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 (
    1420, 1425, 1427, 1428, 1071, 1068, 1374, 
    1376, 1400, 1066, 1103, 1404, 1380, 
    1401, 1079, 1132
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00063

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": "11",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (1420,1425,1427,1428,1071,1068,1374,1376,1400,1066,1103,1404,1380,1401,1079,1132) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}