MySQL performance optimization - index optimization

1. avoid sql Statement does not hit index

Use explain see SQL Implementation plan of ,

explain select * from tb_item where id = 1;

Field	meaning
id	select The serial number of the query , It's a set of numbers , Represents execution in query select Clause or the order of the operation table .
select_type	Express SELECT The type of , Common values are SIMPLE（ A simple watch , That is, no table join or subquery is used ）、PRIMARY（ Main query , That is, the outer query ）、UNION（UNION The second or subsequent query statement in ）、SUBQUERY（ First in subquery SELECT） etc.
table	Output result set table
type	Indicates the connection type of the table , The connection types with good to poor performance are ( system ---> const -----> eq_ref ------> ref -------> ref_or_null----> index_merge ---> index_subquery -----> range -----> index ------> all )
possible_keys	When representing a query , Possible indexes
key	Indicates the index actually used
key_len	Length of index field
rows	Number of scan lines
extra	Description and description of the implementation

  Preparation conditions ：

create table `tb_seller` (
    `sellerid` varchar (100),
    `name` varchar (100),
    `nickname` varchar (50),
    `password` varchar (60),
    `status` varchar (1),
    `address` varchar (100),
    `createtime` datetime,
    primary key(`sellerid`)
)engine=innodb default charset=utf8mb4; 
 
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('alibaba',' Alibaba ',' Ali shop ','e10adc3949ba59abbe56e057f20f883e','1',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('baidu',' Baidu Technology Co., Ltd ',' Baidu store ','e10adc3949ba59abbe56e057f20f883e','1',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('huawei',' Huawei Technology Co., Ltd ',' Huawei store ','e10adc3949ba59abbe56e057f20f883e','0',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('itcast',' Chuanzhi podcast Education Technology Co., Ltd ',' Spreading wisdom Podcast ','e10adc3949ba59abbe56e057f20f883e','1',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('itheima',' Black horse programmer ',' Black horse programmer ','e10adc3949ba59abbe56e057f20f883e','0',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('luoji',' Logitech Technology Co., Ltd ',' Logitech shop ','e10adc3949ba59abbe56e057f20f883e','1',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('oppo','OPPO Technology Co., Ltd ','OPPO Official flagship store ','e10adc3949ba59abbe56e057f20f883e','0',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('ourpalm',' Zhangqu Technology Co., Ltd ',' Zhangqu shop ','e10adc3949ba59abbe56e057f20f883e','1',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('qiandu',' QIANDU Technology ',' QianDu store ','e10adc3949ba59abbe56e057f20f883e','2',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('sina',' Sina Technology Co., Ltd ',' Sina's official flagship store ','e10adc3949ba59abbe56e057f20f883e','1',' The Beijing municipal ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('xiaomi',' Xiaomi Tech ',' Xiaomi's official flagship store ','e10adc3949ba59abbe56e057f20f883e','1',' Xi'an City ','2088-01-01 12:00:00');
insert into `tb_seller` (`sellerid`, `name`, `nickname`, `password`, `status`, `address`, `createtime`) values('yijia',' IKEA ',' IKEA flagship store ','e10adc3949ba59abbe56e057f20f883e','1',' The Beijing municipal ','2088-01-01 12:00:00');
create index idx_seller_name_sta_addr on tb_seller(name,status,address);

2. Full match , Specify specific values for all columns in the index

explain select * from tb_seller where name=' Xiaomi Tech ' and status='1' and address=' The Beijing municipal '\G;

3. The leftmost prefix rule

If you index multiple columns , Follow the leftmost prefix rule . It means that the query starts from the top left of the index , And don't skip columns in the index .

Match the leftmost prefix rule , Go to the index ：

Illegal leftmost prefix rule , Index failure ：

If the law of the left is the best , But there was a jump in a column , Only the leftmost column index takes effect ：

4. Range query right column , Index failure .

explain select * from tb_seller where name=' Xiaomi Tech ' and status>'1' and address=' The Beijing municipal '\G;

According to the first two fields name , status Queries are indexed , But the last condition address No index is used .

5. Do not perform operations on columns of the index , Can cause indexes to fail .

explain select * from tb_seller where substring(name,2,3)=' Technology ';

6. String without single quotes , Index failure .

explain select * from tb_seller where name=' Xiaomi Tech ' and status=1 and address=' The Beijing municipal '\G;

because , The query is , No single quotes for Strings ,MySQL The query optimizer for , It will automatically perform type conversion , Cause index invalidation .

7. Try to use overlay index , Avoid using select * 

Overlay index （ The column queried is the index column ）

If the query column , Out of index columns , It also reduces performance .

8.OR If OR The column before the keyword uses the index , The following fields do not use indexes , The index will not be used .

Example ,name Fields are index columns , and createtime It's not an index column , In the middle is or Connection is not indexed ：

explain select * from tb_seller where name=' Black horse programmer ' or createtime = '2088-01-01 12:00:00'\G; 

9. With % Fuzzy query starting with the , The index will fail . If it's just tail blur matching , The index will not fail ; If it's a fuzzy head match , The index will fail .（ You can solve this problem by overriding the index ）

10. If MySQL Evaluating the use of an index is slower than a full table scan , Will abandon the use of indexes , Perform a full table scan .

11.is null  、is not null Sometimes the index fails .

12.in Use index ,not in No index .

13. Single column index and composite index .

Try to use composite indexes , Use less single column indexes

14.<> I can walk the index

EXPLAIN select fullname from t_user where user_id <> 5;

create index idx_name_sta_address on tb_seller(name, status, address);

 It's like creating three indexes  ： 
	name
	name + status
	name + status + address
 Create a single column index  
create index idx_seller_name on tb_seller(name);
create index idx_seller_status on tb_seller(status);
create index idx_seller_address on tb_seller(address);

The database will choose an optimal index （ The most recognizable index ） To use , Not all indexes will be used .

2. Mass insert data optimization

1. If you need to insert many rows of data into a table at the same time , You should try to use more than one value table insert sentence , This way will greatly reduce the connection between the client and the database 、 Turn off consumption, etc . Make the efficiency ratio separate from the single execution insert Fast sentence .

Example , The original way is ：

insert into tb_test values(1,'Tom');

insert into tb_test values(2,'Cat');

insert into tb_test values(3,'Jerry');

The optimized scheme is ：

insert into tb_test values(1,'Tom'),(2,'Cat'),(3,'Jerry');

• Data insertion in transactions .

start transaction;

insert into tb_test values(1,'Tom');

insert into tb_test values(2,'Cat');

insert into tb_test values(3,'Jerry');

commit;

• Data is inserted in order

insert into tb_test values(4,'Tim');

insert into tb_test values(1,'Tom');

insert into tb_test values(3,'Jerry');

insert into tb_test values(5,'Rose');

insert into tb_test values(2,'Cat');

After optimization

insert into tb_test values(1,'Tom');

insert into tb_test values(2,'Cat');

insert into tb_test values(3,'Jerry');

insert into tb_test values(4,'Tim');

insert into tb_test values(5,'Rose');

3.ORDER BY Statements to optimize

CREATE TABLE `emp` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(100) NOT NULL,
  `age` int(3) NOT NULL,
  `salary` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8mb4;

insert into `emp` (`id`, `name`, `age`, `salary`) values('1','Tom','25','2300');
insert into `emp` (`id`, `name`, `age`, `salary`) values('2','Jerry','30','3500');
insert into `emp` (`id`, `name`, `age`, `salary`) values('3','Luci','25','2800');
insert into `emp` (`id`, `name`, `age`, `salary`) values('4','Jay','36','3500');
insert into `emp` (`id`, `name`, `age`, `salary`) values('5','Tom2','21','2200');
insert into `emp` (`id`, `name`, `age`, `salary`) values('6','Jerry2','31','3300');
insert into `emp` (`id`, `name`, `age`, `salary`) values('7','Luci2','26','2700');
insert into `emp` (`id`, `name`, `age`, `salary`) values('8','Jay2','33','3500');
insert into `emp` (`id`, `name`, `age`, `salary`) values('9','Tom3','23','2400');
insert into `emp` (`id`, `name`, `age`, `salary`) values('10','Jerry3','32','3100');
insert into `emp` (`id`, `name`, `age`, `salary`) values('11','Luci3','26','2900');
insert into `emp` (`id`, `name`, `age`, `salary`) values('12','Jay3','37','4500');

create index idx_emp_age_salary on emp(age,salary);

1. Two ways of sorting use filesort（ Low efficiency ）,use index（ Direct return of ordered data through index ）

2. Query overlay index , The use of use index

3. Multi field sorting one up and one down does not use the index （ Unified ascending descending order ）

4. The sorting field order should be consistent with the index field order

5.filesort Optimize

Two scan algorithm

One scan algorithm

1). The first is to sort the returned data , That is to say filesort Sort , All sorts that do not return sorting results directly through index are called FileSort Sort .

2). The second way is to return ordered data directly through ordered index scanning , This is the case using index, No need for extra sorting , High operating efficiency .

Multi field sorting

Try to minimize the extra ordering , Return ordered data directly through index .where Conditions and Order by Use the same index , also Order By In the same order as the index , also Order by The fields of are all in ascending order , Or in descending order . Otherwise, extra operation is necessary , And then there will be FileSort.

4.group by

group by The bottom layer should also be sorted , If you don't want to sort, add order by null;

stay group by Create an index on the field of

create index index_name on table( Name , Name );

5.OR use UNION Instead of OR

To contain OR Query clause for , If you want to use indexes , be OR Each condition column between must use an index , And you can't use composite indexes ; If there is no index , You should consider adding indexes .

1.or The following fields have no index , The index becomes invalid

or Associated conditions , Every field must have an index

2. Composite indexes cannot be used （ Do not walk in accordance with the index ）

3. Use union Instead of or

select * from table where id = 1 nuion select * from table where id = 10;

select * from table where id = 1 or age = 20;

select * from table where id = 1 union select· * from table where age= 20;

6. Paging optimization

General paging query , Better performance can be achieved by creating indexes . A common and very troublesome problem is limit 2000000,10 , At this time need MySQL Before ordering 2000010 Record , Just go back to 2000000 - 2000010 The record of , Other records discarded , The cost of query sorting is very high .

Optimization idea 1

Complete sort paging operation on Index , Finally, according to the primary key Association, return to the original table to query other column contents .

. Inquire about id, take id Sort , In association query

select * from table a, (select id from table order by id limit 200000,10) b where a.id = b.id;

Optimization idea II

This scheme is applicable to tables with self increasing primary key , You can put Limit The query is converted to a query in a certain location

( Tables with self incrementing primary keys , And the primary key cannot be faulted )

select * from table where id > 200000 limit 10;

7. Use SQL Tips

After the table name in the query statement , add to use index To provide hope MySQL To refer to the index list , You can make MySQL No longer consider other available indexes .

create index idx_seller_name on tb_seller(name);

USE INDEX

select * from table use index（index_name）where

IGNORE INDEX

If the user just wants to let MySQL Ignore one or more indexes , You can use ignore index As hint .

explain select * from tb_seller ignore index(idx_seller_name) where name = ' Xiaomi Tech ';

FORCE INDEX

explain select * from tb_seller force index(idx_seller_name) where name = ' Xiaomi Tech ';