SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    6474, 6508, 6530, 6566, 6596, 6621, 6649, 
    6677, 6694, 6728, 6780, 6803, 6828, 
    6846, 6865
  )

Query time 0.00064

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.02"
    },
    "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": "4.02",
        "eval_cost": "3.00",
        "prefix_cost": "7.02",
        "data_read_per_join": "88K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`webmarco`.`cscart_categories`.`parent_id` in (6474,6508,6530,6566,6596,6621,6649,6677,6694,6728,6780,6803,6828,6846,6865))"
    }
  }
}