle of Composite Index

          When using composite indexes , Need to meet such a principle :  When using the column of the composite index as the condition , The leftmost column must appear as a condition , Otherwise, the combined index will not take effect .

Examples are as follows :  Table has a,b,c 3 Column , by a,b Create a composite index with two columns

Columns such as select * from table where a=’’and b=’’ The index works
        select * from table where b=’’and a=’’ The index works
        select * from table where a=’’and c=’’ The index works
        select * from table where b=’’and c=’’ Index does not work

Full-text index

        When we use fuzzy queries , Use like when , At this time, the general index is invalid , At this time, we need to use full-text indexing

--  Create full text index 
CREATE FULLTEXT INDEX  Index name  ON  Table name ( Field name ) WITH PARSER ngram; 
--  Fuzzy query 
SELECT  structure  FROM  Table name  WHERE MATCH( Name ) AGAINST( Search terms ')

The principle of index creation

          After recognizing the characteristics of index , So imagine , In which case do we need to use indexes , Or not using indexes ? Summarized below

Need index

        First of all, the primary key automatically establishes a unique index , In addition, fields frequently used as query criteria should also be indexed (where The following sentence ) , Fields associated with other tables in the query , Foreign key relationships create indexes , Sort in query , Grouped fields , These are in order , Grouped fields are more efficient to find by index

There is no need to index

        Tables with less data in the table , If there is little data in the table , Then we don't need to use indexes at all , Another is the table that is frequently added, deleted and modified , When we write data , The index will also change , So this maintenance will affect the efficiency . where Do not create an index for fields that are not used after the condition , A table whose data is repeated and evenly distributed , Duplicate data columns , Indexing is not very effective

The data structure of the index

          mysql The default storage engine is InnoDB, It uses B+ The data structure implemented by the tree as an index

      Mysql The index uses B+ Trees , Because indexes are used to speed up queries , and B+ The tree can improve the query speed by sorting the data , Then multiple elements can be stored in one node , So that B+ The height of the tree will not be too high . And there are pointers between leaf nodes , It can well support full table scanning , Scope search, etc SQL sentence .

  Clustered index and non clustered index

Cluster index

      Clustered index refers to : Find the index and find the data , A primary key is a clustered index

Nonclustered index

      Non clustered index refers to : Find index first , No data is found at this time , Because the index and data are stored separately , After finding the index, we need to query the table again according to the index information , To find the data you need , It's also called secondary index

An example of this is as follows : Set up a student table

CREATE TABLE student (
   id BIGINT, 
   NO VARCHAR (20), 
   NAME VARCHAR (20), 
   PRIMARY KEY (`id`), 
   UNIQUE KEY `idx_no` (`no`) 
)

Then we do three kinds of queries : 

 1. Get data through the primary key , Because the primary key is the unique identifier , So you can get all the data directly according to the primary key , It's a clustered index

 2. Look up the number and name according to the number ,  The number itself is a unique index , But the query column contains the student number and student name , When the number index is hit , The primary key is stored in the data of the node of the index ID, According to the primary key ID Check again , So this kind of inquiry no Not clustered index

3. Check the number according to the number ( Determine whether the number exists ) , In this case ,  Return the number directly , Because the data needed is the index , No need to return table query , In this case no It's a clustered index .

        stay mysql The default storage engine for InnoDB in , Data and index are stored together , It's a clustered design ( But it uses non clustered indexes ),  And in the MYISAM In the engine , Data and index are stored separately , Yes no clustering design