#2 is where the choice of Foreign Key comes in (#1 is an aspect of your data model). You can choose to define/enforce the relationship through your own code, OR you can use Foreign Key.

Doing it yourself gives you some benefits like optimization/customization and non-benefits like someone less knowledgeable overriding your code.

Using Foreign Key has the benefit of being automatically enforced/managed by the database. Disadvantage is that it might not be optimized, and can slow down database updates (since each update will be verified by the database).

Conclusion – If your database is constantly updated, maybe it is better to use custom code instead of foreign keys. Otherwise, maybe best to use foreign keys.

I saw your post at StackOverFlow and replied there but no response yet. Anyway, I understand foreign keys may result in lockings and my question is, will this be a “performance” impact for an online game with high reads/writes?

Cheers,
darnpunk

I don’t understand it at all!

Thoughts?

Unless I am the only person who is going to write code against a database, I will not design a database without foreign keys (or some equivalent enforcement).

The reason:
When you design the database (or sections of an existing database) in advance of the application developers use of it, foreign keys give guidance to the developers. Just in case they don’t completely understand the structure (common), foreign keys kick out errors as they do things incorrectly. Then they change what they are doing or they come and ask me questions and I explain it to them.

To develop a system without FKs (or some enforcement), every person who develops against the system has to have a perfect understanding of the relationships – way too much to ask.

Two other notes:
1. ORMs often use the database FK metadata them to create the correct XML that includes relationships.

2. Even if your developers have perfect understanding of the system – what about the next contract that will attach to the database? I see problems without FK constraints.

There’s basically one thing you need to make sure of when not using foreign key’s: the fk id must be a valid id e.g. it must exist in the database and not point to a non-existing record. It’s also very easy to delete an record where a foreign key points to, so you need to check beforehand if there are any foreign keys that point to the record you want to delete, otherwise there will be a lot of inconsistencies.

I now use InnoDB (when using MySQL) or PostgreSQL most of the time and it definitely helps to ease the development.

Cascading updates/deletes are also a nice thing, but you need make sure you understand the whole impact. Deleting a user record might also delete some entries, which might delete some comments, which in turn deletes some other stuff.

One thing I noticed with MyISAM is that it’s a really fast database engine because it’s pretty feature-less.