这个句话的意思是，delete时间我可以从cdc 获取_id 和 op：d?

MongoDB’s change event record doesn’t have update before message. So, we can only convert it to Flink’s UPSERT changelog stream. An upsert stream requires a unique key, so we must declare _id as primary key. We can’t declare other column as primary key, becauce delete operation do not contain’s the key and value besides _id and sharding key.