SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    8327, 8376, 8393, 8403, 8444, 8454, 8432, 
    8497, 8588, 8503, 8331, 8423, 8539, 
    8373, 8664
  )

Query time 0.00076

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "3.92"
    },
    "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": 15,
      "rows_produced_per_join": 15,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "2.43",
        "eval_cost": "1.50",
        "prefix_cost": "3.93",
        "data_read_per_join": "38K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`maniamall_cscart`.`cscart_categories`.`parent_id` in (8327,8376,8393,8403,8444,8454,8432,8497,8588,8503,8331,8423,8539,8373,8664))"
    }
  }
}