Overview

Verify that unique index constraints hold under staged writes without special handling, as described in design note section 3.4.

Background

WiredTigerIndex::_checkDups inserts and removes a prefix key to conflict with concurrent insertions of the same value. Under staging this key enters the WriteSet, so the conflict detector rejects the later committer. The _id index uses one table key per value, so duplicate _id inserts conflict directly. The bounded duplicate search runs against the merged view.

Scope of Work

• Tests: concurrent duplicate inserts on unique secondary and _id indexes, delete then reinsert in one transaction, update that swaps unique values, old unique index formats where applicable.
• Measure WriteSet growth from prefix keys on realistic workloads (open question 9).

Acceptance Criteria

• No duplicate unique keys can be committed under concurrency, or gaps are documented.
• Findings recorded on this ticket.
• Acceptance bar: the full jstest suite passes with featureFlagStagedWrites enabled, except for specific tests excluded with a documented reason tied to prototype scope.

Technical Notes

• Prototype quality; not intended for commit to master.
• Design reference: src/mongo/db/txns/staged_writes_design.md (PR #66680).