MySQL index

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 the top method to sort the values of one or several columns in a table .

 The purpose of indexing is to speed up the search or sorting of records in a table .

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 to create indexes .

 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 .

 Indexing can significantly improve performance when searching and recovering data in a database 

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 .

3. The principle of index creation is based on

 Although index can improve the speed of database query , 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 primary 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 , Every query needs 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 .
 On a regular basis GROUP BY、ORDER BY Index the fields of .
 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 .

4. Classification and creation of index

(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 （ Field ）

Modify tables to create

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

Specify index when creating table

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

(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

Modify tables to create

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

When creating a table, specify

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

(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 .

Modify tables to create

ALTER TABLE  Table name  ADD PRIMARY KEY( Name )

(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 . Need to meet The principle of left , because select Of the statement where The conditions are executed from left to right in turn , So it's using select Statement query where
The order of the fields used by the condition must be the same as the sort in the composite index , Otherwise the index will not take effect .

Modify the table structure to create

mysql> alter table member add index phone_addr_index (phone,address);

(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 FOLLTEXT 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

mysql> create fulltext index remark_index on member(remark);

Modify table structure and create index

Fuzzy query instance

Look at the index

The meaning of each field is as follows ：

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.

Index_type: Used indexing methods （BTREE,FULLTEXT,HASH,RTREE).

Comment: remarks .

Delete index

Delete index directly

DROP INDEX  Index name  ON  Table name ;

Modify the table to delete the index

ALTER TABLE  Table name  DROP INDEX  Index name ;

Delete primary key index

alter table  Table name  drop primary key;

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