Join two tables with the same schema, then union throws `SchemaError(DuplicateUnqualifiedField )`

[ZuoTiJia]

Describe the bug
Join two tables with the same schema, then union throws SchemaError(DuplicateUnqualifiedField )

To Reproduce

create table ta(a bigint) as values(1);
create table tb(a bigint) as values(2);
select * from ta join tb on true
    union all
select * from ta join tb on true;

SchemaError(DuplicateUnqualifiedField { name: "a" })

Expected behavior
query sucess.

Additional context
datafusion v18.0.0

[Jefffrey]

Issue seems to be here:

https://github.com/apache/arrow-datafusion/blob/5ffa8d78f7e5796fa7c2f98b19346576ab2a4bb8/datafusion/expr/src/logical_plan/builder.rs#L1085-L1111

Specifically where it strips the qualifier from the generated DFField

A solution could be to grab the left field qualifier here instead of wiping it.

[yukkit]

Issue seems to be here:

https://github.com/apache/arrow-datafusion/blob/5ffa8d78f7e5796fa7c2f98b19346576ab2a4bb8/datafusion/expr/src/logical_plan/builder.rs#L1085-L1111

Specifically where it strips the qualifier from the generated DFField

A solution could be to grab the left field qualifier here instead of wiping it.

I agree with you