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 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (339445, 339446) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.81"
    },
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "product_id"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "product_id",
        "lang_code"
      ],
      "key_length": "9",
      "rows_examined_per_scan": 2,
      "rows_produced_per_join": 2,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "2.41",
        "eval_cost": "0.40",
        "prefix_cost": "2.81",
        "data_read_per_join": "13K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ],
      "attached_condition": "((`webmarco`.`cscart_product_descriptions`.`product_id` in (339445,339446)) and (`webmarco`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id short_description full_description
339445 <h4>New formulation 100% synthetic latest technology direct flow oil for gasoline engines. Its advanced composition ensures top protection against mechanical damage even in the most modern high performance engines for long change intervals, low oil consumption and fuel economy. Suitable for multi-valve catalytic and conventional gasoline engines with or without turbo.</h4>
339446 <h4>New formulation 100% synthetic latest technology direct flow oil for gasoline engines. Its advanced composition ensures top protection against mechanical damage even in the most modern high performance engines for long change intervals, low oil consumption and fuel economy. Suitable for multi-valve catalytic and conventional gasoline engines with or without turbo.</h4>