MySQL 45 lecture notes (I) execution of an SQL statement

One .Mysql Basic framework

SELECT
	* 
FROM
	T 
WHERE
	ID = 10;

MySQL Can be divided into Server Layer and storage engine layer

1. server Layers include The connector 、 The query cache 、 analyzer 、 actuator 、 Built in functions （ date , Time , Digital and encryption functions, etc ）, All the functions across storage engines are implemented in this layer , such as stored procedure , trigger , View etc. .
2. Storage engine layer be responsible for Data storage and extraction ： Its architecture pattern is plug-in , Support InnoDB,MyISAM,Memory Wait for multiple storage engines
3. The most commonly used engine is InnoDB, from MySQL5.5.5 edition Start default storage engine .

Two . The connector

The connector is responsible for communicating with the client Establishing a connection , Access permissions , Maintenance and management Connect ：

mysql -h$ip -P$port -u$user -p

• User name password authentication passed , The connector will find out the permissions you have in the permission table
• After the user successfully establishes the connection , You use the administrator account to modify the permissions of this user , It will not affect the permissions of existing connections
• After modifying user permissions , You need to create a new connection to use the new permission settings

Long connection to database

• A long connection After the connection is successful , If the client continues to have requests , Always use The same connection （ recommend ）
• Short connection It means to disconnect after a few queries , Create a new one for the next query

Long connection disadvantages

• Too much memory ：Mysql The memory used temporarily during execution is managed in the connection object , These resources will not be released until the connection is disconnected , So long connections accumulate , This leads to too much memory , Forced by the system （OOM）

The solution of using too much memory for long connections

• regular Disconnect the long connection
• perform mysql_reset_connection Reinitialize connection resources

3、 ... and . The query cache

• MYSQL After getting a query request , Meeting Query cache first have a look , Was it before Executed this statement
• The query information will be displayed as key-value On the form of , direct Cache in memory in .
• key Is a query statement ,value Is the query result
• Statement is not in cache , Will continue Subsequent execution writes the results back to the query cache in

Query caching often does more harm than good

• The query cache fails frequently , Any update, modification, deletion , Metropolis Invalidate the query cache
• MySQL8.0 There will be no query cache after version

Four . analyzer

• Lexical analysis
• Syntax analysis

5、 ... and . Optimizer

• The optimizer is when there are multiple indexes in a table , Decide which index to use
• A statement has multiple table associations （join） To determine the connection order of each table

SELECT
   t1.* 
FROM
   t1
   LEFT JOIN t2 ON t2.d = 20 
WHERE
   t1.c = 10

The optimizer will determine whether the index can be moved , Choose the fastest execution method

6、 ... and . actuator

MySQL adopt The analyzer knows what you're going to do , adopt Optimizer know How do you do it? , Enter the actuator stage , Start Execute statement

Execution steps
1. Authority verification - Whether you have permission to query, modify or delete this table
2. Call the storage engine to execute