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 (
    1609, 1610, 1638, 1689, 1693, 1864, 1939, 
    2202, 2361, 2387, 2394, 2396
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00124

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": 12,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (1609,1610,1638,1689,1693,1864,1939,2202,2361,2387,2394,2396) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}