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 (
    15976, 15715, 15716, 15717, 15718, 15719, 
    15720, 15725, 15726, 15727, 15121, 
    15120, 15480, 15138, 15139, 2599, 2745, 
    2691, 2692, 2693, 2694, 2690, 2689, 
    2695, 2696, 13465, 15122, 15425, 15417, 
    15418
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00086

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "21.01"
    },
    "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",
      "index_condition": "((`dev_yourmart_co_bw`.`cscart_product_descriptions`.`product_id` in (15976,15715,15716,15717,15718,15719,15720,15725,15726,15727,15121,15120,15480,15138,15139,2599,2745,2691,2692,2693,2694,2690,2689,2695,2696,13465,15122,15425,15417,15418)) and (`dev_yourmart_co_bw`.`cscart_product_descriptions`.`lang_code` = 'en'))",
      "cost_info": {
        "read_cost": "18.01",
        "eval_cost": "3.00",
        "prefix_cost": "21.01",
        "data_read_per_join": "114K"
      },
      "used_columns": [
        "product_id",
        "lang_code",
        "short_description",
        "full_description"
      ]
    }
  }
}