What is the mysql.db table used for? Related to my MySQL issue 

I have a user with USAGE grant in mysql.user, but in mysql.db this user has Select, Insert, Update, Delete. Thus the user can successfully query the database.

I can't find any information about how this mysql.db works, is it like some kind of cached permission in there ? Will a mysqld restart flush it ? 

mysql&gt; show grants for user@'xx.xx.xx.%';
+-------------------------------------------------------------------------------------------------------------------------------+ 
| Grants for user@xx.xx.xx.%                                      |
+-------------------------------------------------------------------------------------------------------------------------------+ 
| GRANT USAGE ON *.* TO 'user'@'xx.xx.xx.%' IDENTIFIED BY PASSWORD 'xxx' REQUIRE SSL |



 mysql&gt; select host,db,user, select_priv,update_priv,delete_priv  from
 mysql.db where  user='user'; 
 | host         | db   | user    | select_priv | update_priv |  delete_priv | 
 | xx.xx.xx.1   | myDB | user    | Y           | Y           | Y
From the docs, mysql.db is the table that handles database-specific GRANTS. That is to say, if you explicitly indicated a database in your GRANT command, it would show up in this table:

GRANT SELECT, INSERT ON foo.* TO `bar`@`localhost`;


So the user bar@localhost would have SELECT and INSERT marked as 'Y' in the mysql.db table.

To remove entries from this table, likewise you need to specify the database:

REVOKE SELECT, INSERT ON foo.* FROM `bar`@`localhost`;


Issuing a REVOKE INSERT ON *.* statement (all databases) will not affect this table.

Also, a DROP USER statement will clean up all the entries in the various mysql.* tables that deal with user authentication, such as mysql.db.