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 (
    136, 248, 83, 137, 88, 82, 89, 95, 93, 87, 
    96, 94, 90, 47, 138, 85, 84, 139
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00008

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": 18,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (136,248,83,137,88,82,89,95,93,87,96,94,90,47,138,85,84,139) and cscart_product_descriptions.lang_code = 'ja'"
    }
  }
}