SELECT 
  COUNT(cscart_order_details.item_id) 
FROM 
  cscart_order_details 
  LEFT JOIN cscart_orders ON cscart_order_details.order_id = cscart_orders.order_id 
WHERE 
  user_id = 0 
  AND product_id IN (7908) 
  AND cscart_orders.status = 'C'

Query time 0.00083

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "10.18"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_orders",
          "access_type": "index_merge",
          "possible_keys": [
            "PRIMARY",
            "user_id",
            "status"
          ],
          "key": "intersect(status,user_id)",
          "key_length": "3,3",
          "rows_examined_per_scan": 13,
          "rows_produced_per_join": 13,
          "filtered": "93.95",
          "cost_info": {
            "read_cost": "4.33",
            "eval_cost": "1.30",
            "prefix_cost": "5.63",
            "data_read_per_join": "134K"
          },
          "used_columns": [
            "order_id",
            "user_id",
            "status"
          ],
          "attached_condition": "(((`cscartdb`.`cscart_orders`.`user_id` = 0) and (`cscartdb`.`cscart_orders`.`status` = 'C')) and (`cscartdb`.`cscart_orders`.`order_id` is not null))"
        }
      },
      {
        "table": {
          "table_name": "cscart_order_details",
          "access_type": "ref",
          "possible_keys": [
            "o_k"
          ],
          "key": "o_k",
          "used_key_parts": [
            "order_id",
            "product_id"
          ],
          "key_length": "6",
          "ref": [
            "cscartdb.cscart_orders.order_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 13,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "3.25",
            "eval_cost": "1.30",
            "prefix_cost": "10.18",
            "data_read_per_join": "3K"
          },
          "used_columns": [
            "item_id",
            "order_id",
            "product_id"
          ]
        }
      }
    ]
  }
}