Mysql45 talking about infrastructure: how is an SQL query executed?

Server Layers include connectors 、 The query cache 、 analyzer 、 Optimizer 、 Actuators etc. , cover MySQL Most of the core service functions of , And all the built-in functions （ Such as date 、 Time 、 Mathematics and cryptographic functions ）, All cross-storage engine functionality is implemented in this layer , Like stored procedures 、 trigger 、 View etc. .

The storage engine layer is responsible for data storage and extraction .

One 、server layer

1、 The connector

Verify user name and password , Verify reserved permissions , After a user successfully establishes a connection , Even if you use the administrator account to modify the permissions of this user , It does not affect the permissions of existing connections .

The process of establishing a connection is usually more complicated , So I suggest that you try to minimize the action of establishing connection in use , That is, try to use long connections .

But after using all the long connections , You might notice , Sometimes MySQL Memory usage is rising very fast , This is because MySQL Memory temporarily used during execution is managed in connection objects . These resources will be released when the connection is broken . So if long connections accumulate , May cause too much memory , Killed by the system （OOM）, From a phenomenological point of view MySQL Abnormal restart .

How to solve this problem ？ You can consider the following two options .

a、 Regularly disconnect long connections . Use it for a while , Or it can be judged in the program that a large memory consuming query has been executed , disconnect , Then query and reconnect .

b、 If you're using a MySQL 5.7 Or later , You can do this after each large operation , Through execution mysql_reset_connection To reinitialize the connection resources . This process does not require reconnection and re-authorization , But it restores the connection to the state it was in when it was created .

2、 The query cache

characteristic ： If hit, the result will be returned directly , Otherwise execution sql, Cache results .

But for the most part I would advise you not to use the query cache , Why? ？ Because query caching often does more harm than good .
The hit rate for the query cache will be very low . Unless your business is a static table , It takes a long time to update . such as , A system configuration table , Then the query on this table is suitable for the query cache .

Fortunately MySQL This is also provided “ According to the need to use ” The way . You can set the parameters query_cache_type Set to DEMAND, So for the default SQL Statements do not use the query cache . For the statements that you decide to use the query cache , It can be used SQL_CACHE Explicitly specify , Like the following statement ：

mysql> select SQL_CACHE * from T where ID=10;

MySQL 8.0 Version of the query cache directly removed the entire block .

3、 analyzer

The analyzer will do “ Lexical analysis ”. What you enter is a string with multiple Spaces SQL sentence ,MySQL You need to identify the strings in it , What is the .

MySQL From what you typed "select" This keyword recognizes , This is a query statement . It also takes strings “T” Identify a “ Table name T”, Put the string “ID” Identify a “ Column ID”.

After these identifications , Just do it “ Syntax analysis ”. According to the result of lexical analysis , The parser will follow the grammar rules , Judge the one you typed SQL Does the statement satisfy MySQL grammar .

If the grammar is wrong, an error will be reported .

4、 Optimizer

The optimizer is when there are multiple indexes in a table , Decide which index to use ; Or there are multiple table associations in a statement （join） When , Determine the join order of the tables .

The logical result of the two execution methods is the same , But the efficiency of the execution will be different , The role of the optimizer is to decide which scheme to use .

5、 actuator

Judge the permission before execution , If you have authority , Open the table and continue . When I open my watch , The actuator is defined according to the engine of the table , Use the interface provided by the engine .