SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6936, 7309, 6867, 7251, 6953, 6883, 7325, 
    7284, 6919, 7304, 6973, 7340, 7317, 
    7001, 7306, 7347, 7029, 7391, 7336, 
    7357, 8988, 7062, 7368, 7151, 7373, 
    7180, 7381, 7210, 7225, 7356, 7379, 
    7396, 7456
  )

Query time 0.00060

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "23.85"
    },
    "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": 57,
      "rows_produced_per_join": 57,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "12.45",
        "eval_cost": "11.40",
        "prefix_cost": "23.85",
        "data_read_per_join": "337K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (6936,7309,6867,7251,6953,6883,7325,7284,6919,7304,6973,7340,7317,7001,7306,7347,7029,7391,7336,7357,8988,7062,7368,7151,7373,7180,7381,7210,7225,7356,7379,7396,7456))"
    }
  }
}