kosiew commented on issue #16565:
URL: https://github.com/apache/datafusion/issues/16565#issuecomment-3035480855
@adriangb,
> incorporating the struct-aware casting logic from #16371 into `CastExpr`
and `TryCastExpr`
Yes. I think it is a necessary step to explore the feasibility of rewriting
Expr for schema adaptation.
A few thoughts on this:
1. **Reusability of struct-aware casting kernels**: The core struct-aware
casting kernel introduced in #16371 was designed to recursively handle casting
between nested struct types, dealing with missing or reordered fields.
Integrating this logic into `CastExpr` and `TryCastExpr` would unify casting
semantics across physical expressions, ensuring consistent behavior during
predicate rewrite and evaluation.
2. **Improved predicate pushdown**: Predicate pushdown often requires
projecting predicates down into struct subfields. If schemas differ between
scans (e.g., fields missing or reordered), leveraging struct-aware casts would
allow predicates to be adapted instead of dropped—thus generating more
selective filters pushed down to data sources.
3. **Implementation considerations**: We would need to extend `CastExpr` and
`TryCastExpr` to detect when the input/output types are structs and invoke the
struct-aware casting logic appropriately. This means:
- Adding recursive traversal logic in these expressions.
- Handling missing subfields by either filling with NULLs or using
default values as per the kernel's design.
- Handling field reordering and type promotion gracefully.
4. **Challenge**: Rewriting the struct-aware casting logic from #16371 into
manipulating expressions. Your expression rewrite prowess bends my mind.
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