fornwall opened a new pull request, #25282:
URL: https://github.com/apache/datafusion/pull/25282
## Which issue does this PR close?
Closes #25281.
## Rationale for this change
`GROUP BY` and `SELECT DISTINCT` over a group key whose type contains a
zero-field struct (`Struct()`) panicked when the aggregate emitted:
```sql
select arrow_cast(null, 'Struct()') as s, count(*) from (values (1),(2),(3))
group by 1;
-- thread 'tokio-rt-worker' panicked ... dictionary re-encode during emit:
-- InvalidArgumentError("use StructArray::try_new_with_length or
StructArray::new_empty_fields ...")
```
The same panic hit `SELECT DISTINCT` and keys where the empty struct is
nested inside another struct. A zero-field struct is a valid Arrow type and
should group like any other key.
## What changes are included in this PR?
`encode_array_if_necessary` in `group_values/row.rs` rebuilt every struct
column with `StructArray::try_new`, which arrow rejects for zero fields since
there is no child column to derive the length from. It now uses
`StructArray::try_new_with_length` with the source array's length, so
zero-field structs are rebuilt like any other struct. This function is shared
by `GroupValuesRows` and the row-backed `GroupColumn`, so both paths are
covered.
## What is the testing strategy for this PR?
Three new `sqllogictest` cases at the end of `struct.slt` (whole-key,
`DISTINCT`, and nested inside `named_struct`). All three panicked before the
fix and pass after it.
## Are there any user-facing changes?
No, other than the panic being fixed.
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