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

Query time 0.00014

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_absolute_rating",
      "access_type": "ALL",
      "rows": 91,
      "filtered": 100,
      "attached_condition": "cscart_absolute_rating.object_type = 'company'"
    }
  }
}