SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    8608, 8609, 8610, 8619, 8620, 8623, 8624, 
    8631, 8632, 8635, 8638, 8639, 8658, 
    8659, 8661, 8667
  ) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00048

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (8608,8609,8610,8619,8620,8623,8624,8631,8632,8635,8638,8639,8658,8659,8661,8667) and cscart_product_descriptions.lang_code = 'ru'"
    }
  }
}