Overview

Prototype the merged cursor that presents a WiredTiger table as the union of its committed contents (at the transaction's snapshot) and the RecoveryUnit's staged entries for that table. The cursor is opaque to key and value contents; RecordStore, SortedDataInterface and container code above it is unchanged.

Scope of Work

• Semantics per design note section 6.1: exact search, next/prev two-way merge with staged entries winning and tombstones hiding table entries, positioning searches, bounds applied to both sides, reset, get_key/get_value holding ConstSharedBuffer references.
• Table-only operations: largest_key, random cursors, statistics.
• Choose and implement an interposition approach: WT_CURSOR-compatible shim vs. C++ cursor class (open question 4). Unsupported operations must fail loudly.
• Confirm key/value capture from the write helpers works for every key format used (open question 5).
• Unit tests for merge semantics, including bounds and reverse iteration.

Acceptance Criteria

• Merged cursor in the project branch with passing unit tests.
• Findings recorded on this ticket: interposition choice and rationale, unsupported operations.
• 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).