0272 — The compaction_key partial index was dropped once latest_key left it without a consumer

Context. After the read path swapped from the correlated scan to the latest_key lookup, the per-topic partial index (compaction_key, id) WHERE compaction_key IS NOT NULL existed to serve a query that no longer ran.

Decision. Drop it — but only after verifying via EXPLAIN, twice (a keyed row present, and a pure-unkeyed range matching compactionlab’s own shape), that the index appears nowhere in the new predicate’s plan, not even as a rejected candidate, and that the unkeyed-row short-circuit holds identically against latest_key.

Consequences. Keyed publishes stop paying maintenance on a dead index. The verification caught a real staleness bug: compactionlab’s EXPLAIN check had silently drifted to testing the old query after the read-path swap, its comment still claiming to test the exact shape readMessages runs — fixed before deciding. Cost accepted: compactionwidthlab/compactionscalelab, the index’s only remaining consumers, can no longer exactly reproduce their recorded historical numbers on a re-run — acceptable, since those measurements were deliberately frozen.