SELECT 
  object_id, 
  thread_id, 
  type, 
  object_type, 
  company_id 
FROM 
  cscart_discussion 
WHERE 
  1 
  AND object_id = 6 
  AND object_type = 'C' 
  AND cscart_discussion.company_id = 1

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_discussion",
      "access_type": "const",
      "possible_keys": [
        "object_id",
        "company_id"
      ],
      "key": "object_id",
      "used_key_parts": [
        "object_id",
        "object_type",
        "company_id"
      ],
      "key_length": "10",
      "ref": [
        "const",
        "const",
        "const"
      ],
      "rows": 1,
      "filtered": 100
    }
  }
}