MySQL index

One 、MySQL Indexes

1、 The concept of index

An index is a sorted list , In this list are stored the value of the index and the physical address of the row containing the data of this value （ Be similar to C The linked list of language points to the memory address of data record through pointer ）.

After using the index, you can locate the data of a row without scanning the whole table , Instead, we first find the corresponding physical address of the row data through the index table, and then access the corresponding data , Therefore, it can speed up the query speed of the database .

An index is like a catalogue of a book , You can quickly find the required content according to the page number in the table of contents .

Index is a way to sort the values of one or more columns in a table .

The purpose of indexing is to speed up the search or sorting of records in a table .

2、 Functions and disadvantages of index

advantage ：
After setting the appropriate index , The database uses a variety of fast positioning technologies , Can greatly speed up the query speed , This is the main reason for creating all the .

When the table is large or the query involves multiple tables , Using indexes can improve query speed thousands of times .

It can reduce the cost of the database IO cost , And the index can also reduce the sorting cost of the database .

By creating a unique index , It can ensure the uniqueness of each row of data in the data table .

You can speed up the connection between tables .

When using grouping and sorting , Can greatly reduce the grouping and sorting time .

shortcoming ：
Index takes up extra disk space .

about MyISAM In terms of engines , Index files and data files are separate , The address where the index file is used to hold the data record . and InnoDB The table data file of the engine itself is the index file .

It takes more time to insert and modify data , Because the index has to change with it .

3、 The principle of index creation is based on

Indexing can speed up database queries , But it's not always appropriate to create an index . Because the index itself consumes system resources , With an index , The database will perform index query first , Then navigate to the specific data row , If the index is not used properly , On the contrary, it will increase the burden of the database .

Primary Key 、 Foreign key must have index . Because the primary key is unique , The foreign key is associated with the primary key of the child table , You can quickly locate when querying .

Number of records exceeds 300 The table of rows should have an index . If there is no index , You need to traverse the table , Will seriously affect the performance of the database .

A table that is often connected to other tables , Index should be established on the connection field .

Fields with poor uniqueness are not suitable for indexing .

Fields that are updated too frequently are not suitable for creating indexes .

It often appears in where Fields in clause , In particular, the fields of large tables , It should be indexed .

Indexes should be built on fields with high selectivity .

Indexes should be built on small fields , For large text fields or even extra long fields , Don't index .

4、 Classification and creation of index

General index

The most basic index type , There's no such thing as uniqueness .

create index  Index name  on  Table name  ( Name [(length)]);

show create table xy01; # View table structure 

Modify table mode to create index

alter table  Table name  add index  Index name  ( Name );
alter table xy01 add index cardid_index (cardid);

Set index when creating table

create table  Table name  （ Field 1  data type ,... Field n, data type ,index  Index name  （ Name ））;
create table xy03 (id int(5) not null,name char(10),num varchar(20),index id_index (id));

unique index

Similar to a normal index , But the difference is that each value of a column with a unique index is unique .
Null values are allowed for unique indexes （ Note that it's different from the primary key ）, If it's a composite index , The combination of column values must be unique , Add unique
Key will automatically create a unique index .

create unique index  Index name  on  Table name  （ Name ）;
create unique index name_index on xy03 (name);

Modify tables to create

alter table  Table name  add unique  Index name  （ Name ）;
alter table xy03 add unique num_index (num);

Specify a unique index when creating

create table  Table name  ( Field 1  data type ,... Field n  data type ,unique  Index name  ( Name ));
create table xy04 (id int not null primary key,name char(10),hobby varchar(20),unique name_index (name));

primary key
Primary key index is a special unique index , Must be specified as “primary key”.
A table can only have one primary key , No null values are allowed , Adding a primary key will automatically create a primary key index

When creating a table, specify

create table  Table name  ( Field 1  data type ,... Field n  data type ,primary key （ Field name ）);
create table xy05 (fruits char(10) not null primary key,price float（4）);
 or 
create table xy05 (fruits char(10) not null,price float（4）,primary key （fruits）);

Create a primary key index by modifying a table

alter table  Table name  add primary key （ Name ）;

Composite index

A composite index can be an index created on a single column , It can also be indexes created on multiple columns . We need to meet the leftmost principle , because select Of the statement where The conditions are executed from left to right in turn , So it's using select Statement query where The order of the fields used by the condition must be the same as the sort in the composite index , Otherwise the index will not take effect .

Specify the index group when creating the table

create table  Table name  （ Name 1  data type ,... Name n  data type ,index  Index name  （ Name 1,..., Name n））;
create table xy06 (id int,name char,tel int,hobby char,sex char,index xy_index (name,sex));

Create index groups by modifying tables

alter table xy04 add index xy_index (name,hobby);

Full-text index

It is suitable for fuzzy query , Can be used to retrieve text information in an article .
stay MysQL5.6 Version before FULLTEXT The index can only be used for MyISAM engine , stay 5.6 After the version innodb The engine also supports FULLTEXT Indexes . Full text indexing can be done in CHAR、VARCHAR perhaps TEXT Create on column of type .
Only one full-text index per table is allowed .
Create index directly

create fulltext index  Index name  on  Table name  （ Name ）;

Create a full-text index by modifying the table method

alter table  Table name  add fulltext index  Index name  （ Name ）;

Use full text index queries

select * from  Table name  where match( Name ) against(' Query content ');

Look at the index

show index from  Table name ;
show keys from  Table name ;

Parameter meaning ：

Table： The name of the table
Non_unique： If the index can't include repetition , Then for 0; If possible , Then for 1
Key_name： Name of index
seq_in_index： Column ordinal in index , from 1 Start
column_name： Column name
collation： How columns are stored in indexes . stay MySQL in , Valuable ’A( Ascending ） or NULL( No classification )
Cardinality： An estimate of the number of unique values in an index
sub_part: If the column is only partially indexed , Is the number of characters indexed . If the entire column is indexed , Then for NULL
Packed： Indicates how keywords are compressed . If it's not compressed , Then for NULL
Null： If the column contains NULL, It contains YES. without , The column contains NO
lndex_type： Used indexing methods (BTREE,FULLTEXT,HASH,RTREE)
comment： remarks

5 Delete index

Delete index directly
drop index Index name on Table name

Modify the table to delete the index

Delete primary key index
alter table xy02 drop primary key;