SELECT p.product_id
FROM oc_product_to_category p2c
LEFT JOIN oc_product p ON (p2c.product_id = p.product_id)
WHERE p.status = '1'
  AND p.date_available <= NOW()
  AND p2c.category_id = '1564'
ORDER BY p.price ASC
LIMIT 0,
      12
12 rows (0.227 s)

id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	p2c	ref	PRIMARY,category_id	category_id	4	const	26359	100.00	Using temporary; Using filesort
1	SIMPLE	p	eq_ref	PRIMARY	PRIMARY	4	***.p2c.product_id	1	100.00	Using where

SELECT p.product_id
FROM oc_product_to_category p2c
LEFT JOIN oc_product p ON (p2c.product_id = p.product_id)
LEFT JOIN oc_product_description pd ON (p.product_id = pd.product_id)
WHERE p.status = '1'
  AND p.date_available <= NOW()
  AND p2c.category_id = '1564'
ORDER BY          LCASE(pd.name) ASC
LIMIT 0,
      12
12 rows (0.556 s)

id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	p2c	ref	PRIMARY,category_id	category_id	4	const	26359	100.00	Using temporary; Using filesort
1	SIMPLE	p	eq_ref	PRIMARY	PRIMARY	4	***.p2c.product_id	1	100.00	Using where
1	SIMPLE	pd	ref	PRIMARY	PRIMARY	4	***.p2c.product_id	1	100.00	 

  PRIMARY KEY (`product_id`),
  KEY `price` (`price`)

 PRIMARY KEY (`product_id`,`category_id`),
  KEY `category_id` (`category_id`)

 PRIMARY KEY (`product_id`,`language_id`),
  KEY `name` (`name`),
  KEY `language_id` (`language_id`)