Dandandan opened a new issue, #24518:
URL: https://github.com/apache/datafusion/issues/24518
### Is your feature request related to a problem or challenge?
`SortMergeJoinExec` emits every column of both inputs.
We can add projection support to only produce the output columns (saving
some compute / copy).
```sql
select * from t1 right join t2 on t1.c3 = t2.c3 -- c3 needs a cast
```
```text
ProjectionExec: expr=[c1@0 as c1, ..., c4@8 as c4]
SortMergeJoinExec: join_type=Right, on=[(CAST(t1.c3 AS Decimal128(10,
2))@4, c3@2)]
```
The cast column is only there to be joined on, and every operator above the
join
carries it until the projection removes it.
### Describe the solution you'd like
Add projection support to `SortMergeJoinExec`
### Describe alternatives you've considered
_No response_
### Additional context
_No response_
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