Calculate rank for multiple attributes - ruby-on-rails-3

Question

I'm building a motorsport timing app. As car's race around the track, Users input the split times for each Car into the app. The app then ranks each car at each split and displays this in my view.
For example, Car A has a split_1 of 5.00 seconds, split_2 of 15.00 seconds, and split_3 of 25.00 seconds. Car B has a split_1 of 5.50 seconds, split_2 of 14.00 seconds, and split_3 of 23.00 seconds. The ranks displayed in my view for car A would be 1, 2, 2. Car B would be 2, 1, 1.
I wrote an inefficient, database-intensive method to calculate the rank for every single split. Basically the method gets all the split_1 times for every car on the Timesheet and places them into a sorted array, then compares the current Car's split_1 to find its position. It does this for every car, and every lap. It's a nightmare.
class Car < ActiveRecord::Base
has_many :laps
end
class Lap < ActiveRecord::Base
belongs_to :car
belongs_to :timesheet
end
class Timesheet < ActiveRecord::Base
has_many :laps
end
I know I'm missing an easier, more efficient way to calculate each rank. I'm using Postgres 9.2, and with my limited understanding and experience, I think window functions might offer a solution. Is there another way?

Answer 1

With a table like:
CREATE TABLE split_times (car TEXT, split INT, "time" INTERVAL);
INSERT INTO split_times VALUES ('A', 1, '00:00:05');
INSERT INTO split_times VALUES ('A', 2, '00:00:15');
INSERT INTO split_times VALUES ('A', 3, '00:00:25');
INSERT INTO split_times VALUES ('B', 1, '00:00:05.5');
INSERT INTO split_times VALUES ('B', 2, '00:00:14');
INSERT INTO split_times VALUES ('B', 3, '00:00:23');
CREATE INDEX split_times_split_and_time ON split_times (split, "time");
This will get you the split ranks:
SELECT
car, split,
RANK() OVER (PARTITION BY split ORDER BY time) AS rank
FROM
split_times;
You can use this to get the results per car:
WITH x AS (
SELECT
car, split,
RANK() OVER (PARTITION BY split ORDER BY time) AS rank
FROM
split_times ORDER BY split
)
SELECT
car, ARRAY_AGG(rank) AS ranks
FROM
x
GROUP BY car;
SQL Fiddle demo
Window functions documentation