Create physical plan bug: got Arrow schema with 1 and DataFusion schema with 0

[killme2008]

Describe the bug

Create a table and insert some values:

CREATE TABLE integers(i INTEGER);

INSERT INTO integers VALUES (0), (1), (2), (3), (4);

Execute following SQL:

SELECT COUNT(*) FROM (SELECT * FROM integers UNION ALL SELECT * FROM integers LIMIT 7) tbl;

Report error from datafusion:

create_physical_expr expected same number of fields, got got Arrow schema with 1  and DataFusion schema with 0. 
This was likely caused by a bug in DataFusion’s code and we would welcome that you file an bug report
 in our issue tracker

The subquery works:

SELECT * FROM integers UNION ALL SELECT * FROM integers LIMIT 7;

+-----------------------+
| i |
+-----------------------+
| 0                     |
| 1                     |
| 2                     |
| 3                     |
| 4                     |
| 0                     |
| 1                     |
+-----------------------+

[HaoYang670]

It seems that there is a bug in the PushDownProjection.

It seems that the PushDownProjection rewrites the schema of Union but this schema is ignored when generating physical plan.