SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    7914, 7960, 7983, 7994, 8010, 8018, 8040, 
    8058, 8069, 8075, 8100, 8109, 8135, 
    8144, 8160, 8169, 8177, 8188, 8199, 
    8205, 8212, 8221, 8254, 8266, 8276, 
    8288, 8306, 8312
  )

Query time 0.00057

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "12.23"
    },
    "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": 28,
      "rows_produced_per_join": 28,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "6.63",
        "eval_cost": "5.60",
        "prefix_cost": "12.23",
        "data_read_per_join": "166K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (7914,7960,7983,7994,8010,8018,8040,8058,8069,8075,8100,8109,8135,8144,8160,8169,8177,8188,8199,8205,8212,8221,8254,8266,8276,8288,8306,8312))"
    }
  }
}