[Question] Sink Node Reuse for multi-stream join

[Ge Da]

Hi Paimon community,

I'm working on a multi-stream join scenario using Paimon and have a
question about Sink Node Reuse optimization.

Current setup:
- Using partial-update merge engine
- Multiple Kafka streams (order_base_cdc, order_pay_cdc)
- Currently using UNION ALL with NULL placeholders

I've heard about "Sink Node Reuse" optimization that can avoid maintaining
NULL placeholders when table schema changes. Could someone explain:

1. How does Sink Node Reuse work in Paimon?
2. What are the specific configuration parameters needed?
3. Can each stream write only its own fields without NULL placeholders?
4. Any examples or documentation for this feature?

Current DML structure:
```sql
INSERT INTO paimon_table
SELECT orderid, channelName, orderTime, NULL, NULL FROM stream1
UNION ALL
SELECT orderid, NULL, NULL, payAmount, payTime FROM stream2
```

Is there a way to write like this instead?
```sql
-- Stream 1
INSERT INTO paimon_table SELECT orderid, channelName, orderTime FROM
stream1;
-- Stream 2
INSERT INTO paimon_table SELECT orderid, payAmount, payTime FROM stream2;
```

Thanks for any help!