Huge speed difference in two similar queries (MySQL ORDER clause) - mysql

Question

Important updade (explanation):
I realized that my query having single DESC order is 10 times slower that the same query with ASC order. The ordered field has an index. Is it normal behavior?
Original question with queries:
I have a mysql table with a few hundred of product items. It's suprising (for me) how 2 similar sql queries differs in terms of performance. I don't know why. Can you please give me a hint or explain why the difference is so huge?
This query takes 3ms:
SELECT
*
FROM
`product_items`
WHERE
(product_items.shop_active = 1)
AND (product_items.active = 1)
AND (product_items.active_category_id is not null)
AND (has_picture is not null)
AND (price_orig is not null)
AND (category_min_discount IS NOT NULL)
AND (product_items.slug is not null)
AND `product_items`.`active_category_id` IN (6797, 5926, 5806, 6852)
ORDER BY
price asc
LIMIT 1
But the following query takes already 169ms... Only difference is that the order clause contains 2 columns. "Price" value has each product, while "price top" has roughly only 1% of products.
SELECT
*
FROM
`product_items`
WHERE
(product_items.shop_active = 1)
AND (product_items.active = 1)
AND (product_items.active_category_id is not null)
AND (has_picture is not null)
AND (price_orig is not null)
AND (category_min_discount IS NOT NULL)
AND (product_items.slug is not null)
AND `product_items`.`active_category_id` IN (6797, 5926, 5806, 6852)
ORDER BY
price asc,
price_top desc
LIMIT 1
The table structure looks like this:
CREATE TABLE `product_items` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`shop_id` int(11) DEFAULT NULL,
`item_id` varchar(255) DEFAULT NULL,
`productname` varchar(255) DEFAULT NULL,
`description` text,
`url` text,
`url_hash` varchar(255) DEFAULT NULL,
`img_url` text,
`price` decimal(10,2) DEFAULT NULL,
`price_orig` decimal(10,2) DEFAULT NULL,
`discount` decimal(10,2) DEFAULT NULL,
`discount_percent` decimal(10,2) DEFAULT NULL,
`manufacturer` varchar(255) DEFAULT NULL,
`delivery_date` varchar(255) DEFAULT NULL,
`categorytext` text,
`created_at` datetime NOT NULL,
`updated_at` datetime NOT NULL,
`active_category_id` int(11) DEFAULT NULL,
`shop_active` int(11) DEFAULT NULL,
`active` int(11) DEFAULT '0',
`price_top` decimal(10,2) NOT NULL DEFAULT '0.00',
`attention_priority` int(11) DEFAULT NULL,
`attention_priority_over` int(11) DEFAULT NULL,
`has_picture` varchar(255) DEFAULT NULL,
`size` varchar(255) DEFAULT NULL,
`category_min_discount` int(11) DEFAULT NULL,
`slug` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `index_product_items_on_url_hash` (`url_hash`),
KEY `index_product_items_on_shop_id` (`shop_id`),
KEY `index_product_items_on_active_category_id` (`active_category_id`),
KEY `index_product_items_on_productname` (`productname`),
KEY `index_product_items_on_price` (`price`),
KEY `index_product_items_on_discount_percent` (`discount_percent`),
KEY `index_product_items_on_price_top` (`price_top`)
) ENGINE=InnoDB AUTO_INCREMENT=1715708 DEFAULT CHARSET=utf8;
UPDATE
I realized that the difference is mainly in the type of ordering: if I use asc+asc for both columns the query takes around 6ms, if I use asc+desc or desc+asc, the query takes around 160ms..
Thank you.

Answer 1

If creating an index to help ORDER BY doesn't help, try creating an index that helps both WHERE and ORDER BY:
CREATE INDEX product_items_i1 ON product_items (
shop_active,
active,
active_category_id,
has_picture,
price_orig,
category_min_discount,
slug,
price,
price_top DESC
)
Obviously, this is a bit clunky, and you'll have to balance the performance gain for the query with the price of maintaining the index.