SELECT 
  creator_item_id 
FROM 
  cscart_creator_items 
WHERE 
  product_id = 21642 
  AND user_id = 0

Query time 0.00047

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.01"
    },
    "table": {
      "table_name": "cscart_creator_items",
      "access_type": "ALL",
      "rows_examined_per_scan": 5,
      "rows_produced_per_join": 1,
      "filtered": "20.00",
      "cost_info": {
        "read_cost": "0.91",
        "eval_cost": "0.10",
        "prefix_cost": "1.01",
        "data_read_per_join": "40"
      },
      "used_columns": [
        "creator_item_id",
        "user_id",
        "product_id"
      ],
      "attached_condition": "((`cscartdb`.`cscart_creator_items`.`user_id` = 0) and (`cscartdb`.`cscart_creator_items`.`product_id` = 21642))"
    }
  }
}