Database indexing

You add a feature that looks up users by email, and it works fine with a hundred users. At a million users it crawls. Nothing about your code changed, so what happened? The database is doing more work than you think for that lookup, and an index is usually the fix.

An index is a separate, sorted structure the database keeps so it can find rows without examining every one. It is the difference between flipping to the index at the back of a book and reading the whole book to find a topic.

This unit is about the judgment around indexes: what they speed up, the price they quietly charge on writes, and how to decide which columns deserve one. It is one of the most practical wins in all of system design.