SELECT 
  `product_review_id` 
FROM 
  `cscart_product_reviews` 
WHERE 
  `product_id` = 26421 
  AND `user_id` = 0 
LIMIT 
  1

Query time 0.00074

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.60"
    },
    "table": {
      "table_name": "cscart_product_reviews",
      "access_type": "index_merge",
      "possible_keys": [
        "idx_product_id",
        "idx_user_id"
      ],
      "key": "intersect(idx_product_id,idx_user_id)",
      "key_length": "3,3",
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 1,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "0.50",
        "eval_cost": "0.10",
        "prefix_cost": "0.60",
        "data_read_per_join": "1K"
      },
      "used_columns": [
        "product_review_id",
        "product_id",
        "user_id"
      ],
      "attached_condition": "((`dev2`.`cscart_product_reviews`.`user_id` = 0) and (`dev2`.`cscart_product_reviews`.`product_id` = 26421))"
    }
  }
}