SELECT 
  id_path 
FROM 
  cscart_categories c 
  JOIN cscart_category_vendor_product_count p USING(category_id) 
WHERE 
  p.company_id = 3

Query time 0.00003

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "p",
      "access_type": "ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "4",
      "used_key_parts": ["company_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "c",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY", "p_category_id"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["category_id"],
      "ref": ["smscrm_edev.p.category_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}