SELECT 
  a.category_id 
FROM 
  cscart_categories as a 
  LEFT JOIN cscart_categories as b ON b.category_id IN (215) 
WHERE 
  a.id_path LIKE CONCAT(b.id_path, '/%')

Query time 0.00040

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.060074355,
    "nested_loop": [
      {
        "table": {
          "table_name": "b",
          "access_type": "const",
          "possible_keys": ["PRIMARY", "p_category_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["category_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "a",
          "access_type": "index",
          "key": "id_path",
          "key_length": "767",
          "used_key_parts": ["id_path"],
          "loops": 1,
          "rows": 271,
          "cost": 0.060074355,
          "filtered": 100,
          "attached_condition": "a.id_path like <cache>(concat('215','/%'))",
          "using_index": true
        }
      }
    ]
  }
}

Result

category_id
1613
1669
1670
1671
1672
1673
1674
1675
1614
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
337
502
503
504
505
506
339
507
508
509
510
658
1660
1661
1662
1663
1664
1665
1666
1667
1668
659
660
661
662