Optimize JOIN SQL query with additional SELECT - sql

Question

I need a query which will select just one (GROUP BY phi.id_product) image for each product and this image have to be the one with the highest priority (inner SELECT with ORDER BY statement).
The priority is stored in N:M relation table called product_has_image
I've created a query, but it tooks about 3 seconds to execute and I need to optimize it. Here it is:
SELECT p.*, i.id AS imageid
FROM `product` p JOIN `category` c on c.`id` = p.`id_category`
LEFT OUTER JOIN (SELECT id_product, id_image FROM
`product_has_image` ORDER BY priority DESC) phi ON p.id = phi.id_product
LEFT OUTER JOIN `image` i ON phi.id_image = i.id
WHERE (c.`id_parent` = 2 OR c.`id` = 2)
GROUP BY phi.id_product
Indexes which I find to be important in this query are:
image (PRIMARY id)
product_has_image (PRIMARY id_product, id_image; INDEX id_product; INDEX id_image)
product (PRIMARY id, id_category; INDEX id_category)
category (PRIMARY id; INDEX id_parent)
Most of the time takes joining the tables using the SELECT statement which is required for sorting.
Joining with LEFT JOIN [product_has_image] phi ON p.id = phi.id_product is much faster, but doesn't assign the image with the highest priority.
Any help would be appreciated.

Answer 1

Reformatted for sensibility . . .
SELECT p.*, i.id AS imageid
FROM `product` p
INNER JOIN `category` c on (c.`id` = p.`id_category`)
LEFT OUTER JOIN (SELECT id_product, id_image
FROM `product_has_image`
ORDER BY priority DESC) phi
ON (p.id = phi.id_product)
LEFT OUTER JOIN `image` i
ON (phi.id_image = i.id)
WHERE (c.`id_parent` = 2 OR c.`id` = 2)
GROUP BY phi.id_product
Without seeing an execution plan or DDL, I'd guess (shudder) that the problem is likely to be the inner select/sort. If you create a view
create view highest_priority_images as
select id_product, max(priority)
from product_has_image
group by id_product
Then you can replace that inner SELECT...ORDER BY with a SELECT...INNER JOIN on that view. That would reduce the cardinality, so I'd expect it to run faster.
Posting DDL would help.

Answer 2

I would probably try to do it like this:
SELECT p.*, i.id AS imageid
FROM `product` p
INNER JOIN `category` c ON c.id = p.id_category
/* a list of `id_product`s with their highest priorities
from `product_has_image` */
LEFT OUTER JOIN (
SELECT id_product, MAX(priority) AS max_priority
FROM `product_has_image`
GROUP BY id_product
) m ON p.id = m.id_product
/* now joining `product_has_image` again, using
m.`max_priority` for additional filtering */
LEFT OUTER JOIN `product_has_image` phi
ON p.id = phi.id_product AND m.max_priority = phi.priority
/* if you only select `id` from `image`, you can use
phi.`id_image` instead and remove this join */
LEFT OUTER JOIN `image` i ON phi.id_image = i.id
WHERE c.id_parent = 2 OR c.id = 2

Answer 3

Can't test it now, but wouldn't it be possible to do this?
SELECT p.*, i.id AS imageid
FROM `product` p JOIN `category` c on c.`id` = p.`id_category`
LEFT JOIN `product_has_image` phi ON p.id = phi.id_product
LEFT OUTER JOIN `image` i ON phi.id_image = i.id
WHERE (c.`id_parent` = 2 OR c.`id` = 2)
GROUP BY phi.id_product
ORDER BY phi.priority DESC
Do it in a regular join and order by phi.priority.