SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (7, 15, 11, 278, 282, 280) 
  AND cscart_product_descriptions.lang_code = 'ru'

Query time 0.00236

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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"],
          "rows": 6,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (7,15,11,278,282,280) and cscart_product_descriptions.lang_code = 'ru'"
        }
      }
    ]
  }
}