NDB Cluster seems to ignore foreign_key_checks=OFF

Question

According to MySQL documentation on the local foreign_key_checks variable

Beginning with MySQL Cluster NDB 7.3.2, setting this variable has the same effect on NDB tables as it does for InnoDB tables—previously, the setting was ignored and all such checks were enforced (Bug #14095855).

I should be able to import a unordered InnoDB dumpfile with foreign keys.

My NDB version

# ndbd --version
MySQL distrib mysql-5.6.21 ndb-7.3.7, for Linux (x86_64)

But when I am trying to add a structure-only dump beginning with

SET foreign_key_checks=OFF;

--
-- Table structure for table `aclAktion`
--

DROP TABLE IF EXISTS `aclAktion`;
CREATE TABLE `aclAktion` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(64) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT '',
  `editMask` set('read','write','create','delete','super') CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT 'read,write,create,delete',
  PRIMARY KEY (`id`),
  UNIQUE KEY `aclAktion_uIdx_name` (`name`)
) ENGINE=NDBCLUSTER ROW_FORMAT=DYNAMIC;

--
-- Table structure for table `aclGroup`
--

DROP TABLE IF EXISTS `aclGroup`;

SHOW VARIABLES LIKE 'foreign_key_checks';
CREATE TABLE `aclGroup` (  -- this is line #63 in the dumpfile
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `aclRole_id` enum('gast','kunde','owner','owner_boss','red_ext','red_ext_boss','red_int','red_int_boss','verlag_boss','sysop','root') CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT 'gast',
  `numUsers` int(11) NOT NULL DEFAULT '0',
  `name` varchar(64) NOT NULL DEFAULT '',
  `aktiv` tinyint(1) NOT NULL DEFAULT '0',
  `login` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  UNIQUE KEY `aclGroup_uIdx_name` (`name`),
  KEY `aclGroup_idx_aclRole_id` (`aclRole_id`),
  CONSTRAINT `aclGroup_fk_aclRole_id` FOREIGN KEY (`aclRole_id`) REFERENCES `aclRole` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=NDBCLUSTER ROW_FORMAT=DYNAMIC;

--
-- Table structure for table `aclRole`
--

DROP TABLE IF EXISTS `aclRole`;
CREATE TABLE `aclRole` (
  `id` enum('gast','kunde','owner','owner_boss','red_ext','red_ext_boss','red_int','red_int_boss','verlag_boss','sysop','root') CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT 'gast',
  `numGroups` int(11) NOT NULL DEFAULT '0',
  `login` tinyint(1) NOT NULL DEFAULT '0',
  `checkNumLogins` tinyint(1) NOT NULL DEFAULT '0',
  `defaultPrivileges` set('read','write','create','delete','super') CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT 'read',
  `beschreibung` varchar(192) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=NDBCLUSTER ROW_FORMAT=DYNAMIC;

I get an error:

ERROR 1215 (HY000) at line 63: Cannot add foreign key constraint

The error log on the API node shows that NDB actually tries to handle the foreign key correctly

2014-11-03 17:50:05 [NdbApi] INFO     -- Flushing incomplete GCI:s < 10865/8
2014-11-03 17:50:05 [NdbApi] INFO     -- Flushing incomplete GCI:s < 10865/8
2014-11-03 17:50:05 22485 [Note] NDB Binlog: starting log at epoch 10865/8
2014-11-03 17:50:05 22485 [Note] NDB Binlog: ndb tables writable
2014-11-03 17:50:05 22485 [Note] NDB Binlog: Node: 2, subscribe from node 4, Subscriber bitmask 010
2014-11-03 17:50:05 22485 [Note] NDB Binlog: Node: 3, subscribe from node 4, Subscriber bitmask 010
2014-11-03 17:50:08 22485 [Note] NDB Binlog: Node: 2, unsubscribe from node 4, Subscriber bitmask 00
2014-11-03 17:50:08 22485 [Note] NDB Binlog: Node: 3, unsubscribe from node 4, Subscriber bitmask 00
2014-11-03 17:50:16 22485 [Note] NDB Binlog: Node: 2, subscribe from node 4, Subscriber bitmask 010
2014-11-03 17:50:16 22485 [Note] NDB Binlog: Node: 3, subscribe from node 4, Subscriber bitmask 010
2014-11-03 18:01:29 22485 [Note] NDB Binlog: CREATE TABLE Event: REPL$database/aclAktion
2014-11-03 18:01:29 22485 [Note] NDB Binlog: logging ./database/aclAktion (UPDATED,USE_WRITE)
2014-11-03 18:01:29 22485 [Note] NDB FK: Created mock table 'NDB$FKM_20_0_aclRole' referenced by 'aclGroup'

No errors on the ndb node

It seems, the variable foreign_key_checks = off is ignored here. Any hints?

Answer 1

Mock tables were how mysqldump were loaded in older versions of MySQL a long time ago

Issue like this existed

• load of mysqldump script may fail with error 1215 if dest already has tables
• Foreign keys: mysqldump should respect foreign keys

See that last line

2014-11-03 18:01:29 22485 [Note] NDB FK: Created mock table 'NDB$FKM_20_0_aclRole' referenced by 'aclGroup'

Since mysqldumps were in alphabetical order by default, there might be some cases where the parent table of a foreign key relationship would appear after the child table rather than before. As a consequence, the mock table was created to have a parent to adopt the aclGroup table.

Since a child cannot have more than one foreign key reference to the same parent on the same columns, then NDB is ignoring it and still trying to enforce foreign_key_checks=OFF

Under the hood, this is what the NDB storage engine should be doing afterCREATE TABLE aclGroup

ALTER TABLE aclGroup DROP CONSTRAINT aclGroup_fk_aclRole_id;
DROP TABLE `NDB$FKM_20_0_aclRole`;
DROP TABLE IF NOT EXISTS aclRole;
CREATE TABLE aclRole (...) ENGINE=NDBCLUSTER;
ALTER TABLE aclGroup ADD CONSTRAINT aclGroup_fk_aclRole_id
FOREIGN KEY (`aclRole_id`) REFERENCES `aclRole` (`id`) ON DELETE CASCADE ON UPDATE CASCADE;    

If you look at Dump of foreign key constraints with mysqldump not sorted, you will note that this was a feature request for InnoDB long ago. This same feature is probably not properly implemented in NDB. I am sure if you change the NDBCLUSTER to InnoDB in your script and run it, it will work without incident.

So, YES, foreign_key_checks=OFF is being ignored.