SELECT 
  category_id 
FROM 
  cscart_categories 
WHERE 
  id_path LIKE '166/%'

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "ALL",
      "possible_keys": ["id_path"],
      "rows": 83,
      "filtered": 40.96385574,
      "attached_condition": "cscart_categories.id_path like '166/%'"
    }
  }
}