The underlying logical architecture of MySQL

---

1、 The server handles client requests

What does the server process do with the request sent by the client process , To produce the final processing result ？ Here, take the query request as an example to show ：

Let's take a look at

first floor ： adjoining course

System （ client ） visit MySQL In front of the server , The first thing to do is to establish TCP Connect .
After three handshakes, the connection is established successfully , MySQL Server pair TCP The transmitted account and password are used for identity authentication 、 Access .

• Wrong user name or password , I'll get one Access denied for user error , The client program completes execution
• User name password authentication passed , From the permission table, you will find out the permission of the account and the connection Association , The following permission judgment logic , Will depend on the permissions read at this time

TCP After the connection receives the request , Must be assigned to a thread dedicated to the interaction with this client . So there will be a thread pool , Go to the later process . Each connection gets threads from the thread pool , Eliminates the overhead of creating and destroying threads .

The second floor ： Service layer

SQL Interface: SQL Interface

• Receive the user's SQL command , And returns the result of the query that the user needs . such as SELECT … FROM It's called SQL Interface
• MySQL Support DML（ Data operation language ）、DDL（ Data definition language ）、 stored procedure 、 View 、 trigger 、 User defined functions, etc SQL Language interface

Parser: Parser

• In the parser SQL Statement syntax analysis 、 Semantic analysis . take SQL Statements are decomposed into data structures , And pass the structure to the next step , in the future SQL Statements are passed and processed based on this structure . If you encounter an error in the decomposition composition , So that's it SQL The statement is unreasonable .
• stay SQL Commands are validated and parsed by the parser when they are passed to the parser , And create for it Grammar tree , And enrich the query syntax tree according to the data dictionary , Meeting Verify whether the client has permission to execute the query . After creating the syntax tree ,MySQL Will also be right SQl Query syntax optimization , Query rewriting .

Optimizer: Query optimizer

• SQL Statement after syntax parsing 、 Before the query, the query optimizer will be used to determine SQL Statement execution path , Generate a Implementation plan
• The implementation plan indicates that Which index to use The query （ Full table search or index search ）, What is the connection order between tables , Finally, the method provided by the storage engine will be called according to the steps in the execution plan to truly execute the query , And return the query results to the user .
• It USES “ selection - Projection - Connect ” Strategy to query . for example

SELECT id,name FROM student WHERE gender = ' Woman ';

This SELECT The query is based on WHERE The sentence goes on selection , Instead of querying all the tables and then gender Filter . This SELECT The query is based on id and name Make attributes Projection , Instead of taking all the attributes out and filtering them later , These two query criteria Connect Get up and generate the final query results .

Caches & Buffers： Query cache component

• MySQL There's some internal maintenance Cache and Buffer, such as Query Cache Used to cache a SELECT Statement execution result , If the corresponding query result can be found in it , Then there is no need to query and parse 、 Optimization and execution of the whole process , Directly feed back the results to the client .
• The caching mechanism consists of a series of small caches . For example, table caching , Record the cache ,key cache , Authority cache, etc .
• The query can be cached in Sharing between different clients .
• from MySQL 5.7.20 Start , Query caching is not recommended , And in MySQL 8.0 Delete in , Because this is chicken ribs

Little story ：
If I ask you 9+8×16-3×2×17 What's the value of , You may use a calculator to calculate , final result 35. If I ask you again 9+8×16-
3×2×17 What's the value of , Do you still use another silly calculation ？ We have just calculated , Just say the answer directly .

The third level ： Engine layer

Plug in storage engine layer （ Storage Engines）, Really responsible MySQL The storage and extraction of data in , Perform operations on the underlying data maintained at the physical server level , Server pass API Communicating with the storage engine . Different storage engines have different functions , So we can choose according to our actual needs .

MySQL 8.0.25 The storage engines supported by default are as follows ：

Storage layer

All data , database 、 The definition of the table , The contents of each row of the table , Indexes , All exist On the file system , In the form of a file , And complete the interaction with the storage engine . Of course, some storage engines, such as InnoDB, It also supports direct management of bare devices without using the file system , But the implementation of modern file systems makes this unnecessary . Under the file system , You can use a local disk , have access to DAS、NAS、SAN And other storage systems .

---

summary

Let's get familiar with SQL The execution process is convenient , We can simplify it as follows ：

Simplified to a three-tier structure ：

1. adjoining course ： Establish connection between client and server , The client sends SQL To the server side ;
2. SQL layer （ Service layer ）： Yes SQL Statement for query processing ; It has nothing to do with the storage mode of database files ;
3. Storage engine layer ： Dealing with database files , Responsible for data storage and reading .