SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    15700, 15699, 15698, 15697, 15696, 15695, 
    15694, 5958
  ) 
  AND feature_id IN (19842) 
  AND lang_code = 'vi'

Query time 0.00103

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_features_values",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "idx_product_feature_variant_id",
          "key_length": "12",
          "used_key_parts": ["product_id", "feature_id", "lang_code"],
          "rows": 8,
          "filtered": 100,
          "attached_condition": "cscart_product_features_values.feature_id = 19842 and cscart_product_features_values.product_id in (15700,15699,15698,15697,15696,15695,15694,5958) and cscart_product_features_values.lang_code = 'vi'",
          "using_index": true
        }
      }
    ]
  }
}