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 (
    84716, 84738, 92171, 84726, 84742, 84935, 
    84858, 92175, 84740, 84942, 84934, 
    92167, 84737, 84739, 84725, 87980, 
    87983, 84741, 87984, 87970, 88023, 
    84947, 88032, 87985, 84736, 84688, 
    87979, 84960, 87978, 84850
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00095

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "13.52"
    },
    "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": 30,
      "rows_produced_per_join": 30,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "10.52",
        "eval_cost": "3.00",
        "prefix_cost": "13.52",
        "data_read_per_join": "114K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ],
      "attached_condition": "((`nuie_scalesta_net`.`cscart_product_descriptions`.`product_id` in (84716,84738,92171,84726,84742,84935,84858,92175,84740,84942,84934,92167,84737,84739,84725,87980,87983,84741,87984,87970,88023,84947,88032,87985,84736,84688,87979,84960,87978,84850)) and (`nuie_scalesta_net`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id short_description full_description
84688 nuie Bliss Semi Flush to Wall WC
84716 Hudson Reed Maya Flush to Wall Pan, Cistern & Seat
84725 Old London Richmond Low Level Pan, Cistern & Flush Pipe Kit
84726 Old London Richmond High Level Pan, Cistern & Flush Pipe Kit
84736 Old London Richmond Comfort Height Close Coupled WC & Cistern
84737 Old London Richmond Comfort Height Low Level WC & Flush Pipe
84738 Old London Richmond Comfort Height High Level WC & Flush Pipe
84739 Old London Richmond Mid Level Pan, Cistern and Flush Pipe Kit
84740 Old London Richmond Comfort Height Mid Level Pan, Cistern and Flush Pipe Kit
84741 nuie Carlton Low Level Pan, Cistern & Flush Pipe Kit
84742 nuie Carlton High Level Pan, Cistern & Flush Pipe Kit
84850 nuie Lawton Close Coupled WC
84858 Hudson Reed Luna Flush to Wall Pan, Cistern & Seat
84934 Hudson Reed Arlo Flush to Wall Pan, Cistern & Seat
84935 nuie Ambrose Compact Semi Flush to Wall WC
84942 Hudson Reed Arlo Compact Flush to Wall Pan, Cistern & Seat
84947 nuie Provost Semi Flush to Wall WC
84960 nuie Asselby Close Coupled WC
87970 nuie Freya Comfort Height Rimless Toilet Pan, Cistern & Seat - White
87978 nuie Ava Pan, Cistern & Seat
87979 nuie Ava Flush Rimless Toilet Pan, Cistern & Soft Close Seat - White
87980 nuie Ava Comfort Height Toilet Pan, Cistern & Seat - White
87983 nuie Ava Pan, Cistern & Seat
87984 nuie Freya Pan, Cistern & Seat
87985 nuie Freya Pan, Cistern & Seat
88023 nuie Ivo Comfort Height Pan & Cistern
88032 nuie Doc M Pack Comfort Height Pan & Cistern - White
92167 BC Designs Victrion CC Pan & Cistern Inc Fittings - White
92171 BC Designs Victrion Pan, High Level Cistern & Pipe - White
92175 BC Designs Victrion Pan, Mid Level Cistern & Pipe - White