Elide redundant secondary-index rewrites on identical in-place updates

[EnRaiha]

Problem

An update that leaves a secondary index column unchanged still performs remove+insert for that index (2 writes per index instead of 0). The writes are correct — the entry is the same before and after — so this is an optimization opportunity, not a correctness defect.

Proposed solution

Compare extracted old vs new indexed values per field in the KV secondary-index update path; when equal, skip both remove and insert for that index. Changed values keep the existing remove+insert behavior.

Engine / area

Key-Value (secondary indexes).

Alternatives & workarounds

None needed; current behavior is correct, just wasteful.

Before submitting

• I searched existing issues and discussions for this proposal.

[farhan-syah]

Verdict: valid optimization.

• Cause: KvIndexSet::on_put removes old entries and inserts new entries when indexed bytes match. The atomic PUT path repeats this. Composite indexes also perform both writes.
• Implementation: Compare old and new indexed bytes per single-field and composite index. Skip both operations when equal. Keep remove-only behavior for removed fields and insert-only behavior for added fields.
• Scope: Leave sorted indexes unchanged. OrderStatTree::insert already no-ops for an identical sort key.
• Benefit: Reduce redundant index writes and write amplification for unchanged indexed fields.
• Triage: Keep type:chore. Add engine:kv and priority:P3. No severity label applies.