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, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    460402, 438135, 448865, 414574, 434263, 
    439645, 435106, 442622, 452529, 426624, 
    452048, 424261, 436348, 442119, 447602, 
    440833, 448732, 404249, 424146, 417229, 
    424226, 412627, 460205, 436067
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00219

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.04252208,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_descriptions",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "product_id"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["product_id", "lang_code"],
          "loops": 1,
          "rows": 24,
          "cost": 0.04252208,
          "filtered": 100,
          "attached_condition": "cscart_product_descriptions.product_id in (460402,438135,448865,414574,434263,439645,435106,442622,452529,426624,452048,424261,436348,442119,447602,440833,448732,404249,424146,417229,424226,412627,460205,436067) and cscart_product_descriptions.lang_code = 'ja'"
        }
      }
    ]
  }
}