SELECT 
  category_id 
FROM 
  cscart_categories 
WHERE 
  id_path LIKE '1842/%'

Query time 0.00034

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": ["id_path"],
      "key": "id_path",
      "key_length": "767",
      "used_key_parts": ["id_path"],
      "rows": 25,
      "filtered": 100,
      "attached_condition": "cscart_categories.id_path like '1842/%'",
      "using_index": true
    }
  }
}

Result