SELECT 
  a.category_id 
FROM 
  cscart_categories as a 
  LEFT JOIN cscart_categories as b ON b.category_id IN (234) 
WHERE 
  a.id_path LIKE CONCAT(b.id_path, '/%')

Query time 0.00095

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "80.68"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "b",
          "access_type": "ALL",
          "rows_examined_per_scan": 84,
          "rows_produced_per_join": 8,
          "filtered": "10.00",
          "cost_info": {
            "read_cost": "8.41",
            "eval_cost": "0.84",
            "prefix_cost": "9.25",
            "data_read_per_join": "21K"
          },
          "used_columns": [
            "category_id",
            "id_path"
          ],
          "attached_condition": "(`mobikul_delivery_boy_livedemo`.`b`.`category_id` = 234)"
        }
      },
      {
        "table": {
          "table_name": "a",
          "access_type": "ALL",
          "rows_examined_per_scan": 84,
          "rows_produced_per_join": 78,
          "filtered": "11.11",
          "using_join_buffer": "hash join",
          "cost_info": {
            "read_cost": "0.87",
            "eval_cost": "7.84",
            "prefix_cost": "80.68",
            "data_read_per_join": "204K"
          },
          "used_columns": [
            "category_id",
            "id_path"
          ],
          "attached_condition": "(`mobikul_delivery_boy_livedemo`.`a`.`id_path` like concat(`mobikul_delivery_boy_livedemo`.`b`.`id_path`,'/%'))"
        }
      }
    ]
  }
}