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

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.55"
    },
    "table": {
      "table_name": "cscart_absolute_rating",
      "access_type": "ALL",
      "rows_examined_per_scan": 3,
      "rows_produced_per_join": 1,
      "filtered": "33.33",
      "cost_info": {
        "read_cost": "0.45",
        "eval_cost": "0.10",
        "prefix_cost": "0.55",
        "data_read_per_join": "112"
      },
      "used_columns": [
        "object_type",
        "rating"
      ],
      "attached_condition": "(`dev2`.`cscart_absolute_rating`.`object_type` = 'company')"
    }
  }
}