0264 — The compaction_key index is partial, covering only keyed rows
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Context. The correlated latest-per-key scan needed a cheap per-partition lookup on (compaction_key, id) in each message_log_<topic_id> table, but compaction is not the standard consumer setup — most rows carry compaction_key = NULL.
Decision. The index added in createTopicLog is partial — (compaction_key, id) WHERE compaction_key IS NOT NULL — rather than covering every row, so unkeyed traffic pays no index-maintenance write overhead for a feature it does not use.
Consequences. Keyed lookups stay cheap; unkeyed publishes are untouched. Superseded later in the same phase: once the latest_key lookup replaced the correlated scan in the read path, no production query consumed this index and it was dropped outright (see the decision to drop the compaction_key partial index).