SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    7685, 7740, 7750, 7686, 7741, 7751, 7687, 
    7688, 7689, 7690, 7691, 7692, 7693, 
    7694, 7695, 7696, 7697, 7698, 7699, 
    7700, 7702, 7703, 7704, 7705, 7706, 
    7707, 7708, 7709, 7710, 7711, 7712
  )

Query time 0.00083

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "14.23"
    },
    "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": 33,
      "rows_produced_per_join": 33,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "7.63",
        "eval_cost": "6.60",
        "prefix_cost": "14.23",
        "data_read_per_join": "195K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (7685,7740,7750,7686,7741,7751,7687,7688,7689,7690,7691,7692,7693,7694,7695,7696,7697,7698,7699,7700,7702,7703,7704,7705,7706,7707,7708,7709,7710,7711,7712))"
    }
  }
}