What Actually Happens When You Call INSERT?

You call INSERT . The database says OK . You move on. That acknowledgment feels instant. It feels cheap. It feels like the database just... wrote something down. But between your INSERT and that OK , at minimum four distinct things happened that most engineers who use databases every day have never thought about: The write was recorded in a sequential log before it touched any data structure — so a crash wouldn't lose it At least one index was updated — and that update is more expensive than the insert itself on some engines A decision was made about whether to hit the disk synchronously or defer it — a tradeoff with real latency consequences The data was placed into a structure that was chosen years ago by the database designers, and that choice explains almost every performance characteristic you've ever observed This series is about those four things. Not from a textbook — from the inside out, with real benchmark numbers at the end. The Moment of Insertion Let's start with the most