Instrumentation and the cost of Foreign Keys

I occasionally get in to light arguments healthy discussions with students about whether or not to use Foreign Key constraints on InnoDB tables.  My standard response has always been: “it depends on how much of a tradeoff you are willing to make for performance. In some situations the cost can be considerable”.

.. that’s when they expect me to “come up with some real proof” to show them. I do not disagree with their logic or proof being on their list-of-demands.  I support the use of data to make decisions.

The problem is that MySQL has (traditionally) been lacking the instrumentation required to make these decisions easy.  This is getting better  – here is an example we recently added to our InnoDB course:

 CREATE TABLE parent (
 id INT NOT NULL AUTO_INCREMENT,
 bogus_column char(32),
 PRIMARY KEY (id)
) ENGINE=InnoDB;

CREATE TABLE child (
 id INT NOT NULL AUTO_INCREMENT,
 parent_id INT NOT NULL,
 bogus_column char(32),
 PRIMARY KEY (id),
 KEY (parent_id),
 CONSTRAINT child_ibfk_1 FOREIGN KEY (parent_id) REFERENCES parent (id)
) ENGINE=InnoDB;

INSERT INTO parent (bogus_column)
VALUES ('aaa'), ('bbb'), ('ccc'), ('ddd'), ('eee');

INSERT INTO child (parent_id,bogus_column) VALUES
(1, 'aaa'), (2,'bbb'), (3, 'ccc'),
(4, 'ddd'), (5, 'eee');

START TRANSACTION; # session1
START TRANSACTION; # session2

# session1
UPDATE child SET parent_id = 5
 WHERE parent_id = 4;

#session2
UPDATE parent SET bogus_column = 'new!' WHERE id = 4;

#session2
UPDATE parent SET bogus_column = 'new!' WHERE id = 5;

In the last statement, session2 will block waiting on a lock. Want to know where that lock is? Check information_schema.innodb_locks:

mysql> SELECT * FROM information_schema.innodb_locks\G
*************************** 1. row ***************************
    lock_id: 87035:1300:3:6
lock_trx_id: 87035
  lock_mode: X
  lock_type: RECORD
 lock_table: `test`.`parent`
 lock_index: `PRIMARY`
 lock_space: 1300
  lock_page: 3
   lock_rec: 6
  lock_data: 5
*************************** 2. row ***************************
    lock_id: 87034:1300:3:6
lock_trx_id: 87034
  lock_mode: S
  lock_type: RECORD
 lock_table: `test`.`parent`
 lock_index: `PRIMARY`
 lock_space: 1300
  lock_page: 3
   lock_rec: 6
  lock_data: 5
2 rows in set (0.00 sec)

The same example without the foreign key constraints does not block on the last statement.  We also see the expected output change to:

mysql> SELECT * FROM information_schema.innodb_locks;
Empty set (0.00 sec)

This information_schema table is new to InnoDB plugin.  In earlier releases of MySQL you may be able to get the data, but it is not in such an easily digestible form.  Instrumentation is the most under talked about feature in all new releases of MySQL and Percona Server.  See BUG #53336 for more examples of pure awesomeness.

Entry posted by Morgan Tocker | 2 comments

Add to: | | | |