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 (
    16346, 16348, 15749, 16347, 16338, 15753, 
    16340, 15747, 16351, 15750, 15746, 
    16337, 16336, 15745, 15734, 15741
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00099

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.02861176,
    "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": 16,
          "cost": 0.02861176,
          "filtered": 100,
          "attached_condition": "cscart_product_descriptions.product_id in (16346,16348,15749,16347,16338,15753,16340,15747,16351,15750,15746,16337,16336,15745,15734,15741) and cscart_product_descriptions.lang_code = 'en'"
        }
      }
    ]
  }
}