Mysql database index

1. The concept of index

Database index

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, it can not be used 1 Trace the whole table to locate the data of a row , 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. The function of index

● 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 to create indexes .
● 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 I0 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 .
● Indexing can significantly improve performance when searching and recovering data in a database

Side effects of indexing :

● 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

Although index can improve the speed of database query , 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 primary 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 , Every query needs 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 .
● On a regular basis GROUP BY、ORDER BY Index the fields of ;
● 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

create table member (id int (10) , name varchar (10) , cardid int (18) , phone int (11) , address varchar (50) , remark text) ;
(1) General index : The most basic index type , There's no such thing as uniqueness .
● Create index directly
CREATE INDEX Index name ON Table name ( Column name [ (length)]);

( Name (length)): length Optional. , The same below . If you ignore length Value , Then use the value of the entire column as the index . If specified , Use the front of the column length Characters to create the index , This helps to reduce the size of the index file . Without losing accuracy , The shorter the length, the better .

The index name is suggested to be “ _index" ending .

create index name_ index on member (name) ;

● Modify tables to create
ALTER TABLE Table name ADD INDEX Index name ( Name );

● Specify index when creating table
CREATE TABLE Table name ( Field 1 data type , Field 2 data type [,…], INDEX Index name ( Name ));

(2) unique index : Similar to a normal index , But the difference is that each value of a unique index column 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 . Adding a unique key automatically creates a unique index .

● Create a unique index directly :
CREATE UNIQUE INDEX Index name ON Table name ( Name );
create unique index cardid_ index on member (cardid) ;

● Modify tables to create
ALTER TABLE Table name ADD UNIQUE Index name ( Name );

● When creating a table, specify
CREATE TABLE Table name ( Field 1 data type , Field 2 data type [,…], UNIQUE Index name ( Name ));

(3) primary key : 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 ([…], PRIMARY KEY ( Name ));

● Modify tables to create
ALTER TABLE Table name ADD PRIMARY KEY ( Name ) ;
alter table member add primary key (id) ;

(4) Composite index ( Single column index and multi column index ) : It 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 .

CREATE TABLE Table name ( Name 1 data type , Name 2 data type , Name 3 data type , INDEX Index name ( Name 1, Name 2, Name 3));

select * from Table name where Name 1=‘…’ AND Name 2=‘…’ AND Name 3=’ …';

(5) Full-text index (FULLTEXT) : 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 );

● Modify tables to create
ALTER TABLE Table name ADD FULLTEXT Index name ( Name );
alter table member add fulltext remark_ index (remark) ;

● Specify index when creating table
CREATE TABLE Table name ( Field 1 data type [,…], FULLTEXT Index name ( Name ));
# The data type can be CHAR、VARCHAR perhaps TEXT

● Use full text index queries
SELECT * FROM Table name WHERE MATCH( Column name ) AGAINST(‘ Query content ’) ;

insert into member values (1, zhangsan, 123123, 123123, ‘nanjing’, ‘this is member!’) ;
insert into member values(2, ‘lisi’ , 456456, 456456, ‘beijing’, ‘this is vip!’) ;
insert into member values (3, ‘wangwu’, 789789, 78979, ’ shanghai’, ‘this is vip member!’) ;
select * from member where match (remark) against(‘vip’) ;

5. Look at the index

show index from Table name ;
show keys from Table name ;
The meaning of each field is as follows :
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.
Index type: Used indexing methods ( BTREE,FULLTEXT, HASH, RTREE) .
Comment: remarks .

6. Delete index

● Delete index directly
DROP INDEX Index name ON Table name ;

● Modify the table to delete the index
ALTER TABLE Table name DROP INDEX Index name ;

● Delete primary key index
ALTER TABLE Table name DROP PRIMARY KEY;

Case study : For example, a membership card system for a shopping mall . This system has a membership table , There are the following fields :
Membership number INT
Membership name VARCHAR(10)
Membership ID number INT (18)
Membership telephone INT(11)
Member Address VARCHAR (50)
Member notes TEXT

create table member (id int (10) , name varchar (10),cardid varchar (10) , phone int (11) , address varchar (50) , remark text) ;
alter table member add primary key (id) ;
create index name_ index on member (name) ;
create unique index cardid index on member (cardid) ;
alter table member add fulltext remark_ index (remark) ;

Then the member number , A primary key , Use PRIMARY KEY
Membership name , If you want to index , So it's normal INDEX
Membership ID number , If you want to index , Then you can choose UNIQUE ( Unique , No repetition )
Member notes , If you need to index , You can choose FULLTEXT, Full text search .
however FULLTEXT When searching for a long article , The best effect . Used for shorter text , If it's just a line or two , ordinary INDEX It's fine too .