Comments on: Foreign key constraints, DB for real

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By: Jay Pipes

Jay Pipes — Mon, 01 May 2006 14:36:19 +0000

Hi Wolfram!

Just to back up what Lukas was saying, performance of the storage engines depend very much on the application and its needs. InnoDB performance may be better for reads than MyISAM in some circumstances (high concurrency, for instance) and MyISAM can be better for writes in some circumstances (logging applications, for instance). The best advice is to build a benchmarking platform and test, test, test Never take anything you hear for granted!

Cheers!

By: Lukas

Lukas — Mon, 17 Apr 2006 16:36:47 +0000

@Maarten: that depends on what you are doing .. for high concurrency between reads and writes InnoDB is faster due to row level locking and MVCC.

For reads only MyISAM is faster. It also does some aggregates faster because it does not have MVCC.

You have to look at what features you need and how you are interacting with your database to choose the proper table engine.

By: Ivo Jansch

Ivo Jansch — Mon, 17 Apr 2006 16:27:00 +0000

Hmm, you’re right, leaving out the type only seems to work in Oracle. I confused things

By: Maarten Manders

Maarten Manders — Mon, 17 Apr 2006 16:17:01 +0000

Isn’t InnnoDB a whole lot slower than MyISAM?

By: Wolfram

Wolfram — Mon, 17 Apr 2006 16:09:31 +0000

Ivo, but your statement doesn’t work! It always raises:

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘NOT NULL,
title varchar(20) NOT NULL,
PRIMARY KEY pk_subchapter (subchapter_id’ at line 3

seems you can’t leave out the type … even though it would make sense … what’s wrong?

By: Wolfram

Wolfram — Mon, 17 Apr 2006 15:50:52 +0000

ah yes, no type at all is even cooler and even more logical thx

By: Ivo Jansch

Ivo Jansch — Mon, 17 Apr 2006 15:43:01 +0000

Some hints: don’t specify the type of a foreign key at all. It will automatically pick the type of the field it references. Also, I find it cleaner to give the constraints a name so they don’t get an autogenerated key name.

The ON UPDATE CASCADE is not really necessarry for numerical keys as you’re not going to change these ever.

Finally, I think it’s not necessary to have both a key and a foreign key on the same field. As a foreign key is a regular key as well.

Your example for subchapter would then become:

CREATE TABLE subchapter (
subchapter_id int(11) unsigned NOT NULL auto_increment,
chapter_id NOT NULL,
title varchar(20) NOT NULL,
PRIMARY KEY pk_subchapter (subchapter_id),,
FOREIGN KEY fk_subchapter_ch (chapter_id) REFERENCES chapter(chapter_id)
ON DELETE CASCADE
) ENGINE=InnoDB;

Note how chapter_id does not have a type here.