fix: carry struct columns' full names through select, project and aggregate

[nielspardon]

select computed its emit offset from the input's name count instead of its top-level field count. Because a struct column has one field but several depth-first names, the output mapping pointed past the end of the projected fields over any struct-typed input, and infer_plan_schema raised IndexError. select, project and aggregate (grouping expressions) also kept only output_names[0] of each expression, dropping a struct column's nested names. They now keep the full depth-first output_names list, and raise ValueError when an expression carries more names than its output type has fields.

column(..., alias="z") on a struct column used to replace all of its names with ["z"]. A single alias now renames only the top-level field and keeps the nested names. A full alias list still replaces all of them, but now raises ValueError unless it names every field.

Struct-valued expressions other than column() (functions, casts, literals, if_then, aggregate measures) still emit a single name. That needs a naming scheme the spec doesn't define, so it's tracked separately in #312.

Fixes #311

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Reviewing files that changed from the base of the PR and between f24b850 and b5b1f25.

📒 Files selected for processing (4)

• src/substrait/builders/extended_expression.py
• src/substrait/builders/plan.py
• tests/builders/extended_expression/test_column.py
• tests/builders/plan/test_project.py

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Nested struct columns retain their member names when given a single alias; alias lists rename all output fields and must match the field count.
  • Selecting, projecting, or grouping by nested struct columns preserves nested field names in the output, with selection mappings and inferred schemas aligned to the resulting fields.
  • Invalid expressions that provide more output names than fields now raise a ValueError.

Column aliases and plan builders now preserve nested struct output names. select also bases emitted-field indexes on the input struct’s top-level field count.

Changes: Nested Struct Output Handling

Layer / File(s)	Summary
Nested column output names src/substrait/builders/extended_expression.py, tests/builders/extended_expression/test_column.py	A single alias renames a struct column’s top-level output name and preserves nested member names. An alias list replaces the full output-name list when its count matches. Tests cover both cases, invalid alias counts, and an empty string alias.
Plan output names and select mappings src/substrait/builders/plan.py, tests/builders/plan/test_project.py, tests/builders/plan/test_aggregate.py	project, select, and aggregate grouping include all output names from struct-valued expressions. select starts emitted-field indexes at the input struct’s top-level field count. Tests check output names, inferred schemas, and invalid name counts.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to b5b1f

The previously reported invalid output-name count is addressed. No actionable merge-blocking risk remains after normal checks.

Pre-merge checks · 4 · 1

Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 30.43% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 23 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: preserving full names for struct columns across select, project, and aggregate.
Description check		The description explains the failure, the implemented fix, alias behavior, out-of-scope behavior, and linked issue. It provides the required rationale and is sufficiently complete.
Linked Issues check		Issue #311 requires top-level field positions for select emit mappings and full depth-first names for select and project. The reviewed diff uses len(ns.struct.types) for the select offset an…
Out of Scope Changes check		The source changes address struct name propagation, select field offsets, aggregate grouping names, and column alias validation. The reported tests cover these behaviors. The empty-string alias be…

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Inline comments:
Review comments at @src/substrait/builders/extended_expression.py:
- Line 605: Update the alias handling around output_names to reject a multi-name
alias when its length differs from the inferred output_names length, before
replacing the output names. Preserve the existing behavior for matching
multi-name aliases and single-name aliases.

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• Configuration used: Repository: substrait-io/substrait-python/.coderabbit.yaml
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📥 Commits

Reviewing files that changed from the base of the PR and between 8277940 and f24b850.

📒 Files selected for processing (5)

• src/substrait/builders/extended_expression.py
• src/substrait/builders/plan.py
• tests/builders/extended_expression/test_column.py
• tests/builders/plan/test_aggregate.py
• tests/builders/plan/test_project.py

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