chore: clean up and document TODOs in processing module

[devin-ai-integration]

Reference Issues/PRs

N/A — GitHub issues are disabled on this repo. See TODO inventory below.

What does this implement or fix?

Audits and cleans up all 18 TODOs across clause.cpp (14), clause.hpp (2), clause_utils.cpp (1), and unsorted_aggregation.cpp (1) in the processing module.

Changes made:

1. Removed 1 obsolete TODO (clause.cpp:2168): The comment requested exception handling for duplicate source-to-target row matches. This is already implemented at lines 2296-2311 via a matched_target_rows BitSet that raises E_INVALID_USER_ARGUMENT. Replaced with a brief note pointing to the existing validation.

2. Clarified 17 remaining TODOs with improved context:

   • Stripped internal Monday.com ticket references from all comments
   • Fixed misleading wording ("update must be done" → correctly says "insert")
   • Added specific descriptions of what each TODO requires and why

No functional code changes. All edits are comment-only.

---

TODO Inventory (for future tracking)

Category	Files / Lines	Summary
Merge update insert support	clause.cpp:140, 151, 2125, 2386, 2399	Source rows with no target match should be inserted. Currently skipped. Affects both timeseries and rowrange paths, string and non-string columns.
Sparse target columns	clause.cpp:144	for_each_enumerated skips sparse positions, so matched source rows targeting sparse target positions won't be updated.
Parallel MergeClause	clause.cpp:1563	MergeClause flattens all entities into one group (serial). Should be parallelized like ResampleClause.
Arrow input for merge update	clause.cpp:2105	Arrow format not yet supported as merge update input; error raised in process().
Dynamic schema support	clause.cpp:2183, 2533, 2544	Source/target descriptors, field indexes, and type constraints assume static schema.
Performance optimization	clause.cpp:2298, 2453	Duplicate-check loop could be inlined; index matching loop could be inverted when source > target.
PassthroughClause structuring	clause.hpp:118, 124	Two overloads use inconsistent structuring strategies (row-slice vs. passthrough).
Arrow + multi-index concat	clause_utils.cpp:321	Blocked on arrow normalization metadata finalization.
EMPTYVAL in MeanAggregator	unsorted_aggregation.cpp:492	Fully sparse column approach works but may not be optimal; revisit when feature flag is removed.

Any other comments?

These are purely comment/documentation changes — no logic was modified.

Checklist

Checklist for code changes...

• Have you updated the relevant docstrings, documentation and copyright notice?
• Is this contribution tested against all ArcticDB's features? (comment-only changes, no functional impact)
• Do all exceptions introduced raise appropriate error messages? (N/A)
• Are API changes highlighted in the PR description? (N/A — no API changes)
• Is the PR labelled as enhancement or bug so it appears in autogenerated release notes? (N/A — housekeeping)

Link to Devin session: https://app.devin.ai/sessions/7b17aae56da9472ba724c9658ee07783
Requested by: @chrischappell-cog

---

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.