I should also add that addressing performance issues with caching can seriously impact the maintainability of your application, particularly if you end up with complex cache expiry rules and/or systems to pre-populate cache entries. I'd definitely say that the priority should be to first simplify your app as much as possible, then optimise and only when that doesn't work to use caching.
Agreed 100%. I've worked hard to remove all forms of caching from my sites. It simplifies things tremendously.
I've found that database views and functions are one of the easiest ways to improve performance in Rails.
FOR EXAMPLE
To generate user's information shown above each comment for https://img.skitch.com/20120220-pununfygpsaw1cw5gmjin8e95i.p..., I have to get the user's username, the total amount of "points" the user has, the user's profile image, if the user is an admin (admins are formatted differently), etc. This information is stored across around 5 or 6 tables. Enter this view (which calls a few db functions):
CREATE VIEW user_profile_info
AS
SELECT users.id AS user_id,
users.slug AS user_slug,
users.username,
user_stats.total_points,
user_profile_image(users.id) AS profile_image,
is_user_admin(users.id) AS admin
FROM (users
JOIN user_stats
ON (( user_stats.user_id = users.id )));
Now, I can have a simple UserProfileInfo ActiveRecord class that wraps this user_profile_info database view.
Then I can do:
@object.comments.includes(:user_profile_info)
and, very efficiently, I get a list of comments and all the user's information.
If I didn't use this approach, I would have to have a complex caching scheme to avoid the multiple sql queries. The goal is to minimize the amount of data that comes over the wire via sql queries (which also reduces the amount of work ActiveRecord has to do to construct these objects in memory).
BTW, I'm starting to write a book about using postgresql effectively with web applications. I've found that there are tons of web developers (especially in Rails) that don't use the full-power of postgresql correctly which leads to slow and buggy code.
