Optimalize MySQL query - mysql

Question

I have one query, that is comparatively slow. I tried to rewrite it many times, but I cant find better solution. So I want to ask you, if it is written in wrong way from the beginning or it is ok.
SELECT sql_calc_found_rows
present_id, present_id, present_url, present_name, present_text_short, foto_name, price_id, price_price, price_amount, price_dis
FROM a_present
LEFT JOIN
(SELECT price_id, price_present_id, price_supplier_id, price_dis, price_amount,
(CASE WHEN price_dis <> 0 THEN price_dis ELSE price_amount END) as price_price
FROM a_price
WHERE
price_visibility = 1 AND price_deleted <> 1
GROUP BY price_id ) pri
ON pri.price_present_id = present_id
LEFT JOIN _present_fotos ON foto_id = present_title_foto
LEFT JOIN _cate_pres ON cp_present = present_id
WHERE present_visibility = 1 AND present_deleted <> 1 AND price_price > 0 AND present_out <> 1 AND cp_category IN (30,31,232,32)
GROUP BY present_id
ORDER BY price_price
LIMIT 8
Description: price_dis is price after discount, price_amount is price before discount.. Each product (present) has more prices than one.. Is there faster solution to select final price?
If you will find table structure bad, I will be in trouble:)
Thank you very much!
EDIT:
explain select

Answer 1

OK, so I see a couple of things that could be improved.
First of all, you are JOINing with a table derived from a subquery - with subqueries MySQL does not use indexes (hence the slowdown). Instead of joining with a subquery, try JOINing with a table a_price itself, and put that CASE statement in the original (parent) SELECT. It should allow MySQL to use indexes when JOINing, and it is really important, when your subquery returns many rows.
It should look somewhat like this (including MIN() and GROUP BY, as you need minimum price):
SELECT (...), price_amount, price_dis, MIN((CASE WHEN pri.price_dis <> 0 THEN pri.price_dis ELSE pri.price_amount END)) as price_price
FROM a_present
LEFT JOIN a_price pri
ON pri.price_present_id = present_id AND price_visibility = 1 AND price_deleted <> 1
(...)
GROUP BY present_id
Second of all - as EXPLAIN SELECT suggests - MySQL does not use index on table _cate_pres. You should make it use index to JOIN and to select categories you need (since you put some of them in the IN (..) statement).
Try adding an index on _cate_pres.cp_category, and/or maybe a composite index on this table (using two columns - cp_category and cp_present).
Generally, the result you want to achieve (not always it's possible, but in your case I'm pretty sure it is), is to make the following disappear from EXPLAIN SELECT:
[key] NULL - this means no key is used in this particular set
[Extra] Using temporary - this means a temporary table is created to retrieve results, and it is usually bad for performance
[Extra] Using filesort - this means no index is used for sorting, so sorting process is slow.
Read more about indexes in the Mysql docs, and please give much attention to EXPLAIN output.