I hope some suggestions on SQL optimization can help you who are tortured by SQL like me

6.1 Optimize insert sentence

When it comes to data insert During operation , The following optimization schemes can be considered .

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

  The optimized scheme is ：

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

• 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');

6.2 Optimize order by sentence

6.2.1 Environmental preparation

CREATE TABLE `emp` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(100) NOT NULL,
  `age` int(3) NOT NULL,
  `salary` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

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);

6.2.2 Two ways of sorting

1). The first is to sort the returned data , That is to say filesort Sort

tips

Sorting that does not return the sorting result directly through the index is called FileSort Sort .

explain  select  *  from emp  order by  age  desc;
explain  select  *  from emp  order by  age  asc;

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 .

explain  select  id from emp  order by  age  asc;
explain  select  id,age  from emp  order by  age  asc;
explain  select  id,age,salary  from emp  order by  age  asc;

Multi field sorting

--  Multi field sorting 
explain  select  id,age,salary  from emp  order by  age, salary;
explain  select  id,age,salary  from emp  order by  age desc, salary desc;

explain  select  id,age,salary  from emp  order by  salary desc, age desc;
explain  select  id,age,salary  from emp  order by  age desc, salary asc ;

summary ：

I understand MySQL Sort by , The goal of optimization is clear ：

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.

6.2.3 Filesort Optimization principle

tips

Understanding can

By creating the appropriate index , Can reduce the Filesort Appearance , But in some cases , Conditionality cannot allow Filesort disappear , Then we need to speed up Filesort The sorting operation of . about Filesort , MySQL There are two sort algorithms ：

1） Two scan algorithm ：MySQL4.1 Before , Use this method to sort . First, take out the sorting field and row pointer information according to the conditions , Then in the sorting area sort buffer Middle order , If sort buffer Not enough , On the temporary watch temporary table The sorting result is stored in . After sorting , Then read the record according to the row pointer back to the table , This operation may result in a large number of random I/O operation .

2） One scan algorithm ： Take out all the fields that meet the conditions at once , Then in the sorting area sort buffer After sorting, output the result set directly . When sorting, memory overhead is large , But the efficiency of sorting is higher than that of twice scanning algorithm .

MySQL By comparing system variables max_length_for_sort_data The size and Query The total size of the field taken out by the statement , To determine which sort algorithm to use , If max_length_for_sort_data Bigger , Then use the second optimized algorithm ; Otherwise use the first .

Can be improved properly sort_buffer_size and max_length_for_sort_data System variables , To increase the size of the sorting area , Improve the efficiency of sorting .

show  variables like 'max_length_for_sort_data';
show  variables  like 'sort_buffer_size';

6.3 Optimize group by sentence

tips

Continue to use 6.2 Optimize order by Table created in statement emp

Create index （ preparation ）：

create index idx_emp_age_salary on emp(age,salary); 

because GROUP BY In fact, the sorting operation will also be carried out , And with the ORDER BY comparison ,GROUP BY It's mainly about the grouping operation after sorting .

Of course , If some other aggregate functions are used when grouping , So we need to calculate some aggregate functions . therefore , stay GROUP BY During the implementation of , And ORDER BY You can also use the index .

If the query contains group by But users want to avoid the consumption of sorting results , Then you can execute order by null No sorting . as follows ：

drop index idx_emp_age_salary on emp;
explain select age,count(*) from emp group by age;

After optimization

explain select age,count(*) from emp group by age order by null;

As can be seen from the above example , first SQL The statement needs to be "filesort", And the second one. SQL because order by null There is no need for "filesort", As mentioned above Filesort It's very time consuming .

6.4 Optimize nested queries

tips

Continue to use 4.3 Use explain Analyze the tables created in the execution plan

Mysql4.1 After the version , Start supporting SQL Subquery of . This technology can be used SELECT Statement to create a single column query result , Then use this result as a filter in another query . Using subquery can complete many logical steps at once SQL operation , At the same time, transaction or table lock can be avoided , And it's easy to write . however , In some cases , Subqueries can be joined more efficiently （JOIN） replace .

Example , Find all user information with roles :

explain select * from t_user where id in (select user_id from user_role );

The execution plan is :

After optimization :

explain select * from t_user u , user_role ur where u.id = ur.user_id;

Connect (Join) The reason why queries are more efficient , Because MySQL There is no need to create a temporary table in memory to complete this logically two-step query .

6.5 Use SQL Tips

tips：

Continue to use 5.2.1 Prepare the tables created in the environment

SQL Tips , Is an important means to optimize the database , Simply speaking , Is in the SQL Add some human prompts in the statement to optimize the operation .

6.5.1 USE INDEX

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);

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

6.5.2 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_sta_addr) where name = ' Xiaomi Tech ';

6.5.3 FORCE INDEX

Is mandatory MySQL Use a specific index , It can be used in query force index As hint .

create index idx_seller_address on tb_seller(address);
explain select * from tb_seller force index(idx_seller_address) where address = ' The Beijing municipal ';

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