SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6570, 7162, 5896, 8255, 6932, 6452, 8394, 
    7078, 6593, 6045, 6480, 8270, 6989, 
    7107, 8407, 7193, 6636, 6068, 6495, 
    7265, 6094, 6661, 6510, 6116, 6722, 
    6135, 6756, 6158, 6786, 6179, 6195, 
    6219, 6549
  )

Query time 0.00049

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 (6570,7162,5896,8255,6932,6452,8394,7078,6593,6045,6480,8270,6989,7107,8407,7193,6636,6068,6495,7265,6094,6661,6510,6116,6722,6135,6756,6158,6786,6179,6195,6219,6549))"
    }
  }
}