SELECT 
  rating 
FROM 
  cscart_manual_rating 
WHERE 
  `object_id` = 9 
  AND `object_type` = 'company'

Query time 0.00026

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_manual_rating",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "102",
      "used_key_parts": ["object_id", "object_type"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}