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 (
    91617, 91618, 83879, 83881, 83887, 91787, 
    91796, 91797, 91798, 91799, 91800, 
    91801, 91802, 91803, 87870, 83894, 
    91622, 91698, 83885, 83888, 91626, 
    87867, 87868, 87869
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00074

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "10.82"
    },
    "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": 24,
      "rows_produced_per_join": 24,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "8.42",
        "eval_cost": "2.40",
        "prefix_cost": "10.82",
        "data_read_per_join": "91K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ],
      "attached_condition": "((`nuie_scalesta_net`.`cscart_product_descriptions`.`product_id` in (91617,91618,83879,83881,83887,91787,91796,91797,91798,91799,91800,91801,91802,91803,87870,83894,91622,91698,83885,83888,91626,87867,87868,87869)) and (`nuie_scalesta_net`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id short_description full_description
83879 BC Designs Copper Baths Freestanding Boat Bath - Copper Outer/Nickel Inner - 1500mm - Copper/Nickel
83881 BC Designs Copper Baths Freestanding Antique Copper Boat Bath - Nickel Inner - 1500mm - Antique Copper/Nickel
83885 BC Designs Copper Baths Freestanding Boat Bath - Copper Inner/Verdigris Green Outer - 1500mm - Copper/Verdigris Green
83887 BC Designs Copper Baths Freestanding Boat Bath - Nickel Inner - Paint -1500mm - Nickel
83888 BC Designs Copper Baths Freestanding Boat Bath - Copper Inner/Patinata Blue Outer - 1500mm - Copper/Patinata
83894 BC Designs Copper Baths Freestanding Boat Bath - Copper Outer/Copper Inner - 1500mm - Copper
87867 <p>Hudson Reed Grace Round Freestanding Bath & Push Button Waste</p>
87868 Hudson Reed Rose Oval Freestanding Bath & Push Button Waste
87869 Hudson Reed Bella Square Freestanding Bath & Push Button Waste
87870 Old London Farringdon Double Ended Freestanding Bath & Push Button Waste -
91617 BC Designs Senator Curved Square Freestanding Bath 1804mm x 850mm - No Waste - Powder Grey
91618 BC Designs Senator Curved Square Freestanding Bath 1804mm x 850mm - No Waste - Satin Rose
91622 BC Designs Cian Freestanding Slipper 1600mm - Polished White
91626 BC Designs Wivenhoe Freestanding Bath - 1800mm - Polished White
91698 BC Designs Essex Round Freestanding Bath with Leg Stand 1510mm x 760mm - No Waste - Matt White
91787 BC Designs Kurv Bath 1890 x 900 (No Waste) - Marble
91796 BC Designs Portman Bath 1640 x 750 x 500 - Powder Blue
91797 BC Designs Portman Bath 1640 x 750 x 500 - Light Fawn
91798 BC Designs Portman Bath 1640 x 750 x 500 - Gunmetal
91799 BC Designs Portman Bath 1640 x 750 x 500 - Industrial Grey
91800 BC Designs Portman Bath 1640 x 750 x 500 - Khaki Green
91801 BC Designs Portman Bath 1640 x 750 x 500 - Mushroom
91802 BC Designs Portman Bath 1640 x 750 x 500 - Powder Grey
91803 BC Designs Portman Bath 1640 x 750 x 500 - Satin Rose