MySQL-02. Understanding of indexes at work

Indexes

1. What is index ？

• help MySQL Data structure for efficient data acquisition
• It's equivalent to a catalog , It's a document , Need to occupy physical space

2. Use scenarios

• where Back field , Index this field （alter table Table name add index（ Field name ））.
• order by Later fields , If there is no index , The execution plan will use external sorting for all the queried data , Affect performance , After using sorting , The index itself is ordered , Therefore, the data can be extracted one by one according to the index order and mapping relationship .
• Yes join Statement matching relationship on The involved fields are indexed to improve efficiency .
• As much as possible in select Write only the necessary query fields after , Increase the probability of index coverage .

3. Three ways to create indexes

3.1 In execution CREATE TABLE Create index

CREATE TABLE user_index2 (
	id INT auto_increment PRIMARY KEY,// primary key 
	first_name VARCHAR (16),
	last_name VARCHAR (16),
	id_card VARCHAR (18),
	information text,
	KEY name (first_name, last_name),// General index 
	FULLTEXT KEY (information),// Full-text index 
	UNIQUE KEY (id_card)// unique index 
);

3.2 Use ALTER TABLE Command to add index

ALTER TABLE table_name ADD INDEX index_name (column_list);

3.3 Use CREATE INDEX Command to create

CREATE INDEX index_name ON table_name (column_list);

3.4 Delete index

alter table user_index drop KEY name;
alter table user_index drop KEY id_card;
alter table user_index drop KEY information;

4. Avoid index invalidation

• Contrary to " The best left prefix rule ", Leading brother can't die , The middle brother can't break
• Do nothing on the index column （ Calculation 、 function ）
• Use ！= perhaps <> Causes a full table scan
• is null,is not null You can't use indexes
• String without single quotes