SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6339, 6334, 6335, 6336, 6337, 6338, 6644, 
    15307, 28615, 6765, 6793, 14789, 15306, 
    15309, 15311
  )

Query time 0.00034

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "3.74"
    },
    "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": 15,
      "rows_produced_per_join": 15,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "2.24",
        "eval_cost": "1.50",
        "prefix_cost": "3.74",
        "data_read_per_join": "38K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`maniamall_cscart`.`cscart_categories`.`parent_id` in (6339,6334,6335,6336,6337,6338,6644,15307,28615,6765,6793,14789,15306,15309,15311))"
    }
  }
}