One of the things we’ve added to our applications in the last few months is a little gem that (among other things) adds a comment to each MySQL query that is generated by one of our applications.

Now, when we look at our Rails or slow query logs, our MySQL queries include the application, controller, and action that generated them:

Account Load (0.3ms)  SELECT `accounts`.* FROM `accounts` 
WHERE `accounts`.`queenbee_id` = 1234567890 

When we’re trying to improve a slow query, or identify a customer problem, we never have to go digging to understand where the query came from—it’s just right there. This comes in handy in development, support, and operations – we used it during a pre-launch review of unindexed queries in the brand new Basecamp which launched a couple months ago. If you combine this with something like pt-query-digest, you end up with a powerful understanding of how each Rails action interacts with MySQL.

It’s easy to add these comments to your Rails application in a relatively unintrusive way. We’ve released our approach that works in both Rails 2.3.x and Rails 3.x.x apps as a gem, marginalia.

marginalia (mar-gi-na-lia, pl. noun) — marginal notes or embellishments

We’ve been using this in production on all of our apps now since December, ranging from Rails 2.3.5 to Rails master and Ruby 1.8.7 to 1.9.3. You should be able to have it running in your application in a matter of minutes.

It’s worth acknowledging that anytime you modify the internals of something outside your direct control there are risks, and that every function call adds some overhead. In our testing, these have both been well worth the tradeoff, but I absolutely encourage you to consider the tradeoff you’re making for yourself every time you instrument or log something. You may certainly have a different set of tradeoffs, and you should absolutely test on your own application.

Have a suggested improvement to our sample code or another way to do this? We’d love to hear it.

Thanks to Taylor for the original idea, and to Nick for helping to extract it into its own gem.