SELECT 
  product_id, 
  short_description, 
  full_description 
FROM 
  cscart_product_descriptions 
WHERE 
  product_id IN (18, 16, 21) 
  AND lang_code = 'en'

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_descriptions",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "product_id"],
          "key": "product_id",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "rows": 3,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (18,16,21)",
          "attached_condition": "cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}