When upserts don't update but still write: Debugging Postgres performance at scale

Datadog engineers encountered unexpected performance degradation, including doubled disk writes and quadrupled WAL syncs, after implementing a new Postgres upsert to track host activity. Through investigation with `pg_walinspect`, they discovered that even when the query's `WHERE` clause prevented actual data updates, the `ON CONFLICT` mechanism was still recording row locks in the Write-Ahead Log.