We ran into this around CI pipelines after we split the monolith. I expected a tooling problem. It was an assumption in the schema. EXPLAIN and a short rollback window saved the afternoon. Writes slowed first, then a handful of reads timed out. Dropping the unused index and batching the backfill unblocked the queue. I now review write paths with the same care as the happy-path query.
I used to think more indexes always meant faster queries. Production taught me about write amplification and table bloat instead. We had three indexes that nothing queried, slowing every insert. EXPLAIN ANALYZE finally showed which plans actually used which indexes. After dropping the dead ones, writes got healthier without hurting reads. Now I review unused indexes in the same ritual as reviewing slow queries.
No spike in CPU. Error budgets looked fine at a glance. Users still reported blank pages in a thin slice of traffic. Logs only showed upstream resets with no clear application exception. The culprit was a stale keep-alive timeout between nginx and the app. Aligning idle timeouts stopped the intermittent 502s within an hour. We also added a dashboard for upstream reset reasons so the next page is faster.