Remember the SQL tuning questions of MySQL asked in the interview

Then add a drumstick 2020-11-07 16:43:52
remember sql tuning questions mysql


Catalog

  • 1、 Scene restoration
  • 2、 A term is used to explain
  • 3、 In practice
  • 4、 Personal view
  • 5、 Extended question

1、 Scene restoration

interviewer : Young man , I heard you would SQL tuning , I have one here SQL, You can help me fine tune .SQL as follows : SELECT * FROM T LIMIT 899999, 10;

The table structure is as follows :

id int(10) primary key,
Other fields ……
I :¿¿¿( Slowly put out the counter question mark ), This article SQL What to do , Random search 10 Data ?
interviewer : Oh , sorry , Forget to add the order SELECT * FROM T ORDER BY id LIMIT 899999, 10;
I : If id If it is continuous, it can be written as follows . SQL as follows :SELECT * FROM T where id >= 899999 LIMIT 0, 10;
interviewer : Well, if id It's not continuous ?
I :( A little thought ) Single from SQL At the optimization level, I can't optimize .
interviewer :…… Let's move on to the next topic ……
I :……
interviewer : So in the end, do you have any questions you want to ask me ?
I : The one just now SQL from SQL On the level of how to optimize ?
interviewer : You could write it like this :SELECT * FROM T t where t.id >= (SELECT f.id FROM T f ORDER BY id DESC LIMIT 899999, 1) LIMIT 0, 10;
I :( Pretend to think a little bit ) Are you sure it's better than the one in figure one SQL:SELECT * FROM T ORDER BY id LIMIT 899999, 10; Faster ? In Figure 1 SQL It is already through the primary key index to query data , You're in picture two SQL It doesn't change the original search method . If you had SQL It's sorted by other fields , Using a non primary key index , for example :SELECT * FROM T ORDER BY createdTime LIMIT 899999, 10; So your second SQL It's better than the first one SQL Much faster , Because in MySQL The difference between the non primary key index and the primary key index in search is , A non primary key index stores the values of index columns and primary key columns , To find the specific value, it needs to go back to the table again , The primary key index stores the entire row of data , No need to... Again Back to the table [^①] lookup , Reduced a search process .
interviewer :……
I :……

2、 A term is used to explain

Back to the table

Refers to searching through a non primary key index ( Nonclustered index ) Find the primary key of the corresponding record , Still need to search again according to the primary key , This is a MySQL The index structure of . stay MySQL in , primary key ( Cluster index ) It stores a whole row of data , Not the primary key index ( Nonclustered index ) It stores the value of the primary key column .

If there is such a table

CREATE TABLE person (
id int(11) NOT NULL,
name varchar(32) NOT NULL,
age int(11) NOT NULL,
PRIMARY KEY (id),
KEY idx_person_age (age)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

The index of this table is shown as follows :

primary key :

Non primary key index :

So when we need to find age=30 When the data is , His search process is as follows :

3、 In practice

Now I have such a watch :
CREATE TABLE file (
id int(11) NOT NULL AUTO_INCREMENT,
name varchar(255) DEFAULT NULL,
created_time datetime NOT NULL,
updated_time datetime NOT NULL,
PRIMARY KEY (id),
KEY idx_file_created_time (created_time) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=954185 DEFAULT CHARSET=utf8mb4;
The number of data in the table is :
Let's take a look at what the interviewer said

It can be seen that , There is no big difference in query time , This is because of these two SQL All of them are primary key indexes , There's no big difference in the way
And what I've listed , Sort by time field , The process of generating back table operations .

It can be seen that , There is a big gap in query time , This is due to the back table operation .

4、 Personal view

This scene happened in my interview last week , I don't know whether the interviewer deliberately left a hole or what kind of situation , But when he gives his answer later , It really surprised me , I hope it's a mistake of the moment , If he is right about SQL It's not familiar with understanding itself , It would be ……………………

5、 Extended question

(1) stay file In the table , If the data column I query is only id、created_time And updated_time, So what else can be done ?
(2) Really only through id When sorting and paging , What should we do ?
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