thanks for this important correction. I’ve updated the text. I have to check how I came to that conclusion.

In your sample code above, it is best if you specified ENGINE=InnoDB for those who have MyISAM as default storage engine.

When doing the above with MyISAM, no error is ever produced. Moreover, trying to ALTER tables to InnoDB still works fine, since the foreign key was silently dropped as we were using MyISAM. Will check if there’s an open bug on this.

Regards,
Shlomi

USE t2

after the

create database t2;

But, it works across schemata nonetheless….

“An interesting point, though, is exposed here: trigger-based integrity can work cross-schemata, whereas InnoDB’s foreign keys only work withing a given schema.”

I don’t think this is true.

mysql> use t1
Database changed
mysql> create table t1 (id int primary key);
Query OK, 0 rows affected (0.23 sec)

mysql> create database t2;
Query OK, 1 row affected (0.00 sec)

mysql> create table t2 (id int, foreign key(id) references t1.t1 (id));
Query OK, 0 rows affected (0.11 sec)

mysql> insert into t2 values (1);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`t1`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`id`) REFERENCES `t1` (`id`))

mysql> insert into t1.t1 values (1);
Query OK, 1 row affected (0.13 sec)

mysql> insert into t2 values (1);
Query OK, 1 row affected (0.09 sec)

mysql> delete from t1.t1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`t1`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`id`) REFERENCES `t1` (`id`))

So, it works for me.

Can you please elaborate on this?
Regards

PS, there are more parts to this post

Thanks. You are right. Using triggers on MyISAM (and I really don’t know yet what of Maria and Falcon) does not provide you with real integrity. Using LOCK IN SHARE MODE on InnoDB will probably cost a lot.

Nevertheless I still find the DELETE CASCASE simulation to be helpful: it helps in taking out the garbage (orphaned rows).
Otherwise, and with no real FK, cascading deletes must be performed on the application level, or by periodic pruning.

And given the lack of transactons with MyISAM, is there really a point in trying to enforce referential integrity?

Indeed, very useful stuff. I’ll take note to check up on your scripts and utilities. There’s many of them, and it’s hard to keep up!

Shlomi