SELECT 
  *, 
  amount AS pure_amount 
FROM 
  cscart_reward_points 
WHERE 
  object_id = 6 
  AND object_type = 'C' 
  AND company_id = 0 
  AND amount > 0 
  AND usergroup_id IN(0, 1) 
ORDER BY 
  amount_type ASC, 
  amount ASC 
LIMIT 
  1

Query time 0.00026

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "cscart_reward_points.amount_type, cscart_reward_points.amount",
        "table": {
          "table_name": "cscart_reward_points",
          "access_type": "range",
          "possible_keys": ["unique_key"],
          "key": "unique_key",
          "key_length": "13",
          "used_key_parts": [
            "object_id",
            "usergroup_id",
            "object_type",
            "company_id"
          ],
          "rows": 2,
          "filtered": 66.66666412,
          "index_condition": "cscart_reward_points.object_id = 6 and cscart_reward_points.company_id = 0 and cscart_reward_points.object_type = 'C' and cscart_reward_points.usergroup_id in (0,1)",
          "attached_condition": "cscart_reward_points.amount > 0"
        }
      }
    }
  }
}