kosiew opened a new pull request, #19955:
URL: https://github.com/apache/datafusion/pull/19955
## Which issue does this PR close?
* Closes #19841.
## Rationale for this change
Struct-to-struct casting previously fell back to **positional mapping** when
there was **no field-name overlap** and the number of fields matched. That
behavior is ambiguous and can silently produce incorrect results when
source/target schemas have different field naming conventions or ordering.
This PR makes struct casting **strictly name-based**: when there is no
overlap in field names between the source and target structs, the cast is
rejected with a clear planning error. This prevents accidental, hard-to-detect
data corruption and forces callers to provide explicit field names (or align
schemas) when casting.
## What changes are included in this PR?
* Removed the positional fallback logic for struct casting in
`cast_struct_column`; child fields are now resolved **only by name**.
* Updated `validate_struct_compatibility` to **error out** when there is
**no field name overlap**, instead of allowing positional compatibility checks.
* Updated unit tests to reflect the new behavior (no-overlap casts now fail
with an appropriate error).
* Updated SQLLogicTest files to construct structs using **explicit field
names** (e.g. `{id: 1}` / `{a: 1, b: 'x'}` or `struct(… AS field)`), avoiding
reliance on positional behavior.
* Improved error messaging to explicitly mention the lack of field name
overlap.
## Are these changes tested?
Yes.
* Updated existing Rust unit tests in `nested_struct.rs` to assert the new
failure mode and error message.
* Updated SQLLogicTest coverage (`struct.slt`, `joins.slt`) to use named
struct literals so tests continue to validate struct behavior without
positional casting.
## Are there any user-facing changes?
Yes — behavioral change / potential breaking change.
* Casting between two `STRUCT`s with **no overlapping field names** now
fails (previously it could succeed via positional mapping if field counts
matched).
* Users must provide explicit field names (e.g. `{a: 1, b: 'x'}` or
`struct(expr AS a, expr AS b)`) or ensure schemas share field names.
* Error messages are more explicit: casts are rejected when there is “no
field name overlap”.
## LLM-generated code disclosure
This PR includes LLM-generated code and comments. All LLM-generated content
has been manually reviewed and tested.
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