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 (
    129, 127, 131, 124, 188, 115, 122, 232, 
    231, 230, 187, 170, 186, 177, 185, 179, 
    181, 236, 235, 237, 233, 234, 152, 160
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00055

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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"],
      "rows": 24,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (129,127,131,124,188,115,122,232,231,230,187,170,186,177,185,179,181,236,235,237,233,234,152,160) and cscart_product_descriptions.lang_code = 'ja'"
    }
  }
}