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SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 1310 
  AND lang_code = 'en'

Query time 0.00056

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	111	Using where

Result

feature_id	variant_id	value_int
42	149
178	6311
30	6124
19	106	1.000
59	5877	4.000
135	5964
24	107
127	5895
128	5899
129	5903
134	5960
135	5963
152	6093
152	6092
152	6091
152	6090
152	6094
152	6089
152	6088
152	6095
152	6096
152	6100
152	6101
152	6105
152	6106
176	6261
135	5968
136	5972
135	5967
136	5976