MySQL server won't start with foreign_key_checks=0

Question

I'm running Ubuntu with MySQL version 5.5.44

mysqld Ver 5.5.44-0ubuntu0.14.10.1 for debian-linux-gnu on x86_64 ((Ubuntu))

I have a DB with foreign keys. I would like to have the foreign key check disabled at the system level (not at the session).

I saw the system var foreign_key_checks that seems to control this behavior but when I set it in /etc/mysql/my.cnf (by adding foreign_key_checks = 0) the server won't start. The error log looks like:

150729 15:38:18 [Warning] Using unique option prefix myisam-recover instead of myisam-recover-options is deprecated and will be removed in a future release. Please use the full name instead.
150729 15:38:18 [Note] Plugin 'FEDERATED' is disabled.
150729 15:38:18 InnoDB: The InnoDB memory heap is disabled
150729 15:38:18 InnoDB: Mutexes and rw_locks use GCC atomic builtins
150729 15:38:18 InnoDB: Compressed tables use zlib 1.2.8
150729 15:38:18 InnoDB: Using Linux native AIO
150729 15:38:18 InnoDB: Initializing buffer pool, size = 128.0M
150729 15:38:18 InnoDB: Completed initialization of buffer pool
150729 15:38:18 InnoDB: highest supported file format is Barracuda.
150729 15:38:18  InnoDB: Waiting for the background threads to start
150729 15:38:19 InnoDB: 5.5.44 started; log sequence number 56348937
150729 15:38:19 [ERROR] /usr/sbin/mysqld: unknown variable 'foreign_key_checks=0'
150729 15:38:19 [ERROR] Aborting

How can I configure my MySQL server to start with foreign key checks off at the system level?

Edit: behavior is the same with MySQL 5.6.25:

mysqld Ver 5.6.25-0ubuntu0.15.04.1 for debian-linux-gnu on x86_64 ((Ubuntu))

Answer 1

Root Cause

foreign_key_checks is not a global option you can preset at startup. Why ?

When you click on that link to the Documentation on foreign_key_checks, there is no chart that says it is global or session scope. Other options will specify the scope in a chart

• ft_boolean_syntax says Variable Scope Global ; Dynamic Variable Yes
• innodb_doublewrite says Variable Scope Global ; Dynamic Variable No
• max_allowed_packet says Variable Scope Global ; Dynamic Variable Yes
• sort_buffer_size says Variable Scope Global,Session ; Dynamic Variable Yes

With foreign_key_checks, it is session scope only.

Further proof of this is when you go to the command line and run this

mysqld --help --verbose > /tmp/mysqld.txt
vi /tmp/mysqld.txt

When you look inside the text file, you do not see --foreign-key-checks as a command line startup option. The other four variables I showed you can be used on the command line and in my.cnf.

If you have data to load, you can set that before loading:

SET foreign_key_checks = 0;
source mydata.sql

Proof of this is a standard mysqldump header ( first 10 lines )

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;

Please note line 8

/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;

What does the bottom of a mysqldump look like ? Like this:

/*!40101 SET SQL_MODE=@OLD_SQL_MODE */;
/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;
/*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;
/*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */;

Please note the line

/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;

Epilogue

Bottom Line: foreign_key_checks is not meant of use in my.cnf. That make the most sense because (if allowed) starting mysqld with that would be damaging to referential integrity from startup.