Does introducing foreign keys to MySQL reduce performance

Question

I'm building Ruby on Rails 2.3.5 app. By default, Ruby on Rails doesn't provide foreign key contraints so I have to do it manually. I was wondering if introducing foreign keys reduces query performance on the database side enough to make it not worth doing. Performance in this case is my first priority as I can check for data consistency with code. What is your recommendation in general? do you recommend using foreign keys? and how do you suggest I should measure this?

Answer 1

Assuming:

1. You are already using a storage engine that supports FKs (ie: InnoDB)
2. You already have indexes on the columns involved

Then I would guess that you'll get better performance by having MySQL enforce integrity. Enforcing referential integrity, is, after all, something that database engines are optimized to do. Writing your own code to manage integrity in Ruby is going to be slow in comparison.

If you need to move from MyISAM to InnoDB to get the FK functionality, you need to consider the tradeoffs in performance between the two engines.

If you don't already have indicies, you need to decide if you want them. Generally speaking, if you're doing more reads than writes, you want (need, even) the indicies.

Stacking an FK on top of stuff that is currently indexed should cause less of an overall performance hit than implementing those kinds of checks in your application code.

Answer 2

Generally speaking, more keys (foreign or otherwise) will reduce INSERT/UPDATE performance and increase SELECT performance.

The added benefit of data integrity, is likely just about always worth the small performance decrease that comes with adding your foreign keys. What good is a fast app if the data within it is junk (missing parts or etc)?

Found a similar query here: Does Foreign Key improve query performance?