MySQL index classification

（1）、 primary key
After setting it as the primary key, the database will automatically create an index ,innodb Index for clustering
（1） If the table has a primary key defined , be PK It's a clustered index ;
（2） If the table does not have a primary key defined , Then the first non empty unique index (not NULL unique) Columns are clustered indexes ;
（3） otherwise ,InnoDB Will create a hidden row-id As a clustered index ;( This row-id yes 6 Bit , So try to specify the primary key )

# Index with table ：
CREATE TABLE customer (id INT(10) UNSIGNED  AUTO_INCREMENT ,customer_no VARCHAR(200),customer_name VARCHAR(200),
  PRIMARY KEY(id) 
);
# Use AUTO_INCREMENT Keyword columns must have indexes ( As long as there is an index ).
CREATE TABLE customer2 (id INT(10) UNSIGNED,customer_no VARCHAR(200),customer_name VARCHAR(200),
  PRIMARY KEY(id) 
);
# Create a separate primary key index ：
ALTER TABLE customer add PRIMARY KEY customer(customer_no);  
# Delete the primary key index ：
ALTER TABLE customer drop PRIMARY KEY ;  
# Modify the primary key index ：
# You must delete (drop) The original index , New again (add) Indexes 

（2）、 Single value index
That is, an index contains only a single column , A table can have multiple single-column indexes

# Index with table ：
CREATE TABLE customer (id INT(10) UNSIGNED  AUTO_INCREMENT ,customer_no VARCHAR(200),customer_name VARCHAR(200),
  PRIMARY KEY(id),
  KEY (customer_name)  
);
# Indexes built with tables   Same as index name   Name (customer_name)
# Create a single value index separately ：
CREATE INDEX idx_customer_name ON customer(customer_name); 
# Delete index ：
DROP INDEX idx_customer_name ;

（3）、 unique index
The value of the index column must be unique , But you can have an empty value

# Index with table ：
CREATE TABLE customer (id INT(10) UNSIGNED  AUTO_INCREMENT ,customer_no VARCHAR(200),customer_name VARCHAR(200),
  PRIMARY KEY(id),
  KEY (customer_name),
  UNIQUE (customer_no)
);
# establish   The unique index must ensure that all values are unique （ except null）, If there are duplicate data , Will report a mistake .   
# Create a unique index ：
CREATE UNIQUE INDEX idx_customer_no ON customer(customer_no); 
# Delete index ：
DROP INDEX idx_customer_no on customer ;

（4）、 Composite index
That is, an index contains multiple columns

During database operations , Composite indexes require less overhead than single valued indexes ( Index the same multiple columns )

A composite index can be used when the number of rows in a table is much larger than the number of indexed columns

# Index with table ：
CREATE TABLE customer (id INT(10) UNSIGNED  AUTO_INCREMENT ,customer_no VARCHAR(200),customer_name VARCHAR(200),
  PRIMARY KEY(id),
  KEY (customer_name),
  UNIQUE (customer_name),
  KEY (customer_no,customer_name)
);
# Index separately ：
CREATE INDEX idx_no_name ON customer(customer_no,customer_name); 
# Delete index ：
DROP INDEX idx_no_name  on customer ;

Basic grammar

establish ：ALTER mytable ADD [UNIQUE ] INDEX [indexName] ON (columnname(length))

Delete ：DROP INDEX [indexName] ON mytable;

see ：SHOW INDEX FROM table_name\G

There are four ways to add indexes to a data table ：

ALTER TABLE tbl_name ADD PRIMARY KEY (column_list): This statement adds a primary key , This means that the index value must be unique , And cannot be NULL.

ALTER TABLE tbl_name ADD UNIQUE index_name (column_list): The value of the index created by this statement must be unique （ except NULL Outside ,NULL There may be many times ）.

ALTER TABLE tbl_name ADD INDEX index_name (column_list): Add a normal index , Index values can appear multiple times .

ALTER TABLE tbl_name ADD FULLTEXT index_name (column_list): The statement specifies that the index is FULLTEXT , For full-text indexing .

Which situations need to be indexed

Primary key automatically creates unique index

Fields that are frequently used as query criteria should be indexed (where Subsequent statements )

Fields associated with other tables in the query , Index foreign key relationship

Single key / The choice of Composite Index ,who？( Tend to create composite indexes at high and low levels )

Fields sorted in the query , If the sorting field is accessed by index, the sorting speed will be greatly improved

Statistics or grouping fields in query

When not to create indexes

There are too few records

Often add or delete tables ： Improved query speed , At the same time, it will reduce the speed of updating the table , Such as on the table INSERT、UPDATE and DELETE. Because when updating tables ,MySQL Not only to save data , And save the index file

Where The fields that are not used in the condition are not indexed

Duplicate and evenly distributed table fields , So you should index only the most frequently queried and sorted data columns , A data column contains many duplicates , Indexing doesn't have much practical effect .