tshauck opened a new issue, #516:
URL: https://github.com/apache/datafusion-comet/issues/516
### What is the problem the feature request solves?
It's come up a couple of times recently that expressions need scalar tests,
but it's not clear it's possible to have Comet execute something like `SELECT
trim('123 ')`.
Doing a little digging, I think it's because in Spark these queries include
`OneRowRelation` as the scan, which isn't supported by Comet.
```
spark-sql (default)> SELECT trim('123 ');
24/06/04 10:37:51 WARN CometSparkSessionExtensions$CometExecRule: Comet
cannot execute some parts of this plan natively because Scan OneRowRelation is
not supported
123
Time taken: 0.066 seconds, Fetched 1 row(s)
```
My thinking is that if this SparkPlan is supported, then it should be
possible to run scalar queries. If this isn't the case, then feel free to close
this issue.
### Describe the potential solution
Happy to pursue a different approach if there's a better one, but I wanted
to open this issue first to see if it's a problem and if this is an appropriate
solution.
Potential solution:
1. `OneRowRelation` (which is an `RDDScanExec`) could be implemented a
native operator. DataFusion has a similar
[`PlaceHolderExec`](https://github.com/apache/datafusion/blob/1c85d6abd46fd37fc5f6af8e558f19049c73de4c/datafusion/physical-plan/src/placeholder_row.rs#L41).
2. Update
https://github.com/apache/datafusion-comet/blob/a668a8657a16496781075a014c6009d038c3fa1b/spark/src/main/scala/org/apache/comet/CometSparkSessionExtensions.scala#L299-L712
to match on a ProjectExec with a `OneRowRelation` child scan, and have it use
the aforementioned operator.
### Additional context
_No response_
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