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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 (
    86924, 86925, 86926, 86981, 86982, 86983, 
    86984, 86985, 86986, 86987, 87042, 
    87043
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00056

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "6.73"
    },
    "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": 12,
      "rows_produced_per_join": 12,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "5.53",
        "eval_cost": "1.20",
        "prefix_cost": "6.73",
        "data_read_per_join": "45K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ],
      "attached_condition": "((`nuie_scalesta_net`.`cscart_product_descriptions`.`product_id` in (86924,86925,86926,86981,86982,86983,86984,86985,86986,86987,87042,87043)) and (`nuie_scalesta_net`.`cscart_product_descriptions`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id	short_description	full_description
86924	Old London Floor Standing 2 Door Vanity Unit with 3 Tap Hole Black Marble Top 800mm - Storm Grey
86925	Old London Floor Standing 2 Door Vanity Unit with 3 Tap Hole White Marble Top 800mm - Storm Grey
86926	Old London Floor Standing 2 Door Vanity Unit with 3 Tap Hole Grey Marble Top 800mm - Storm Grey
86981	Old London Floor Standing 2 Door Vanity Unit with 1 Tap Hole Black Marble Top 800mm - Twilight Blue
86982	Old London Floor Standing 2 Door Vanity Unit with 1 Tap Hole White Marble Top 800mm - Twilight Blue
86983	Old London Old London Twilight Blue Floor Standing 2 Door Vanity Basin Unit with Square Basin & 1 Tap Hole Marble Top, 800mm - Twilight Blue
86984	Old London Floor Standing 2 Door Vanity Unit with 1 Tap Hole Grey Marble Top 800mm - Twilight Blue
86985	Old London Floor Standing 2 Door Vanity Unit with 3 Tap Hole Black Marble Top 800mm - Twilight Blue
86986	Old London Floor Standing 2 Door Vanity Unit with 3 Tap Hole White Marble Top 800mm - Twilight Blue
86987	Old London Floor Standing 2 Door Vanity Unit with 3 Tap Hole Grey Marble Top 800mm - Twilight Blue
87042	Old London Floor Standing 2 Door Vanity Unit with 1 Tap Hole Black Marble Top 800mm - Timeless Sand
87043	Old London Floor Standing 2 Door Vanity Unit with 1 Tap Hole White Marble Top 800mm - Timeless Sand