PlaceholderRowExec shown when select from union results. about arrow-datafusion HOT 2 CLOSED

> insert into t3 values (1,2), (2,3)
;
+-------+
| count |
+-------+
| 2     |
+-------+
1 row(s) fetched. 
Elapsed 0.004 seconds.

> explain select count(*) from ((select distinct c1, c2 from t3 order by c1 ) union all (select distinct c2, c1 from t4 order by c1));
+---------------+-------------------------------------------------------------------------------------------+
| plan_type     | plan                                                                                      |
+---------------+-------------------------------------------------------------------------------------------+
| logical_plan  | Aggregate: groupBy=[[]], aggr=[[COUNT(Int64(1)) AS COUNT(*)]]                             |
|               |   Union                                                                                   |
|               |     Projection:                                                                           |
|               |       Sort: t3.c1 ASC NULLS LAST                                                          |
|               |         Projection: t3.c1                                                                 |
|               |           Aggregate: groupBy=[[t3.c1, t3.c2]], aggr=[[]]                                  |
|               |             TableScan: t3 projection=[c1, c2]                                             |
|               |     Projection:                                                                           |
|               |       Sort: t4.c1 ASC NULLS LAST                                                          |
|               |         Projection: t4.c1                                                                 |
|               |           Aggregate: groupBy=[[t4.c2, t4.c1]], aggr=[[]]                                  |
|               |             Projection: t4.c2, t4.c1                                                      |
|               |               TableScan: t4 projection=[c1, c2]                                           |
| physical_plan | AggregateExec: mode=Final, gby=[], aggr=[COUNT(*)]                                        |
|               |   CoalescePartitionsExec                                                                  |
|               |     AggregateExec: mode=Partial, gby=[], aggr=[COUNT(*)]                                  |
|               |       RepartitionExec: partitioning=RoundRobinBatch(14), input_partitions=28              |
|               |         UnionExec                                                                         |
|               |           ProjectionExec: expr=[]                                                         |
|               |             AggregateExec: mode=FinalPartitioned, gby=[c1@0 as c1, c2@1 as c2], aggr=[]   |
|               |               CoalesceBatchesExec: target_batch_size=8192                                 |
|               |                 RepartitionExec: partitioning=Hash([c1@0, c2@1], 14), input_partitions=14 |
|               |                   RepartitionExec: partitioning=RoundRobinBatch(14), input_partitions=1   |
|               |                     AggregateExec: mode=Partial, gby=[c1@0 as c1, c2@1 as c2], aggr=[]    |
|               |                       MemoryExec: partitions=1, partition_sizes=[1]                       |
|               |           ProjectionExec: expr=[]                                                         |
|               |             AggregateExec: mode=FinalPartitioned, gby=[c2@0 as c2, c1@1 as c1], aggr=[]   |
|               |               CoalesceBatchesExec: target_batch_size=8192                                 |
|               |                 RepartitionExec: partitioning=Hash([c2@0, c1@1], 14), input_partitions=1  |
|               |                   AggregateExec: mode=Partial, gby=[c2@0 as c2, c1@1 as c1], aggr=[]      |
|               |                     MemoryExec: partitions=1, partition_sizes=[0]                         |
|               |                                                                                           |
+---------------+-------------------------------------------------------------------------------------------+
2 row(s) fetched. 
Elapsed 0.018 seconds.

> select count(*) from ((select distinct c1, c2 from t3 order by c1 ) union all (select distinct c2, c1 from t4 order by c1));
+----------+
| COUNT(*) |
+----------+
| 2        |
+----------+
1 row(s) fetched. 
Elapsed 0.019 seconds.