Rails / SQL ... better to access database once and store data in array?

Question

The heart of my app is a multi-conditional comparison using an input array and parameters stored in a few database tables.

I'm trying to make the process most efficient ... and I think this could lead to some good conversation about using memory vs. accessing a database.

Here's one example:

1. I have a Merchant, MerchantUserRelation, and User table.
2. Most of the data I need to store in a temporary array is from MerchantUserRelation, BUT at one point, I need to check if it's the user's birthday (user.birthdate.today?).

To me, it seems there are two options:

1. Create a temporary array with only data from UserMerchantRelation and then access the database separately for the user.birthdate.today? method (2 hits to the database), --or--
2. Create a slightly-larger temporary array with both the data from UserMerchantRelation AND User (and thus hit the database only once)

For this example I recognize the differences are EXTREMELY small (read: negligible), but what if the array sizes and # of database accesses required were much larger?

Thank you for any references and/or insights!

Answer 1

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil - Donald Knuth

As you said, the difference is negligible. And there is no absolute solution: memory or db hits. So why care them now? If your app grows and really met bottleneck, you can always profile and refactor to overcome it, by whatever workable methods.