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SELECT 
  product_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  group_id IN (13)

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_product_variation_group_products	ref	idx_group_id	idx_group_id	3	const	19