MySQL - searching a self join and ranges or data - mysql

Question

I'm tasked with my local community center to build a 'newlywed' type game in time for Valentines day, so no rush!
So, We've got 50 odd couples who know each other quite well are going to be asked 100 questions before time. Each question has the users response and a range at which allow a margin of error (this range quota will be limited). and they can then select what they think their partners answer will be, with the same range for margin of error.
EG (I'll play a round as me and my GF):
Question: Do you like fruit?
I am quite fussy about fruit so I'll put a low score out of 100.. say 20. But what I do like, I LOVE and think that my GF might think I will put a higher answer, so my margin of error I'll allow is going to be 30.
I think she loves fruit and will put at least 90.. but she enjoys alot of foods so may just rank it lower, so I'll give her a margin of 20.
Ok, repeat that process for 100 questions and 50 couples.
I'm left with a table like this:
u_a = User answer
u_l = user margin of error level
p_a = partner answer
p_l = partner margin of error level
CREATE TABLE IF NOT EXISTS `large` (
`id_user` int(11) NOT NULL,
`id_q` int(11) NOT NULL,
`u_a` int(11) NOT NULL,
`u_l` int(11) NOT NULL,
`p_a` int(11) NOT NULL,
`p_l` int(11) NOT NULL,
KEY `id_user` (`id_user`,`id_q`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='Stackoverflow Test';
So my row will be in the previous example:
(1, 1, 20, 30, 90, 20)
my mission is to search ALL users to see who the best matches are out of the 50.. (and hope that couples are good together!).
I'll want to search the DB for all users where my answer for my partner matches their answer, but for every user.
Here's what I've got so far (Note I've commented out some code, that's cause I'm trying two ways, not sure what's best):
SELECT
match.id_user,
count(*) as count
from `large` `match`
INNER JOIN `large` `me` ON me.id_q = match.id_q
WHERE
me.id_user = 1 AND
match.id_user != 1 AND
GREATEST(abs(me.p_a - match.u_a), 0) <= me.p_l
AND
GREATEST(abs(match.p_a - me.u_a), 0) <= match.p_l
#match.u_a BETWEEN GREATEST(me.p_a - me.p_l, 0) AND (me.p_a + me.p_l)
#AND
#me.u_a BETWEEN GREATEST(match.p_a - match.p_l, 0) AND (match.p_a + match.p_l)
GROUP BY match.id_user
ORDER BY count DESC
My question today is :
This query takes AGES! I'd like to do it during the game and allow users a chance to change answers on the night and get instant results, so this has to be quick. I'm looking at 40 seconds when looking up all matches for me (user 1).
I'm reading about DB engines and indexing now to make sure I'm doing all that I can... but suggestions are welcome!
Cheers and PHEW!

Answer 1

Your query shouldn't be taking 40 seconds on a smallish data set. The best way to know what is going on is to use explain before the query.
However, I suspect the problem is the condition on me. The MySQL engine might be creating all possible combinations for all users and then filtering you out. You can test this by modifying this code:
from `large` `match` INNER JOIN
`large` `me`
ON me.id_q = match.id_q
WHERE me.id_user = 1 AND
match.id_user != 1 AND . . . .
To:
from `large` `match` INNER JOIN
(select me.*
from `large` `me`
where me.id_user = 1
) me
ON me.id_q = match.id_q
WHERE match.id_user != 1 AND . . . .
In addition, the following indexes might help the query: large(id_user, id_q) and large(id_q).