My MySQL Cookbook

Over the years I’ve been accumulating notes on how to do quick operations in a variety of technologies. These have been, up to now, for my eyes only, and they aim to be the documentation I wish I found online on how to quickly get something done in tech X. Today I decided to share some notes of one of those technologies: MySQL.

Disclaimer: These notes are valid at the moment of writing and not for all versions of MySQL (I am testing 5.7.17 on macOS) as things continuously change.

Disclaimer 2: You are responsible for your data. Read everything written here with a critical mind and only apply it once you tested it. And I can never emphasise this too much: do backups!

What you can learn from this post

First things, first

How to install, config and start MySQL server on macOS

  1. If you are just trying these out and don’t happen to have a MySQL server to connect to you can download it from here, uncompress the DMG file and click next until it’s installed.
  2. Unfortunately the package does not alias the commands we need for direct use on the terminal so add the following to your .bash_profile:
echo "alias mysql=/usr/local/mysql/bin/mysql" >> ~/.bash_profile
echo "alias mysqladmin=/usr/local/mysql/bin/mysqladmin" >> ~/.bash_profile
echo "alias mysqldumpslow=/usr/local/mysql/bin/mysqldumpslow" >> ~/.bash_profile
echo "alias mysqldump=/usr/local/mysql/bin/mysqldump" >> ~/.bash_profile
source ~/.bash_profile
  1. The only configuration we are going to make for now is to set a little file called .my.cnf to have our username and password (which was generated during the installation step). This is obviously not safe for production environments but for a playground it saves us typing the username and password every single time. So do the following: vim ~/.my.cnf and add the following to the file:

For security’s sake please change the permissions on the file so that only you can see it: chmod 0600 ~/.my.cnf.

  1. Now let us start the server. Go to System Preferences and you can see that there is a new icon on the bottom line called MySQL, click there and then on Start MySQL Server (input your password when it asks you)

  2. Go back to the terminal and type mysql and voilá! We’re in!

  3. The password that was generated by the install package is temporary, so I would advise changing it once you’re logged in like this:

    mysql> SET PASSWORD FOR 'root'@'localhost' = PASSWORD('myNewPassword');
    Query OK, 0 rows affected, 1 warning (0.00 sec)

Quick admin commands

# Show all databases
show databases;

# See the SQL for the creation of a table
show create table <table>;

# See all the environment variables
show global status;

# Set the contents of a global variable
set global <VARIABLE_NAME>=<VALUE>;

# Get the contents of all variables starting with 'wait'
show global variables like 'wait%';

# Find all users on this database
select User, Host from mysql.user;

Who am I?

Not to get all philosophical on you but if you at one point forget the user you are logged in as do this to find out:


USER() reports how you attempted to authenticate in MySQL CURRENT_USER() reports how you were allowed to authenticate in MySQL

On queries

How to see running queries and stats

mysqladmin proc stat --verbose
| Id | User | Host      | db | Command | Time | State    | Info                  |
| 10 | root | localhost |    | Query   | 0    | starting | show full processlist |
Uptime: 404  Threads: 1  Questions: 11  Slow queries: 0  Opens: 106  Flush tables: 1  Open tables: 99  Queries per second avg: 0.027

This command allows us to see what’s going on without actually having to log in to the MySQL CLI. You get all the running queries, the uptime of the server, how many of those queries are slow, how many queries are being executed per second, if a query has gone zombie on you and a few others.

As you can see in the Info bit, one of the command being encapsulated by this one is

mysql> show full processlist;

This means that you can do the same things in the MySQL CLI.

How to kill a hanging query

Imagine that, from the list above, you have one query on your list that is taking forever (and by forever I mean a completely subjective number of seconds in the Time column). Then you can simply kill it like you would a process on your Mac.

mysql> show full processlist;
| Id | User | Host      | db   | Command | Time | State      | Info                  |
| 11 | root | localhost | NULL | Query   |    0 | starting   | show full processlist |
| 12 | root | localhost | NULL | Query   |   17 | User sleep | select sleep(100)     |
2 rows in set (0.00 sec)
mysql> kill 12;
Query OK, 0 rows affected (0.00 sec)

How to find slow queries

MySQL not only comes with an opinion on what a slow query” means, but also comes with a nifty little tool to parse the slow query log files and show you a summary called mysqldumpslow.

From the MySQL docs:

The slow query log consists of SQL statements that took more than long_query_time seconds to execute and required at least min_examined_row_limit rows to be examined. The minimum and default values of long_query_time are 0 and 10, respectively.

And a caveat:

By default, administrative statements are not logged, nor are queries that do not use indexes for lookups. This behavior can be changed using log_slow_admin_statements and log_queries_not_using_indexes

By default, slow queries are not logged. So let’s activate them!

# Activate slow query log
mysql> set global slow_query_log='ON';
# max 5 seconds
mysql> set global long_query_time='5';
# These values are valid ONLY until the database server is restarted! As an alternative do it in the config file
# If not already created, create the file
sudo touch /etc/my.cnf
# Change owner to yourself
sudo chown `whoami` /etc/my.cnf
# Add content
echo "[mysqld]" >> /etc/my.cnf
echo "slow_query_log=1" >> /etc/my.cnf
# You can also activate the following, but beware that the logfile may grow fast
echo "log_queries_not_using_indexes=1" >> /etc/my.cnf
echo "log_slow_admin_statements=1" >> /etc/my.cnf

If you want to find out where the log is going to:

mysql> show global variables like 'slow_query_log_file';
| Variable_name       | Value                                        |
| slow_query_log_file | /usr/local/mysql/data/<YOUR_LAPTOP>-slow.log |
1 row in set (0.00 sec)

So now you can test this!

mysql> select sleep(11);
| sleep(11) |
|         0 |
1 row in set (11.01 sec)

And then sudo tail -f /usr/local/mysql/data/<YOUR_LAPTOP>-slow.log

/usr/local/mysql/bin/mysqld, Version: 5.7.17 (MySQL Community Server (GPL)). started with:
Tcp port: 3306  Unix socket: /tmp/mysql.sock
Time                 Id Command    Argument
# Time: 2017-04-09T12:57:07.073262Z
# [email protected]: root[root] @ localhost []  Id:     5
# Query_time: 11.005275  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 0
SET timestamp=1491742627;
select sleep(11);

Now you can use mysqldumpslow to show you a nice summary of the slow queries of your database (once you have a few):

sudo /usr/local/mysql/bin/mysqldumpslow /usr/local/mysql/data/<YOUR_LAPTOP>-slow.log

# Or the top 10 slow queries by count
sudo /usr/local/mysql/bin/mysqldumpslow -t 10 -s c /usr/local/mysql/data/<YOUR_LAPTOP>-slow.log

# Or the top 10 slow queries by time spent
sudo /usr/local/mysql/bin/mysqldumpslow -t 10 -s t /usr/local/mysql/data/<YOUR_LAPTOP>-slow.log

How many queries are running and how many are sleeping connections?

# Running queries
mysqladmin processlist |wc -l 
# Sleeping queries
mysqladmin processlist |grep -i sleep|wc -l 


How to set the timeout for queries

# What is the default duration that a query can run for?
mysql> show global variables like 'wait_timeout';
| Variable_name | Value |
| wait_timeout  | 28800 |
1 row in set (0.01 sec)
# This means 8 hours!
# Let's set it to 10 minutes
mysql> set global wait_timeout=600;
Query OK, 0 rows affected (0.00 sec)

How to set the max connections in MySQL CLI

mysql> show global variables like 'max_connections';
| Variable_name   | Value |
| max_connections | 151   |
1 row in set (0.00 sec)

mysql> set global max_connections=170;
Query OK, 0 rows affected (0.00 sec)

mysql> show global variables like 'max_connections';
| Variable_name   | Value |
| max_connections | 170   |
1 row in set (0.00 sec)

# This value is valid ONLY until the database server is restarted!

How to set the max connections in the configuration file

# If you're on a Mac, you have to create the file and add the max_connections to it
sudo touch /etc/my.cnf
# Change owner to yourself
sudo chown `whoami` /etc/my.cnf
# Add content
echo "[mysqld]" >> /etc/my.cnf
echo "max_connections=505" >> /etc/my.cnf
# Go to System Preferences and restart MySQL
# And query again
mysql> show global variables like 'max_connections';
| Variable_name   | Value |
| max_connections | 505   |
1 row in set (0.00 sec)

How to find the maximum concurrent user connections that happened on this server

mysql> show global status like "%max_used_connections%";

How to configure the Innodb buffer pool

Ideally you want the buffer pool size to be equal to the size of your innodb data files. If you can’t get to that:

On a dedicated database server, you might set the buffer pool size to 80% of the machine’s physical memory size.

You can set the buffer pool size in the my.cnf config file like so:

innodb_buffer_pool_size = 2G

Or dynamically (from MySQL v.5.7.5) on the MySQL CLI like so:

# 2G x 1024 x 1024 x 1024 to get bytes
mysql> set global innodb_buffer_pool_size=2147483648;

How to set the character set in MySQL

# Add to [mysqld] section of /etc/my.cnf:
collation-server = utf8_unicode_ci
init-connect='SET NAMES utf8'

Getting info

How to get the top 10 biggest tables in size

SELECT TABLE_SCHEMA, TABLE_NAME,data_length/1024*1024 FROM `information_schema`.`TABLES` WHERE `TABLE_SCHEMA` !='information_schema' AND `TABLE_SCHEMA` !='mysql' ORDER BY `data_length` DESC LIMIT 10; 

How to get the oldest tables with respect to update times


How to get the data size (in MB) of the different databases

SELECT table_schema "Database Name", sum( data_length + index_length ) / 1024 / 1024 "Data Base Size in MB" FROM information_schema.TABLES GROUP BY table_schema ;


How to get all users in MySQL and their permissions


How to get the permissions for user MYUSER


How to get the permissions for user website’ at IP’

SHOW GRANTS FOR [email protected]''; 

How to give full permission for user myuser’ at localhost on the database magento’

GRANT ALL PRIVILEGES ON magento.* TO 'myuser'@'localhost';

How to revoke permissions

REVOKE SELECT,INSERT,UPDATE,DELETE ON mydatabase.* FROM 'myuser'@'localhost';

How to reset the password for user MYUSER

use mysql;
set password for '<MYUSER>'@'%' = PASSWORD('myPass');
flush privileges;

How to reset the root password

# Add this line to the beginning of /etc/my.cnf ([mysqld] section):

# Create file /var/lib/mysql/mysql.init with the following contents:
SET PASSWORD FOR 'root'@'localhost's=PASSWORD('mynewpassword');

# Change the owner to the local MySQL user
chown <MYSQL_USER> /var/lib/mysql/mysql.init

# Restart the server
# Delete the mysql.init file and remove the init_file line from /etc/my.cnf

How to set user quotas

This is done through the GRANTS that the user has on the database. You can control MAX_CONNECTIONS_PER_HOUR, MAX_QUERIES_PER_HOUR, MAX_UPDATES_PER_HOUR, MAX_USER_CONNECTIONS.

One example:



How to import bulk data faster into MySQL

# The trick here is to disable indexes for the duration of the bulk import, so...
# ...import...

How to export a table to a csv file

mysql <MYDATABASE> -e "SELECT * FROM <MYTABLE>" | sed 's/\t/","/g;s/^/"/;s/$/"/;' > <MYTABLE>.csv

How to get the table DDL (definitions file)

mysqldump -f -d --single-transaction <MYTABLE> > ~/Desktop/createMyTable.sql

How to do backups

# Backup all databases
mysqldump --all-databases > ~/Desktop/fullBackup.sql

# Backup just mydb
mysqldump --databases mydb > ~/Desktop/mydbBackup.sql

# Backup just mytable in mydb
mysqldump mydb mytable > ~/Desktop/mytableBackup.sql

How to log a session in MySQL

mysql> tee mysession.log
Logging to file 'mysession.log'


How to check if a table is healthy, fragmented and how to repair it

mysql> check table <MYTABLE>;
mysql> repair table <MYTABLE>;

I hope this helps!

If you find mistakes with these recipes, feel free to raise an issue here.