The execution process of a select statement in MySQL

It's on it mysql The architecture of the figure

Divided into Server layer , And storage engine layer

First step , The connector

You will connect to this database first , It's the connector that receives you . The connector is responsible for establishing a connection with the client 、 Access permissions 、 Maintaining and managing connections .

Connect... In the command mysql It's a client tool , Used to establish a connection with the server . In the completion of the classic TCP After shaking hands , The connector is about to start authenticating your identity , This is the user name and password you entered .

Long connection and short connection

If the client does not move for a long time , The connector will automatically disconnect it . This time is determined by the parameter wait_timeout The control of the , The default value is 8 Hours .

If you ask again after eight hours, you will ask for reconnection , But the connection process is usually complicated , So try to use long connections , But long connections can also cause problems .

After all using long connection , 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
1. Regularly disconnect long connections , To use reconnection

2.mysql5.7 And above , It can be done after larger operations . 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 .

The first 2 Step , The query cache

I'll check the cache first , Executed statements and their results may be key-value On the form of , Is directly cached in memory . If it is not in the cache, go to the next step .

But most don't query the cache , Because the advantages outweigh the disadvantages

Query cache failures are very frequent , As long as there is an update to a table , All query caches on this table will be cleared . So it's likely that you're struggling to save the results , It's not in use yet , It's all cleared by an update . For databases that are under pressure to update , 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 .

mysql 8.0 The query cache function above version has been directly deleted

The first 3 Step , analyzer

If the query cache is not hit , It's about time to actually execute the statement .

In the following order ：

1.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”.

2. Judge sql Does the statement satisfy mysql grammar

Step four , 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 .

Step five , actuator

At the beginning of execution , Let's first judge what you do to this watch T Do you have permission to execute the query , without , An error with no permissions is returned , 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 .

For tables with indexes , The execution logic is similar . The first time I call this “ Let's take the first row that satisfies our condition ” This interface , And then we loop “ That satisfies the next row ” This interface , These interfaces are all defined in the engine .