SELECT 
  * 
FROM 
  `cscart_product_reviews` 
WHERE 
  `product_id` = 26521 
  AND `reply_id` = 0 
  AND `status` = 'A' 
  OR `product_id` = 26521 
  AND `user_id` = 0 
  AND `reply_id` = 0 
  AND `status` = 'D' 
ORDER BY 
  `product_review_id` DESC 
LIMIT 
  0, 30

Query time 0.00048

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.10"
    },
    "ordering_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "cscart_product_reviews",
        "access_type": "ref",
        "possible_keys": [
          "idx_product_id",
          "idx_user_id"
        ],
        "key": "idx_product_id",
        "used_key_parts": [
          "product_id"
        ],
        "key_length": "3",
        "ref": [
          "const"
        ],
        "rows_examined_per_scan": 1,
        "rows_produced_per_join": 0,
        "filtered": "33.33",
        "backward_index_scan": true,
        "cost_info": {
          "read_cost": "1.00",
          "eval_cost": "0.03",
          "prefix_cost": "1.10",
          "data_read_per_join": "560"
        },
        "used_columns": [
          "product_review_id",
          "product_id",
          "user_id",
          "reply_id",
          "name",
          "advantages",
          "disadvantages",
          "comment",
          "rating_value",
          "ip_address",
          "is_buyer",
          "product_review_timestamp",
          "country_code",
          "city",
          "reply_user_id",
          "reply",
          "reply_timestamp",
          "helpfulness",
          "status",
          "storefront_id",
          "images_count",
          "vote_up",
          "vote_down"
        ],
        "attached_condition": "(((`dev2`.`cscart_product_reviews`.`reply_id` = 0) and (`dev2`.`cscart_product_reviews`.`status` = 'A')) or ((`dev2`.`cscart_product_reviews`.`reply_id` = 0) and (`dev2`.`cscart_product_reviews`.`user_id` = 0) and (`dev2`.`cscart_product_reviews`.`status` = 'D')))"
      }
    }
  }
}