Mysql database - index

1 Indexes

1.1 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 .

1.2 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 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 .

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 .

1.3 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 .

1.4 Classification and creation of index

1.4.1 General index

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

● Create index directly

CREATE INDEX  Index name  ON  Table name  ( Name [(length)]);

#( Name (length))：length Optional. . If you ignore  length  Value , Then use the value of the entire column as the index . If specified, use the... Before the column  length  Characters to create the index , This helps to reduce the size of the index file .
# The index name is suggested to be “_index” ending .

 example ：create index phone_index on member (phone);
select phone from member;
show create table member;

● Modify tables to create

ALTER TABLE  Table name  ADD INDEX  Index name  ( Name );

 example ：alter table member add index id_index (id);
select id from member;
select id,name from member;

Specify index when creating table

CREATE TABLE  Table name  (  Field 1  data type , Field 2  data type [,...],INDEX  Index name  ( Name ));

 example ：create table test(id int(4) not null,name varchar(10) not null,cardid varchar(18) not null,index id_index (id));
show create table test;

1.4.2 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 );

 example ：select * from member;
create unique index address_index on member 	(address);
create unique index name_index on member (name);
show create table member;

Modify tables to create

ALTER TABLE  Table name  ADD UNIQUE  Index name  ( Name );

 example ：alter table member add unique cardid_index (cardid);

● Specify index when creating table

CREATE TABLE  Table name  ( Field 1  data type , Field 2  data type [,...],UNIQUE  Index name  ( Name ));

 example :create table amd2 (id int,name varchar(20),unique id_index (id));
show creat table amd2;

1.4.3 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 ));

 example ：create table test1 (id int primary key,name varchar(20));
create table test2 (id int,name varchar(20),primary key (id));

show create table test1;
show create table test2;

● Modify tables to create

ALTER TABLE  Table name  ADD PRIMARY KEY ( Name ); 

1.4.4 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 .

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='...';

 example :create table amd1 (id int not null,name varchar(20),cardid varchar(20),index index_amd (id,name));
show create table amd1;
insert into amd1 values(1,'zhangsan','123123');
select * from amd1 where name='zhangsan' and id=1;

 Query index usage select Query notes where The leftmost principle of . The order of the query fields should be consistent with the composite index , To take effect .

1.4.5 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 );

 example ：select * from member;
create fulltext index remark_index on member (remark);

● Modify tables to create

ALTER TABLE  Table name  ADD FULLTEXT  Index name  ( Name );

● Specify index when creating table

CREATE TABLE  Table name  ( Field 1  data type [,...],FULLTEXT  Index name  ( Name ));
1
# The data type can be  CHAR、VARCHAR  perhaps  TEXT

● Use full text index queries

SELECT * FROM  Table name  WHERE MATCH( Name ) AGAINST(' Query content ');

 example ：select * from member where match(remark) against('this is vip');

2 Look at the index

show index from  Table name ;
show index from  Table name \G;  Display table index information vertically 
show keys from  Table name ;
show keys from  Table name \G;

Field	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

3 Delete index

Besides deleting the primary key index , The way to delete other indexes is the same .

Delete primary key index

alter table  Table name  drop primary key;

Delete non primary key index

drop index  Index name  on  Table name ;     # Delete index directly 
 ​
alter table  Table name  drop index  Index name ;   # How to modify the table to drop the index 

summary

General index

create index  Index name  on  Table name  ( Field );
alter table  Table name  add index  Index name  ( Field );
create table  Table name  ( Field .... , index  Index name  ( Field ）);

unique index

create unique index XXX index on  Table name （ Field ）;
alter table  Table name  add unique xxx index  Index name （ Field ）;
create table Table name （ Field ....,unique xxx index（ Field ））;

primary key

alter table  Table name  add primary key（ Field ）;
create table Table name （ Field ...…,primary key（ Field ））;

Composite index （ Single column 、 Multi column index ）

create index XXx index on  Table name （ Field 1, Field 2,.., Field n）;
alter table  Table name add index XXx_index（ Field 1, Field 2,.…, Field n）;

Pay attention to where The leftmost principle of

select*from  Table name where  Field 1=xxx and  Field 2=xxx and....

Full-text index

create fulltext index on  Table name （ Field ）;
alter table  Table name  add fulltext XXX index（ Field ）;
create table  Table name （ Field ...,fulitext xxx index（ Field ））;

Fuzzy query

select * from  Table name  where match（ Field ）against（' Query string '）;

Query index

show index from  Table name ;
show keys from Table name ;
show create table Table name ; Only the fields and names of the index can be viewed