SELECT 
  thread_id 
FROM 
  cscart_discussion 
WHERE 
  object_id = 23457 
  AND object_type = 'P' 
  AND type IN ('B', 'R')

Query time 0.00033

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "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,
          "index_condition": "cscart_discussion.object_type = 'P'",
          "attached_condition": "cscart_discussion.`type` in ('B','R')"
        }
      }
    ]
  }
}