Creation and use of MySQL index

Catalog

One 、 Index basis

Two 、 Scenarios for indexing

3、 ... and 、 Create index

Four 、 Delete and view index

5、 ... and 、 Use index

        6、 ... and 、 Efficient indexing policy

---

One 、 Index basis

• Index Introduction

Database index is like Pinyin when looking up a Chinese dictionary 、 Radicals

An index is a page that stores the primary key and index fields Orderly surface , And points to the record of the entity table

Indexes are divided into single column indexes and composite indexes , Single index —— An index contains one column ; Composite index —— An index contains multiple columns

Multiple single column indexes of a table ≠ Composite index

• advantage

Improve query speed

• shortcoming

Index files that take up disk space

Data operation statement （ increase INSERT、 Delete DELETE、 Change UPDATE） The processing time will be longer , That is, reduce the update speed of the table

 

Two 、 Scenarios for indexing

1. The amount of data exceeds 300 The table of should have an index

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

 

3、 ... and 、 Create index

grammar ：[UNIQUE|FULLTEXT] INDEX|KEY [index_Name](column_Name[(length)] [ASC|DESC]) [USING Index method ]

INDEX and KEY It's the same thing

There is only one single column index column_Name; There are multiple composite indexes column_Name, commas , Use the leftmost matching principle

The index method defaults to B+TREE

Left most matching principle ：cloumn_Name by (a,b,c) when ,WHERE You can query (a) or (a,b) or (a,b,c) or (a,c)

1. General index

Is the most basic index , No restrictions , Index values can appear multiple times

• Create index on table creation

grammar ：INDEX [index_Name] (column_Name)

CREATE TABLE student
(Sno char(5),
Sname varchar(20) not null,
INDEX st_name (Sname));

st_name For index name , If the user does not specify , be MySQL The index name will be automatically specified 

 The first 4 Lines can be replaced with 
KEY st_name (Sname)
INDEX (Sname)
KEY (Sname)

• Create index after creating table

grammar ：CREATE INDEX index_Name ON table_Name(column_Name);

• Create index when modifying table

grammar ：ALTER TABLE table_Name ADD INDEX [index_Name] (column_Name);

 

2. unique index

A unique index is a unique key , The value of a unique index column must be unique , It can be for NULL, And There can be multiple NULL

If it's a composite index , The combination of column values must be unique

• Create index on table creation

grammar ：UNIQUE [index_Name] (column_Name)

• Create index after creating table

grammar ：CREATE UNIQUE INDEX [index_Name] ON table_Name(column_Name);

• Create index when modifying table

grammar ：ALTER TABLE table_Name ADD UNIQUE [index_Name] (column_Name);

 

3. primary key

The primary key index does not need to be added manually , The primary key index is automatically created when the primary key is created , So a table can only have one primary key index

Index values must be unique , And Not for NULL

grammar ： take UNIQUE INDEX Change it to PRIMARY KEY

 

4. Full-text index

Use... In search engines ,MySQL Chinese full text index is not supported , adopt sphinx To do Chinese full-text index

grammar ： take UNIQUE Change it to FULLTEXT

 

Four 、 Delete and view index

1. Delete index

grammar ：DROP INDEX [index_Name] ON table_Name;

Use ALTER

grammar ：ALTER TABLE table_Name DROP INDEX index_Name;

          ALTER TABLE table_Name DROP PRIMARY KEY;

2. Show index information

SHOW INDEX  List index information in the table

\G  Format output

grammar ：SHOW INDEX FROM table_Name [\G];

3. Query whether the index is used

grammar ：EXPLAIN Query statement \G;

explain Meaning of output information

key： The final index evaluated by the optimizer （ The query does not use an index NULL）

Extra： Additional information on

 

/****Extra Four possibilities ****/

（1）using index： It means that the query requirements can be met by overwriting the index , No need to go back to the table , Therefore, the efficiency is higher

（2）using index;using where： First, the storage engine returns the search results through index retrieval （ There is still no need to go back to the table ）, And then in Server The layer passes through where Statement to filter the search results . This filter does not need to be returned to the table , Therefore, the efficiency is also very high

（3）using where： Express MySQL The results extracted by the storage engine layer will be filtered , The filter condition field has no index .using where Itself has nothing to do with whether to use indexes

（4）using index condition： yes MySQL 5.6 A new feature introduced in , Applicable to secondary indexes only , It usually occurs in the scenario where the query field cannot be overwritten by the secondary index , In this scenario, you often need to go back to the table . adopt ICP, It can reduce the row records returned by the storage engine , This reduces IO operation

For more information, please take a look https://blog.csdn.net/Saintyyu/article/details/99694649

 

/*use index*/
explain
select Sname
from student
where Sname='Li Zhang' and Sage=19 \G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: ref
possible_keys: st_info
          key: st_info
      key_len: 82
          ref: const
         rows: 1
     filtered: 20.00
        Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)

/*don't use index*/
explain
select *
from student
where Ssex=1 and Sage=19 \G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 5
     filtered: 20.00
        Extra: Using where
1 row in set, 2 warnings (0.00 sec)

 

5、 ... and 、 Use index

/*create table*/
create table student
(Sno char(5),
Sname varchar(20) not null,
Ssex char(2),
Sage tinyint,
Sdept varchar(10),
primary key (Sno));

/*insert data*/

/*create index*/
create index st_name on student(Sname,Ssex,Sage);

/*show index*/
show index from student \G;

/*use index*/
select *
from student
where Sname='he Liu' and Ssex=0;

/*drop index*/
drop index st_name on student;

 

6、 ... and 、 Efficient indexing policy

1. Hash index and prefix index

Sometimes you need to index long character columns , This will increase the storage space of the index and reduce the efficiency of the index , You can use hash index or prefix index to make the index more efficient

• Hash index

hash Index just hash Value order , It has nothing to do with table data , Cannot be applied to order by

hash The index can only be used to compare queries = or IN, Invalid other query range , The essence is that no data is stored

• Prefix index

The prefix index is the prefix of the selected character column n Characters as index , This can greatly save index space , To improve index efficiency

MySQL Cannot use prefix index to do ORDER BY、GROUP BY And using prefix index to do coverage scanning

2. Select the appropriate index column order

3. Clustered index and nonclustered index

• Clustered index

 

• Nonclustered indexes

Unlike a clustered index, a nonclustered index does not determine the physical ordering of data on disk , And in B-Tree Contains indexes but no row data , Row data is simply saved in B-Tree The corresponding pointer of the index in to point to the row data , Such as ： It's up there （Sname,Ssex,Sage） An index built on is a nonclustered index .

4. Overlay query

If an index contains the values of all the fields to be queried , It is called overlay index , Overwriting indexes can greatly improve access performance

Such as ：select Sname,Sage from student where Sname='he Liu';

Fields to query （Sname,Sage） Are contained in the index columns of the composite index （Sname,Ssex,Sage） in

 

---

Reference material

1.MySQL Indexes

2.mysql Novice introduction to index

3.MySQL Creation and use of index

4.MySql Detailed introduction and correct use of the index

5.MySQL in explain Field meaning and extra Detailed explanation

6.MySQL in Explain Of Extra field value Using index and Using index;Using where and Using where as well as Using index condition The difference between