Motivation

This series of blogs is designed to understand the inner working of relational databases. The choice of database would be MySQL since it's something i am currently working with.

How does MySQL work?

What happens if you run a sql query like

SELECT
    *
FROM
    table

The details behind running a SQL query are illustrated in the architecture below.

MySQL follows the client-server architecture and It contains three parts:

client-side
server layer
storage engine layer

Main Components

Client-side

On the client side, each user uses GUI or MySQL shell to talk to the server via TCP/IP. client-side mainly does the following things

	Description	Comment
Authentication	you need to provide a password for authentication although the authentication process is needed	`docker run --name some-mysql -e MYSQL_ROOT_PASSWORD=my-secret-pw -d mysql:tag` This is why you have to pass in `MYSQL_ROOT_PASSWORD` environment variables when you spin up a container.
Security	check user privilege. Same idea as Access Control Lists (ACLs)

To connect to the service, MySQL uses TCP protocol for this. It went through TCP 3-way handshake like this

In your terminal, mysql -u root -p then you are trying to connect to your local MySQL service.

Pool technology is frequently used in programming languages, database and networking. The concept is to create a bunch of free resources in "idling mode". When client side wants something, instead of creating new resources and then destroying them, we just call them to work.

After the 3-way handshake, it goes to connection pool as illustrated in the figure below

When client-side needs something from the server side, if we don't have the connection pooling, the server side has to:

create a process for handling connection
do your thing
destroy the process

connection pool is just like you always see a line of cabs at the airport. They are waiting for their "clients" so you don't have to call a cab at spot. Technology is very similar to the real world.

After you pass in the password, Now, you are connected! Run the following commands to see your connection

show processlist;

You will see your connection as a process in ur server

Now, that's enough details about client-side.

Server layer

Server layer is the brain of the database. After you went through the connector, you are in the parser.

Parser

Parser is a component that built a data structure (called parse tree) of given input . Before parsing lexical analysis is done i.e. input is broken into several tokens. After the data is available in the smaller elements parser performs Syntax Analysis, Semantics Analysis after that parse tree is generated as output.

If your query is

SELECT
    movieTitle
FROM
    StarsIn, MovieStar
WHERE
    starName = name AND birthdate LIKE '%1960';

The parser will turn the SQL query above into a query pass tree as illustrated in the figure below.

Now you should have a parse tree. In the parser, it just parses SQL query and it doesn't care whether it's right or wrong.

Execution (pre-processor, optimizer, executor)

After you have a parse tree, it goes pre-processor, optimizer and executor as illustrated in the figure below

The main functionality of each component is summarized in the figure below.

Component	Description
Pre-processor	Analyzes the parse tree (contains information about query), checks for syntax errors, and prepares the statements for optimization and execution. `check syntax error` error you often see in our console is returned here.
Optimizer	Analyzes the parsed SQL statements and generates an optimized execution plan that is used to retrieve or modify data in the database.
Executor	Executes the optimized execution plan generated by the optimizer, retrieves or modifies data in the database, and returns the results to the client.

Storage Engine Layer

This layer has two components

storage engine: manages the storage of data. For managing data files, indexes and other related structures.
data: actual data, index, log etc.

Summary

Now you should have a better understanding of the overall processes if you run select * from table

connector: client talks to the connector to establish a connection, user authentification.
parser: parse the SQL query into a parse tree for making it easier for subsequent execution plan
execution plan
- pre-processor: check if a table or column exists; if it's valid; expand the select * 's * into all columns.
- optimizer: based on cost consideration, use the minimal cost for retrieving the same data set
- executor: according to the execution plan, talk to the storage engine via API to grab data and return the results to the client.

Try it with Docker!

Let's play around MySQL with Docker

docker container run \                                                                                                                                                                                                    (base) 
                 --name test_mysql \
                 -e MYSQL_ROOT_PASSWORD=123 \
                 -d \
                 mysql

docker container exec <hashcode_for_container> bash

Now you should be inside the container. You can check the version with

mysql --version

# output
# mysql  Ver 8.0.33 for Linux on aarch64 (MySQL Community Server - GPL)

Then you can talk to the server layer

# access server layer as root user and provide password. 
# -h: specify ur IP, if connect to local MYSQL service, ignore this
# -u user, root is the db admin
# -p for password
mysql -u root -p

# enter ur password following prompt

Now it goes through the TCP 3-way handshake as we discussed above. You can try more commands here to play around!

Reference

classic geekforgeek, very general tho kinda hard to understand if you come from another background.

For those who want to dig deep into the parser, please go here and the full course here.

MYSQL, what happens after select ?

select * from table; What i do

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