Cherry-pick: Propagate orderings through struct-producing projections (#39)

[zhuqi-lucas]

Summary

Cherry-pick of #39 from branch-51 to branch-52.

• Propagates orderings through struct-producing projections (e.g. named_struct)
• Adds StructFieldMapping trait to ScalarUDFImpl so UDFs can declare struct field→input mappings
• Uses these mappings in ProjectionExpr::project_orderings to preserve sort order through struct columns

Original PR by @rkrishn7.

Conflicts resolved

• named_struct.rs: import merge (ScalarValue added)
• projection.rs: import merge (DataType, internal_err, ScalarFunctionExpr, RecordBatch/RecordBatchOptions additions)

[Copilot]

Pull request overview

Cherry-pick of #39 to preserve sort order through struct-producing projections (e.g. named_struct) by teaching projection/equivalence logic how struct fields relate back to input expressions, enabling sort elimination for ORDER BY s['field'].

Changes:

• Add StructFieldMapping to the UDF API (ScalarUDFImpl) so struct-producing UDFs can describe field→input-arg mappings.
• Extend ProjectionMapping::try_new to decompose struct-producing projections into field-level mapping entries (e.g. col(a) → get_field(col(s), "a")).
• Add coverage via sqllogictest and equivalence-properties tests for ordering propagation through named_struct.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
datafusion/sqllogictest/test_files/order.slt	Adds EXPLAIN assertions for sort elimination / retention when ordering by struct fields
datafusion/physical-expr/src/projection.rs	Adds struct-field decomposition into ProjectionMapping and adjusts test helper to skip non-Column targets
datafusion/physical-expr/src/equivalence/properties/dependency.rs	Adds a unit test validating ordering propagation through named_struct projections
datafusion/functions/src/core/named_struct.rs	Implements struct_field_mapping for named_struct using get_field as the accessor
datafusion/expr/src/udf.rs	Introduces StructFieldMapping and plumbs it through ScalarUDF / ScalarUDFImpl (incl. alias wrapper)
datafusion/expr/src/lib.rs	Re-exports StructFieldMapping from the expr crate

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[Copilot]

internal_err is imported but not used anywhere in this module, which will fail builds that treat warnings as errors (or clippy -D warnings). Please remove the unused import or use it where intended.

Suggested change

	Result, ScalarValue, assert_or_internal_err, internal_datafusion_err, internal_err, plan_err,
	Result, ScalarValue, assert_or_internal_err, internal_datafusion_err, plan_err,

[Copilot]

func_expr.args()[*source_arg_idx] can panic if a UDF returns a StructFieldMapping with an out-of-bounds argument index. Since struct_field_mapping is part of the public UDF API, this should be handled defensively (e.g., validate indices and skip/return an internal error) rather than panicking during planning/optimization.

Suggested change

	for (accessor_args, source_arg_idx) in &field_mapping.fields {
	for (accessor_args, source_arg_idx) in &field_mapping.fields {
	// `source_arg_idx` comes from UDF `struct_field_mapping` and may
	// be out of bounds for `func_expr.args()`. Guard against this
	// to avoid panicking during planning/optimization.
	if *source_arg_idx >= func_expr.args().len() {
	continue;
	}