Improving MySQL query performance - possible index problems - php

Question

I've seen many questions like mine, but after reading them all I got rather confused.
To sum up - I've got a query that select products from a table and adds more information about them from other tables.
Query:
SELECT
p.product_id,
p.product_name,
p.product_seo_url,
p.product_second_name,
p.product_intro_plain,
p.product_price,
p.product_price_promo,
p.product_promo_expire_date,
p.product_views,
p.product_code,
p.product_exquisite,
p.product_rating,
p.product_votes,
p.product_date_added,
p.product_returned,
p.product_price_returned,
( SELECT gal.image_filelocation
FROM 3w_products_gallery gal
WHERE gal.product_id = p.product_id
ORDER BY show_order ASC
LIMIT 1 ) image_filelocation,
m.man_image_location,
m.man_name,
m.man_seo_url
FROM
3w_products p
LEFT JOIN 3w_manufacturers m
ON p.man_id = m.man_id
LEFT JOIN 3w_products_cat_rel pcr
ON p.product_id = pcr.product_id
WHERE
pcr.ctg_id = '19'
AND p.man_id = '190'
ORDER BY
p.product_id DESC
LIMIT
0, 24
The strange things that happen are that the query sometimes executes for 0.001 sec. and sometimes for 30+ seconds.
There is what EXPLAIN shows:
http://i.stack.imgur.com/ZNtBX.png
I assume the problem lies in the indexes of the tables. Can you tell me how to setup them?
Let me know if you need any more information about the tables or whatever!
Best,
Dimitar

Answer 1

IF your "ID" columns are actually numeric, remove the quotes around them implying strings... even though it would to an implied conversion. If numeric, keep it numeric.
As stated by another in the comments, your LEFT JOIN via the "pcr" alias with its criteria in the WHERE clause turns this into an inner join.
FROM
3w_products p
LEFT JOIN 3w_manufacturers m
ON p.man_id = m.man_id
LEFT JOIN 3w_products_cat_rel pcr
ON p.product_id = pcr.product_id
AND pcr.ctg_id = 19
WHERE
AND p.man_id = 190
Field-level queries can kill performance since the select for each field (your image location) is done once for every record. To at least help this performance, table 3w_products_gallery should have an index ON ( product_id, show_order )
Your main 3w_products table should have an index on (man_id, product_id )... The Man_ID to optimize the WHERE clause by manufacturer, but also the product ID to help optimize the ORDER BY criteria.
Your 3w_manufacturers table, I would suspect already has a valid index on (man_id), since it appears to be the primary key to the table.
Additionally, being web-based content, you might be better to DE-NORMALIZE your product table by adding one new column for "GalleryShowOrder". Then, add a trigger to your Gallery table that any insert or update will then push the first "showOrder" value back to the product table. This way, when you query, you can just add another join to that table on the product and KNOWN show order. If your gallery is returning 1000's of records, even though you are only limiting to 24, it still needs to get all record before the order by is applied. Thus 1000's of subqueries for each of the gallery images.
and your field selection would just become
gal.image_filelocation,
and your JOIN would add the following
LEFT JOIN 3w_products_gallery gal
on p.product_id = gal.product_id
AND p.GalleryShowOrder = gal.show_order