MySQL 45 talk | 05 explain the index in simple terms (Part 2)

MySQL 45 speak | 05 Index in simple terms （ Next ）

I didn't understand it at first ,B The station looked for two videos and thought it was very good , You can learn from the article .

Overlay index ( Joint index ) Optimize back to table query 、 Leftmost prefix principle
What is? MySQL Index push down ？

Raise questions

Here's the watch T in , If I execute select * from T where k between 3 and 5, You need to perform several tree searches Cable operation , How many lines will be scanned ？

mysql> create table T ( 
    ID int primary key, 
    k int NOT NULL DEFAULT 0, 
    s varchar(16) NOT NULL DEFAULT '', 
    index k(k)) engine=InnoDB;

• InnoDB The index structure of :

• This article SQL The execution process of query statement ：select * from T where k between 3 and 5

  1. stay k Found on index tree k=3 The record of , obtain ID = 300;
  2. Until then ID Index tree found ID=300 Corresponding R3;
  3. stay k Index tree takes next value k=5, obtain ID=500;
  4. Back to ID Index tree found ID=500 Corresponding R4;
  5. stay k Index tree takes next value k=6, Not meeting the conditions , The loop ends .

  In the process , Go back to the process of searching the primary key index tree , We call it Back to the table . You can see , This query process read k Of the index tree 3 Bar record （ step 1、3 and 5）, I went back to my watch twice （ step 2 and 4）.

• Because the data required for query results Only on the primary key index , So I have to go back to my watch . that , Is there any
  Possibly index optimized , Avoid the return process ？

Overlay index

• In this query , Indexes k already “ covers ” Our query needs , We call it Overlay index （ Joint index ）.

• Overlay index Can reduce the number of tree searches , Significantly improve query performance , Is a common means of performance optimization .

problem ： On a citizen information sheet , Is it necessary to identify On a citizen information sheet , Is it necessary to identify
The certificate number and name are established Joint index ？

CREATE TABLE `tuser` (  
    `id` int(11) NOT NULL, 
    `id_card` varchar(32) DEFAULT NULL,  
    `name` varchar(32) DEFAULT NULL, 
    `age` int(11) DEFAULT NULL,  
    `ismale` tinyint(1) DEFAULT NULL,  
    PRIMARY KEY (`id`), 
    KEY `id_card` (`id_card`),  
    KEY `name_age` (`name`,`age`) ) ENGINE=InnoDB

Build another one （ ID number 、 full name ） Joint index of , Is it a waste of space ？

• If there is one now High frequency requests , He should check his name according to the ID number of the citizen. , This Joint index It makes sense . It can use overlay index on this high frequency request , No need to go back to the table Check the whole line of records , Reduce statement execution time .

• Of course , The maintenance of index fields always comes at a cost , A trade-off is needed when building redundant indexes to support coverage indexes .

Leftmost prefix principle

Create a separate one for an infrequent request （ ID number , Address ） The index is a bit wasteful . What should be done ？

• B+ Trees This index structure , Available with index “ Left most prefix ”, To locate records .

• use （name,age） This joint index is used to analyze ：

  • （name,age） Index diagram ：

  

  • You can see , Index entry Is the field that appears in the index definition Order Of .

    1. When your logical need is to find out all the names are “ Zhang San ” When people are , Can quickly locate ID4, Then go back and get all the results you need .
    2. If you're looking up all the names, the first word is “ Zhang ” People who , Yours SQL The condition of the statement is "where name like ‘ Zhang %’". At this time , You can also use this index , The first eligible record found is ID3, And then go back through , Until the conditions are not met .

  • This Left most prefix It can be Joint index Of Leftmost left N A field , It can also be String index Of Leftmost left M Characters .

problem ： When building a federated index , How to arrange When building a federated index , How to arrange the order of the fields in the index .

• The evaluation criteria are , Index reusability .

  • The first principle is , If by adjusting the order , One less index can be maintained , So this order is often the one that needs to be prioritized .

  • There are only b The sentence of , Can't use (a,b) This joint index .（ The leftmost prefix is in order ）

  • The second principle is space .

Index push down

Those parts that don't match the leftmost prefix , What will happen ？

• Or is it based on the joint index of citizen table （name, age） For example . If there is a need now ： Search out the “ First name A word is Zhang , And the age is 10 All the boys at the age of ”.

  that ,SQL This is how the statement is written ：

  mysql> select * from tuser where name like ' Zhang %' and age=10 and ismale=1;
  

  • I already know Prefix index The rules , So this statement is used to search the index tree , Only use “ Zhang ”, Find the first satisfaction Record of conditions ID3. Of course , It's not bad , Better than full scan .

  • Then judge whether other conditions are met .

• stay MySQL 5.6 Before , Only from ID3 Start back to watch one by one . Find the data row on the primary key index , Then compare the field values .

• MySQL 5.6 Introduced Index push down Optimize （index condition pushdown), During index traversal , Opposite rope The fields contained in the citation are judged first , direct Filter Delete records that do not meet the conditions , Reduce the number of times to return to the table .

  • No index push down execution process ：

  

  • Index push down execution process ：

  

• In these two pictures , Each dashed arrow indicates Back to the table once .

  chart 3 in , stay (name,age) I took it out of the index age Value , This process InnoDB Not going to see age Value , Just put... In order “name The first word is ’ Zhang ’” Take out the records one by one and return them to the table . therefore , need Back to the table 4 Time .

  chart 4 Follow chart 3 Is the difference between the ,InnoDB stay (name,age) Inside the index, we judge age Is it equal to 10, For not equal to 10 Of Record , Judge directly and skip . In our case , Only need to ID4、ID5 These two records get data from the table break , Just need Back to the table 2 Time .

Summary

• Overlay index
• Prefix index
• Index push down
• One of the important principles of database design is to access resources as little as possible .