mysql 8 (innodb) foreign key constraints on newly created indexes

Question

Suppose I have a table items
with columns id (PRIMARY), name(VARCHAR), section_id (BIGINT), updated_at (DATETIME) ,

and a table sections with id (PRIMARY) .

Naturally, items.section_id is a foreign key that refers to sections.id .

Suppose there is an index on items of the columns (section_id, name) . I believe that if you tried to drop this index, you would get an error that it is needed in a foreign key constraint . I can accept this.

Now, I want to create a new index, like create index ix_section_id_id_updated_at on items (section_id, id, updated_at) . MySQL lets me do this, but if I go to drop this table, I get that same error: it fails, because it is needed in a foreign key constraint .

Why should this be? It already has one index that can be used for this foreign key check. Further, the error does NOT go away with set FOREIGN_KEY_CHECKS=0; . Is there a way to force MySQL to not associate the new index with the foreign key, so that it is quick to drop? This is necessary because I will be running the migration on a production server with temporary downtime, and need to be able to quickly revert the migration in case of anything going wrong afterwards.

Answer 1

I can reproduce your issue if I don't create an index on section_id and allow mysql to do so on the creation of a foreign key(as described in the manual). Adding a new index drops the auto generated key and if you then drop the new index an error is generated because of the requirement to have a key, and mysql does not auto generate one on a drop.. . If you manually generate a key on section_id this problem does not happen..and the newly created compound index drops successfully.

drop table if exists items;
drop table if exists sections;

create table items(id int PRIMARY key, name varchar(3), section_id BIGINT, updated_at DATETIME);

create table sections(id bigint primary key);

alter table items
    add foreign key fk1(section_id) references sections(id);
    
show create table items;

CREATE TABLE `items` (  `id` int(11) NOT NULL,  
`name` varchar(3) DEFAULT NULL,  
`section_id` bigint(20) DEFAULT NULL, 
 `updated_at` datetime DEFAULT NULL,  
PRIMARY KEY (`id`),  
KEY `fk1` (`section_id`),  
CONSTRAINT `fk1` FOREIGN KEY (`section_id`) REFERENCES `sections` (`id`)) ENGINE=InnoDB DEFAULT CHARSET=latin1;

alter table items
    add key key1(section_id, name);

show create table items;

CREATE TABLE `items` (
  `id` int(11) NOT NULL,
  `name` varchar(3) DEFAULT NULL,
  `section_id` bigint(20) DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `key1` (`section_id`,`name`),
  CONSTRAINT `fk1` FOREIGN KEY (`section_id`) REFERENCES `sections` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

and with manually generated key

drop table if exists items;
drop table if exists sections;

create table items(id int PRIMARY key, name varchar(3), section_id BIGINT, updated_at DATETIME);

create table sections(id bigint primary key);


alter table items
    add key sid(section_id);
alter table items
    add foreign key fk1(section_id) references sections(id);
    
show create table items;

CREATE TABLE `items` (
  `id` int(11) NOT NULL,
  `name` varchar(3) DEFAULT NULL,
  `section_id` bigint(20) DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `sid` (`section_id`),
  CONSTRAINT `fk1` FOREIGN KEY (`section_id`) REFERENCES `sections` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

alter table items
    add key key1(section_id, name);

show create table items;

CREATE TABLE `items` (
  `id` int(11) NOT NULL,
  `name` varchar(3) DEFAULT NULL,
  `section_id` bigint(20) DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `sid` (`section_id`),
  KEY `key1` (`section_id`,`name`),
  CONSTRAINT `fk1` FOREIGN KEY (`section_id`) REFERENCES `sections` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;