Adding an Index Made This Query Slower

Postgres can choose a slower plan after you add an index, because the planner gains another execution path and can misjudge its cost. Here, an index on created_at DESC looked cheap for an ORDER BY ... LIMIT 10 query, but its scan discarded 495,944 rows and the query went from 16.5ms to 241ms. A composite index on (user_id, created_at DESC) fixes it in 0.067ms. By now, we've all seen examples of AI