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 (
    312456, 312443, 312448, 312451, 312444, 
    312455, 312449, 312460, 461271, 461010, 
    461009, 312446, 312447, 461013, 312441, 
    461017, 312452, 312453, 400931
  ) 
  AND cscart_product_descriptions.lang_code = 'ja'

Query time 0.00055

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.03382813,
    "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": 19,
          "cost": 0.03382813,
          "filtered": 100,
          "attached_condition": "cscart_product_descriptions.product_id in (312456,312443,312448,312451,312444,312455,312449,312460,461271,461010,461009,312446,312447,461013,312441,461017,312452,312453,400931) and cscart_product_descriptions.lang_code = 'ja'"
        }
      }
    ]
  }
}