SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    1321, 1326, 1332, 657, 1341, 1347, 1378, 
    1402
  )

Query time 0.00058

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "22.71"
    },
    "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": 46,
      "rows_produced_per_join": 46,
      "filtered": "100.00",
      "index_condition": "(`unicomps`.`cscart_categories`.`parent_id` in (1321,1326,1332,657,1341,1347,1378,1402))",
      "cost_info": {
        "read_cost": "18.11",
        "eval_cost": "4.60",
        "prefix_cost": "22.71",
        "data_read_per_join": "188K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ]
    }
  }
}