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 (
    91877, 91878, 91879, 91880, 91881, 91882, 
    91883, 85555, 85594, 86362, 86363, 
    87993, 88931, 91427, 91429, 91984, 
    85564, 85649, 85650, 85651, 86809, 
    90025, 82382, 82383
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00065

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "12.97"
    },
    "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": "10.57",
        "eval_cost": "2.40",
        "prefix_cost": "12.97",
        "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 (91877,91878,91879,91880,91881,91882,91883,85555,85594,86362,86363,87993,88931,91427,91429,91984,85564,85649,85650,85651,86809,90025,82382,82383)) and (`nuie_scalesta_net`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id short_description full_description
82382 nuie Shower Accessories Square Body Jet - Chrome
82383 nuie Shower Accessories Round Body Jet - Chrome
85555 Easyclean Sprung Plug Bath Wst Min OF
85564 nuie Wastes & Extras Pop Up Bath Waste - Chrome
85594 nuie Wastes & Extras Push Button Bath Waste - Chrome
85649 Hudson Reed Wetroom Screen Support Arm - Polished Chrome
85650 Hudson Reed Wetroom Screen Support Arm - Matt Black
85651 Hudson Reed Wetroom Screen Support Arm - Brushed Brass
86362 nuie Heating Accessories Straight Radiator Valve Pack (Pairs) - Chrome
86363 nuie Heating Accessories Angled Thermostatic Radiator Valve Pack - Chrome
86809 nuie Shower Kits Slide Rail Kit - Chrome
87993 nuie Toilet Seats Square Top Fix Soft Close Toilet Seat - White
88931 nuie Shower Accessories Square Brass Handset - Chrome
90025 nuie Baths Bath Leg Set
91427 nuie Toilet Seats Round Soft Close Sandwich Top Fix Toilet Seat - For use with Freya Toilets - White
91429 nuie Toilet Seats Square Wrapover Soft Close Top Fix Toilet Seat - For use with Ava Toilets - White
91877 BC Designs Victrion Double Robe Hook - Brushed Chrome
91878 BC Designs Victrion Double Robe Hook - Brushed Copper
91879 BC Designs Victrion Double Robe Hook - Brushed Gold
91880 BC Designs Victrion Double Robe Hook - Brushed Nickel
91881 BC Designs Victrion Double Robe Hook - Copper
91882 BC Designs Victrion Double Robe Hook - Gold
91883 BC Designs Victrion Double Robe Hook - Nickel
91984 BC Designs Victrion Straight Wall Mounted Shower Arm - Chrome