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SELECT 
  SQL_CALC_FOUND_ROWS products.product_id, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.product, descr1.product
  ) as product, 
  SUM(
    cscart_also_bought_products.amount
  ) amnt, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.youtube_link, products.youtube_link
  ) AS youtube_link, 
  cscart_aval_statuses_descr.descr as aval_descr 
FROM 
  cscart_products as products 
  LEFT JOIN cscart_product_descriptions as descr1 ON descr1.product_id = products.product_id 
  AND descr1.lang_code = 'en' 
  LEFT JOIN cscart_product_prices as prices ON prices.product_id = products.product_id 
  AND prices.lower_limit = 1 
  INNER JOIN cscart_products_categories as products_categories ON products_categories.product_id = products.product_id 
  INNER JOIN cscart_categories ON cscart_categories.category_id = products_categories.category_id 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
  LEFT JOIN cscart_ult_product_descriptions shared_descr ON shared_descr.product_id = products.product_id 
  AND shared_descr.company_id = 1 
  AND shared_descr.lang_code = 'en' 
  LEFT JOIN cscart_also_bought_products ON cscart_also_bought_products.related_id = products.product_id 
  LEFT JOIN cscart_aval_statuses ON cscart_aval_statuses.aval_status_id = products.aval_status 
  LEFT JOIN cscart_aval_statuses_descr ON cscart_aval_statuses.aval_status_id = cscart_aval_statuses_descr.aval_status_id 
  AND cscart_aval_statuses.status = 'A' 
  AND cscart_aval_statuses_descr.lang_code = 'en' 
WHERE 
  1 
  AND cscart_categories.company_id = 1 
  AND (
    products.usergroup_ids = '' 
    OR FIND_IN_SET(0, products.usergroup_ids) 
    OR FIND_IN_SET(1, products.usergroup_ids)
  ) 
  AND products.status IN ('A') 
  AND prices.usergroup_id IN (0, 0, 1) 
  AND cscart_also_bought_products.product_id = 672 
GROUP BY 
  cscart_also_bought_products.related_id 
ORDER BY 
  SUM(
    cscart_also_bought_products.amount
  ) desc, 
  product_id ASC 
LIMIT 
  0, 3

Query time 0.00640

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_also_bought_products	ref	PRIMARY	PRIMARY	3	const	133	Using where; Using temporary; Using filesort
1	SIMPLE	products	eq_ref	PRIMARY,status	PRIMARY	3	mahm3t_cs443.cscart_also_bought_products.related_id	1	Using where
1	SIMPLE	cscart_aval_statuses	eq_ref	PRIMARY	PRIMARY	4	mahm3t_cs443.products.aval_status	1
1	SIMPLE	cscart_aval_statuses_descr	ref	aval_status_id	aval_status_id	10	mahm3t_cs443.cscart_aval_statuses.aval_status_id,const	1	Using where
1	SIMPLE	prices	ref	usergroup,product_id,lower_limit,usergroup_id	usergroup	3	mahm3t_cs443.cscart_also_bought_products.related_id	1	Using where; Using index
1	SIMPLE	shared_descr	eq_ref	PRIMARY,product_id,company_id	PRIMARY	13	mahm3t_cs443.cscart_also_bought_products.related_id,const,const	1	Using where
1	SIMPLE	descr1	eq_ref	PRIMARY,product_id	PRIMARY	9	mahm3t_cs443.cscart_also_bought_products.related_id,const	1	Using where
1	SIMPLE	products_categories	ref	PRIMARY,pt	pt	3	mahm3t_cs443.cscart_also_bought_products.related_id	3
1	SIMPLE	cscart_categories	eq_ref	PRIMARY,c_status,p_category_id,idx_category_id	PRIMARY	3	mahm3t_cs443.products_categories.category_id	1	Using where

Result

product_id	product	amnt	aval_descr
504	Atlas Uni-Grunt \|fast drying priming emulsion	180	<p style="color: #1abc9c;">Less than 20 available.<br /> Ships within 1-3 business days after receipt of your payment.</p>
658	Vinyl coated glass fibre mesh 165g/m²	59	<p>Ships within 2-4 business days after receipt of your payment</p>
659	PVC corner batten with net 10x10x250 cm	48