SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(266, 2577)

Query time 0.00058

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "17.04"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 40,
      "rows_produced_per_join": 40,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "9.04",
        "eval_cost": "8.00",
        "prefix_cost": "17.04",
        "data_read_per_join": "237K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (266,2577))"
    }
  }
}

Result

category_id parent_id
327 266
341 266
344 266
356 266
394 266
443 266
715 266
930 266
990 266
1498 266
1584 266
1907 266
1936 266
2171 266
2178 266
2373 266
2404 266
2433 266
2465 266
2577 266
2591 266
2611 266
2615 266
9692 266
9830 266
9840 266
9844 266
9926 266
9936 266
10631 266
10632 266
10633 266
10667 266
2578 2577
2604 2577
2623 2577
9867 2577
9920 2577
10247 2577
10248 2577