An index made our query faster. It slowly suffocated our database.

Hello, I'm Tuan. When backend engineers encounter a slow query, the first instinct is often something like: "Check the WHERE and ORDER BY, then just add a composite index." I used to think the same way. And to be fair, in many cases, that approach works perfectly fine. But once, a seemingly correct optimization turned into a production incident. The read query became significantly faster, the EXPLAIN plan looked clean, and everything seemed perfect. Yet slowly, the entire production system began