Catalog

Introduce

adjoining course

Service layer

Engine layer

summary

Introduce

We usually write one SQL,MySQL How to receive SQL, perform SQL, And return the query results to the client . How is this set of processes executed , Let's talk about MySQL The logical structure of .

Overview map

   Specific processing flow  

We can see from the picture that ,MySQL The program receives the request from the client through three parts , Connection management 、 Analysis and optimization 、 Storage engine .

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 .

Service layer

Caches & Buffers： Query cache component

     Server If you find this in the query cache SQL sentence , The result will be returned directly to the client ; without , We're in the parser phase . It should be noted that , Because query caching is often inefficient , So in MySQL8.0 After that, I abandoned this function .

        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 .
This query cache can be shared between different clients .
        from MySQL 5.7.20 Start , Query caching is not recommended , And in MySQL 8.0 Delete in .

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 a syntax tree for it , And enrich the query syntax tree according to the data dictionary , Will verify that 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 an execution plan .
        This execution plan indicates which indexes should be used for queries （ 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 .

actuator

Before execution, you need to judge whether the user Have authority . without , A permission error will be returned . If you have permission , Is executed SQL Query and return the result . stay MySQL8.0 The following version , If query cache is set , At this time, the query results will be cached .

Engine layer

Up to now , There's no real access to real data tables .

MySQL The server encapsulates the data storage and extraction operations into a file called Storage engine In the module of . We know surface It consists of line by line records , But it's just a logical concept , How to represent records physically , How to read data from a table , How to write data to specific physical memory , This is all Storage engine Responsible things .

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 , The service layer through API Communicating with the storage engine .

So in MySQL server After query optimization , You only need to call the API, Just get the data and return it to the client .

summary

SQL Statements in MySQL The process in is ： SQL sentence → adjoining course → The query cache → Parser → Optimizer → actuator → Storage engine extracts and stores data .

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