SELECT 
  thread_id 
FROM 
  cscart_discussion 
WHERE 
  object_type = 'P' 
  AND object_id = 63815

Query time 0.00042

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_discussion",
      "access_type": "ref",
      "possible_keys": ["object_id"],
      "key": "object_id",
      "key_length": "6",
      "used_key_parts": ["object_id", "object_type"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "cscart_discussion.object_type = 'P'",
      "using_index": true
    }
  }
}

Result