ahlag opened a new issue, #26528:
URL: https://github.com/apache/datafusion/issues/26528
### Describe the bug
`TRY_CAST` rejects some struct-to-struct casts at planning time even though
`CAST` accepts them.
`CAST` validates struct casts by field name, allowing missing nullable
fields in the target struct to be filled with `NULL`. In contrast, `TRY_CAST`
uses Arrow's `can_cast_types`, which requires the source and target structs to
have the same number of fields.
The runtime `TryCastExpr` already handles struct casts by field name. The
inconsistency is therefore in the planning-time validation, which is stricter
than necessary.
For example, casting `STRUCT<x BIGINT>` to `STRUCT<x BIGINT, y VARCHAR>`
succeeds with `CAST` but fails during planning with `TRY_CAST`.
### To Reproduce
Reproduced with DataFusion 55.2.0.
**CAST succeeds:**
```sql
SELECT cast(named_struct('x', 1) AS STRUCT<x BIGINT, y VARCHAR>) AS s;
```
Result:
```
{x: 1, y: }
```
**TRY_CAST fails during planning:**
```sql
SELECT try_cast(named_struct('x', 1) AS STRUCT<x BIGINT, y VARCHAR>) AS s;
```
Error:
```
This feature is not implemented: Unsupported TRY_CAST from Struct("x":
Int64) to Struct("x": Int64, "y": Utf8View)
```
### Expected behavior
`TRY_CAST` should accept this struct cast and return the same result as
`CAST`:
```
{x: 1, y: }
```
More generally, `TRY_CAST` should accept at planning time every struct cast
supported by `CAST`, while preserving its existing null-on-failure semantics.
### Additional context
This issue was encountered in
[[delta-rs](https://github.com/delta-io/delta-rs/pull/4889#issuecomment-6290083554)](https://github.com/delta-io/delta-rs/pull/4889#issuecomment-6290083554),
where a merge with schema evolution and `safe_cast` enabled failed because of
this behavior.
As a workaround, [[delta-rs PR
#4917](https://github.com/delta-io/delta-rs/pull/4917)](https://github.com/delta-io/delta-rs/pull/4917)
uses `CAST` instead of `TRY_CAST` for widened fields.
Ion Koutsouris also noted that the same issue occurs in DataFusion 55.1.
### Willingness to contribute
I plan to contribute a fix for this bug independently
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