SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (278, 279, 280, 281, 282, 283, 284) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.01296265,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_descriptions",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "product_id"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["product_id", "lang_code"],
          "loops": 1,
          "rows": 7,
          "cost": 0.01296265,
          "filtered": 100,
          "attached_condition": "cscart_product_descriptions.product_id in (278,279,280,281,282,283,284) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}