SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    7857, 7858, 7859, 7860, 7861, 7862, 7863, 
    7864, 7865, 7866, 7867, 7868, 7869, 
    7870, 7871, 7872, 7873, 7874, 7875, 
    7876, 7877, 7878, 7879, 7880, 7881, 
    7882, 7883, 8104
  )

Query time 0.00104

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "20.24"
    },
    "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": 48,
      "rows_produced_per_join": 48,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "10.64",
        "eval_cost": "9.60",
        "prefix_cost": "20.24",
        "data_read_per_join": "284K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (7857,7858,7859,7860,7861,7862,7863,7864,7865,7866,7867,7868,7869,7870,7871,7872,7873,7874,7875,7876,7877,7878,7879,7880,7881,7882,7883,8104))"
    }
  }
}