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 (
    390240, 389494, 389571, 388421, 388520, 
    388384, 415523, 415513, 388742, 435291, 
    389504, 389611, 435297, 388803, 388675, 
    388946, 415704, 415533, 388858, 390293
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00098

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "28.04"
    },
    "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": 20,
      "rows_produced_per_join": 20,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "24.04",
        "eval_cost": "4.00",
        "prefix_cost": "28.04",
        "data_read_per_join": "136K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ],
      "attached_condition": "((`webmarco`.`cscart_product_descriptions`.`product_id` in (390240,389494,389571,388421,388520,388384,415523,415513,388742,435291,389504,389611,435297,388803,388675,388946,415704,415533,388858,390293)) and (`webmarco`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}