SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    8984, 2784, 2814, 2778, 2794, 2820, 2826, 
    2808, 2805, 2810, 2831, 2830, 2834, 
    2838, 2839, 2844, 2845, 2865, 2854, 
    2860, 2864, 8942, 9670, 9665, 9658, 
    9666
  )

Query time 0.00059

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 (8984,2784,2814,2778,2794,2820,2826,2808,2805,2810,2831,2830,2834,2838,2839,2844,2845,2865,2854,2860,2864,8942,9670,9665,9658,9666))"
    }
  }
}