[I] # Eliminate positional fallback in struct casting [datafusion]

["kosiew (via GitHub)" <gi...@apache.org>]

kosiew opened a new issue, #19841:
URL: https://github.com/apache/datafusion/issues/19841

   ## Problem
   
   Currently, DataFusion's struct casting includes a **positional fallback** mechanism that allows casting between structs with completely different field names if the field counts match. This violates DuckDB's semantics and can silently corrupt data.
   
   **Example of problematic behavior:**
   ```rust
   // This currently succeeds but should fail
   source: struct<left int, right varchar>
   target: struct<alpha int, beta varchar>
   
   // Positional fallback causes:
   // left → alpha, right → beta
   // Despite zero matching field names!
   ```
   
   ## Current Behavior
   
   The implementation in [`validate_struct_compatibility()`](datafusion/common/src/nested_struct.rs) currently allows casting when:
   - No field name overlap exists AND
   - Field counts match
   
   The casting correctly fails only when there's no overlap **and** counts differ with the error:
   ```
   Cannot cast struct with X fields to Y fields without name overlap; positional mapping is ambiguous
   ```
   
   ## Proposed Solution
   
   Align with **DuckDB's semantics** by requiring at least one matching field name for any struct cast to succeed:
   
   1. **Require name-based matching** — at least one field name must match between source and target
   2. **Remove positional fallback** — even when field counts are equal
   3. **Clear error message** when no field names match
   
   
   ## Impact
   
   - **Breaking change** — Any code relying on positional casting with no name overlap will fail
   - **Safety improvement** — Prevents silent data corruption from accidental field misalignment
   - **Consistency** — Aligns with DuckDB's well-designed struct casting semantics and the principle of "at least one field name must match"
   
   
   #19674 [discussion](https://github.com/apache/datafusion/pull/19674#issuecomment-3757969719)
   
   


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Re: [I] # Eliminate positional fallback in struct casting [datafusion]

["adriangb (via GitHub)" <gi...@apache.org>]

adriangb closed issue #19841: # Eliminate positional fallback in struct casting
URL: https://github.com/apache/datafusion/issues/19841


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