SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.short_description, 
  IF(
    cscart_product_descriptions.short_description = '' 
    OR cscart_product_descriptions.short_description IS NULL, 
    cscart_product_descriptions.full_description, 
    ''
  ) AS full_description, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    309, 308, 307, 306, 305, 304, 303, 302, 
    300, 298, 297, 296
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00055

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": 12,
          "filtered": 100,
          "index_condition": "cscart_product_descriptions.product_id in (309,308,307,306,305,304,303,302,300,298,297,296)",
          "attached_condition": "cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}