SELECT 
  id_path 
FROM 
  cscart_categories 
WHERE 
  category_id IN (3, 15)

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.41"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "p_category_id"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "category_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 2,
      "rows_produced_per_join": 2,
      "filtered": "100.00",
      "index_condition": "(`betaatif_store`.`cscart_categories`.`category_id` in (3,15))",
      "cost_info": {
        "read_cost": "1.21",
        "eval_cost": "0.20",
        "prefix_cost": "1.41",
        "data_read_per_join": "5K"
      },
      "used_columns": [
        "category_id",
        "id_path"
      ]
    }
  }
}