SELECT 
  category_id, 
  parent_id 
FROM 
  q_categories 
WHERE 
  parent_id IN(
    250, 251, 252, 253, 254, 255, 256, 271, 
    433, 434, 459, 484, 533, 535, 579, 595
  )

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "52.41"
    },
    "table": {
      "table_name": "q_categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 26,
      "rows_produced_per_join": 26,
      "filtered": "100.00",
      "index_condition": "(`portal`.`q_categories`.`parent_id` in (250,251,252,253,254,255,256,271,433,434,459,484,533,535,579,595))",
      "cost_info": {
        "read_cost": "47.21",
        "eval_cost": "5.20",
        "prefix_cost": "52.41",
        "data_read_per_join": "68K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ]
    }
  }
}