SELECT 
  MAX(rating) 
FROM 
  cscart_absolute_rating 
WHERE 
  `object_type` = 'company'

Query time 0.00032

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.01034841,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_absolute_rating",
          "access_type": "ALL",
          "loops": 1,
          "rows": 2,
          "cost": 0.01034841,
          "filtered": 100,
          "attached_condition": "cscart_absolute_rating.object_type = 'company'"
        }
      }
    ]
  }
}