Simply use MySQL index

MySQL Indexes

1 What is index

• In the database table , Indexing fields can greatly improve query speed . By making good use of these indexes , Can make MySQL And
  Run more efficiently .
• If the index is properly designed and used MySQL It's a Lamborghini , So there's no index designed and used MySQL Namely
  A man-made tricycle . Take the catalog page of the Chinese Dictionary （ Indexes ） Let's say , We can Pinyin 、 stroke 、 Partial radicals Etc
  record （ Indexes ） Quickly find the words you need

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2 Common index classification

The index name	explain
primary key （primary key）	Primary key is a unique index , Each table can only have one primary key , Used to identify each record in the data table
unique index （unique）	The unique index refers to All of the index columns Values can only appear once , Must be unique .
General index （index）	The most common index , Role is Speed up data Access speed

MySql Save the indexes of a table in the same index file , If you add, delete or modify the data in ,MySql Will automatically change
New index .

2.1 primary key (PRIMARY KEY)

characteristic : Primary key is a unique index , Each table can only have one primary key , Used to identify a record in the data table .

• A table can have no primary key , But there can only be one primary key at most , also The primary key value cannot contain NULL.

• Create a database table with only fields demo01

CREATE TABLE demo01(
	did INT,
	dname VARCHAR(20),
	hobby VARCHAR(30)
);

Create primary key index

• Grammar format

• When creating a table, directly add the primary key index ( The most commonly used )

CREATE TABLE  Table name (
	--  Add primary key  ( A primary key is a unique index , Not for null, Can't repeat ,)
	 Field name   type  PRIMARY KEY,
);

• Modify table structure Add primary key index

ALTER TABLE  Table name  ADD PRIMARY KEY (  Name  )

• by demo1 Add primary key index to the table

ALTER TABLE demo01 ADD PRIMARY KEY ( did )

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2.2 unique index (UNIQUE)

characteristic : All values of an index column can only appear once , Must be unique .

• Unique index can ensure the uniqueness of data records . in fact , on many occasions , People often do not create unique indexes for Improve access speed , Just to avoid data duplication .

Create unique index

Grammar format

• When creating a table, directly add the primary key index

CREATE TABLE  Table name (
	 Name   type ( length ),
	--  Add unique index 
	UNIQUE [ The index name ] ( Name )
);

• Use create Sentence creation : Create an index on an existing table

create unique index  Index name  on  Table name ( Name ( length ))

• Modify table structure to add index

ALTER TABLE  Table name  ADD UNIQUE (  Name  )

• by hobby Add a unique index to the field

create unique index ind_hobby on demo01(hobby)

test

• Insert data into table

INSERT INTO demo01 VALUES(1,' Zhang San ','DBJ');
#  Report errors Duplicate entry 'DBJ' for key 'hobby'
#  The unique index ensures the uniqueness of data , The efficiency of indexing has also improved 
INSERT INTO demo01 VALUES(2,' Li Si ','DBJ');

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3 General index (INDEX)

• General index （ By keyword KEY or INDEX Index of definitions ） Your only task is Speed up access to data .
• therefore , It should only be for those that most often appear in query conditions （WHERE column=） Or sort by （ORDERBY column） Create data columns in
  Indexes .

Create a normal index

Grammar format

• Use create index Sentence creation : Create an index on an existing table

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

• Modify table structure to add index

ALTER TABLE  Table name  ADD INDEX  Index name  ( Name )

• to dname Add index to field

#  to dname Add index to field 
alter table demo01 add index dname_indx(dname);

4 Delete index

• Because the index will occupy a certain amount of disk space , therefore , In order to avoid affecting the performance of the database , You should delete the unused cables in time
  lead

• Grammar format

ALTER TABLE table_name DROP INDEX index_name;

• Delete demo01 The name in the table is dname_indx The general index of

ALTER TABLE demo01 DROP INDEX dname_indx;

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3 Index performance test

1 Import data table

• Download tests sql

• Find... Under the software folder Test index .sql file , Click on the perform

• The following problems occurred during operation:

• Err 1153 - Got a packet bigger than ‘max_allowed_packet’ bytes

• Because of the large amount of data ,mysql It can be used for single table data with large amount SQL Make restrictions ,10w The string of data exceeds max_allowed_packet

  The allowable range of .

terms of settlement

find ：max_allowed_packet Sure ctrl+f Search for Change the default value to 419430400 Probably 400M

Check whether we have successfully modified

Let's get down to business

• Inquire about test_index The total number of records in the table · Table has 500 Ten thousand data

SELECT COUNT(*) FROM test_index;

• test

Without adding an index , Use dname Field to query

# Index not added , Group query 
SELECT * FROM test_index GROUP BY dname;

by dname Field add normal index

• Be careful : Generally, we are creating tables Determine the fields that need to be indexed

# Add index 
ALTER TABLE test_index ADD INDEX dname_indx(dname);

4 Summary of the advantages and disadvantages of the index

• Adding an index should first be considered in where And order by Index the columns involved .
• The advantages of indexing
  1. Greatly improve the query speed
  2. The sorting time and grouping in the query can be significantly reduced .
• Disadvantages of indexes
  1. Indexing takes time to create and maintain , And the larger the amount of data, the longer the time
  2. When adding data in the table , modify , When deleting , The index should also be maintained at the same time , Reduced data maintenance
     Speed