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 (
    21386, 21390, 21393, 21399, 21389, 21387, 
    21391, 21392, 21401, 21388, 21413, 
    21422, 21397, 21398, 21406, 21394, 
    21439, 21408, 21417, 21414, 21440, 
    21426, 21424, 21411, 21428, 21437, 
    21423, 21419, 21433, 21403, 21420
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00118

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "54.05"
    },
    "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": 31,
      "rows_produced_per_join": 31,
      "filtered": "100.00",
      "index_condition": "((`cscartdb`.`cscart_product_descriptions`.`product_id` in (21386,21390,21393,21399,21389,21387,21391,21392,21401,21388,21413,21422,21397,21398,21406,21394,21439,21408,21417,21414,21440,21426,21424,21411,21428,21437,21423,21419,21433,21403,21420)) and (`cscartdb`.`cscart_product_descriptions`.`lang_code` = 'en'))",
      "cost_info": {
        "read_cost": "50.96",
        "eval_cost": "3.10",
        "prefix_cost": "54.06",
        "data_read_per_join": "118K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ]
    }
  }
}