MySQL: When is Flush Privileges in MySQL really needed? - mysql

Question

When creating new tables and a user to go along with it, I usually just invoke the following commands:
CREATE DATABASE mydb;
GRANT ALL PRIVILEGES ON mydb.* TO myuser#localhost IDENTIFIED BY "mypassword";
I have never ever needed to utilize the FLUSH PRIVILEGES command after issuing the previous two commands. Users can log in and use their database and run PHP scripts which connect to the database just fine. Yet I see this command used in almost every tutorial I look at.
When is the FLUSH PRIVILEGES command really needed and when is it unnecessary?

Answer 1

Privileges assigned through GRANT option do not need FLUSH PRIVILEGES to take effect - MySQL server will notice these changes and reload the grant tables immediately.
From MySQL documentation:
If you modify the grant tables directly using statements such as
INSERT, UPDATE, or DELETE, your changes have no effect on privilege
checking until you either restart the server or tell it to reload the
tables. If you change the grant tables directly but forget to reload
them, your changes have no effect until you restart the server. This
may leave you wondering why your changes seem to make no difference!
To tell the server to reload the grant tables, perform a
flush-privileges operation. This can be done by issuing a FLUSH
PRIVILEGES statement or by executing a mysqladmin flush-privileges or
mysqladmin reload command.
If you modify the grant tables indirectly using account-management
statements such as GRANT, REVOKE, SET PASSWORD, or RENAME USER, the
server notices these changes and loads the grant tables into memory
again immediately.

Answer 2

TL;DR
You should use FLUSH PRIVILEGES; only if you modify the grant tables directly using statements such as INSERT, UPDATE, or DELETE.

Answer 3

Just to give some examples. Let's say you modify the password for an user called 'alex'. You can modify this password in several ways. For instance:
mysql> update* user set password=PASSWORD('test!23') where user='alex';
mysql> flush privileges;
Here you used UPDATE. If you use INSERT, UPDATE or DELETE on grant tables directly you need use FLUSH PRIVILEGES in order to reload the grant tables.
Or you can modify the password like this:
mysql> set password for 'alex'#'localhost'= password('test!24');
Here it's not necesary to use "FLUSH PRIVILEGES;"
If you modify the grant tables indirectly using account-management statements such as GRANT, REVOKE, SET PASSWORD, or RENAME USER, the server notices these changes and loads the grant tables into memory again immediately.

Answer 4

2 points in addition to all other good answers:
1:
what are the Grant Tables?
from dev.mysql.com
The MySQL system database includes several grant tables that contain information about user accounts and the privileges held by them.
clarification: in MySQL, there are some inbuilt databases , one of them is "mysql" , all the tables on "mysql" database have been called as grant tables
2:
note that if you perform:
UPDATE a_grant_table SET password=PASSWORD('1234') WHERE test_col = 'test_val';
and refresh phpMyAdmin , you'll realize that your password has been changed on that table but even now if you perform:
mysql -u someuser -p
your access will be denied by your new password until you perform :
FLUSH PRIVILEGES;