Encountered in the work of 99% SQL optimization, this can give you solutions

The previous articles describes the underlying data structure and mysql mysql optimization artifact explain. Some friends said backstage Xiaoqiang only introduce the concept of normal use or look ignorant, urged a little strong to combat sql optimization, two days over the weekend to organize and summarize, sql optimization combat freshly baked, we usually study and work experience 90% of the sql optimization will be introduced to the mind is too long and is divided into three articles of Kazakhstan.

CREATE TABLE `employees` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名',
  `age` int(20) NOT NULL DEFAULT '0' COMMENT '年龄',
  `position` varchar(20) NOT NULL DEFAULT '' COMMENT '职位',
  `hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '入职时间',
  PRIMARY KEY (`id`),
  KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='员工表';

insert into employees(name,age,position,hire_time) values('LiLei', 22, 'manager', NOW())
insert into employees(name,age,position,hire_time) values('HanMeimei', 23, 'dev', NOW())
insert into employees(name,age,position,hire_time) values('Lucy', 23, 'dev', NOW())

Full value match

Index field type is varchar (n): 2-byte string length storage, if it is utf-8, then the length is 3n + 2

EXPLAIN select * from employees where name='LiLei';

EXPLAIN select * from employees where name='LiLei' AND age = 22;

EXPLAIN select * from employees where name='LiLei' AND age = 22 AND position = 'manager';

The most left-prefix rule

If the index is a multi-column, to the most leftmost prefix rule. It refers to a query from the leftmost in the forefront of the beginning and not skip index column in the index. The following two sql According to the most left-prefix rule, will not take the index.

EXPLAIN select * from employees where age = 22 AND position='manager';
EXPLAIN select * from employees where position ='manager';

Failure Index

Do not do anything (computing, functions, type conversion) column on the index will lead to failure while the index turned full table scan.

EXPLAIN select * from employees where name='LiLei';

EXPLAIN select * from employees where left(name, 3)='LiLei';

Hire_time to a general increase in the index:

alter table `employees` ADD INDEX `idx_hire_time`(`hire_time`) USING BTREE;
EXPLAIN select * from employees where date(hire_time) = '2019-08-25';

Restore the original state index

ALTER TABLE `employees` DROP INDEX `idx_hire_time`;

Storage engine can not use the index in the right column of the range conditions

-- EXPLAIN SELECT * FROM employees WHERE name ='LiLei' AND age=22 AND position ='manager';
EXPLAIN SELECT * FROM employees WHERE name ='LiLei' AND age>22 AND position ='manager';

Key_len see this index length is 78, that is, only to the first two fields name and age, postition not to use the index.

Covering index

Try to use a covering index (an index only access to query (the index column contains a query column)), reduce selelct * statement.

EXPLAIN SELECT name,age,position FROM employees WHERE name ='LiLei' AND age=22 AND position ='manager';

Conditional

mysql is not equal in use (! = or <>) can not be used when the index will lead to a full table scan

EXPLAIN SELECT * FROM employees WHERE name !='LiLei' ;

Analyzing null

is null, is not null can not use the index

EXPLAIN SELECT * FROM employees WHERE name is null;

like

Wildcard like to begin ( '$ abc') mysql index table scans failure becomes perfected

EXPLAIN SELECT * FROM employees WHERE name LIKE '%Lei';

String failure without single quotation marks index

EXPLAIN SELECT * FROM employees WHERE name ='1000';
EXPLAIN SELECT * FROM employees WHERE name =1000;

String without single quotation marks, mysql uses cust underlying function to convert it to a string, then indexes failure.

or & in less use

Or less or in, use it query, mysql not necessarily use the index, mysql internal optimizer will use an index based on an overall assessment of whether a number of factors index ratio, table size.

EXPLAIN SELECT * FROM employees WHERE name ='LiLei' or name='HanMeimei';

Range query optimization

Add a single index value for the age

ALTER TABLE `employees`ADD INDEX `idx_age`(`age`) USING BTREE;
EXPLAIN select * from employees where age > 1 and age <= 2000;

There is no reason to go indexes: mysql internal optimizer will use an index based on an overall assessment of whether a number of factors retrieve ratio, table size.
This example does not take the index may be due to a single large amount of data query optimizer chose not lead to taking the index.
Optimization method: a large range may be split into a plurality of small ranges.

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