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 (90, 47, 138, 85, 84, 139) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00041

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": 6,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (90,47,138,85,84,139)",
      "attached_condition": "cscart_product_descriptions.lang_code = 'ja'"
    }
  }
}