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SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 10070 
  AND lang_code = 'en'

Query time 0.00039

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	cscart_product_features_values	ref	lang_code,product_id,product_id_idx,product_feature_idx	product_id	3	const	52	Using where

Result

feature_id	variant_id	value	value_int
30	129
42	6256
127	5895
128	5899
129	5902
135	5968
24	107
135	5965
178	6312
176	6263
136	5973
136	5972
136	5977
136	5976