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 (
    139, 140, 141, 142, 143, 144, 145, 146, 
    147, 148, 149, 150, 151, 152, 153, 154, 
    155, 156, 157, 158, 159, 160, 161, 162
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00071

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id"],
      "key": "product_id",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 24,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162)",
      "attached_condition": "cscart_product_descriptions.lang_code = 'ja'"
    }
  }
}