What actually happens in a database index (and why half of them do nothing)

Same query. Same table. Same million rows. One day it takes 4 seconds . The next day, 4 milliseconds . Nothing changed in the data. The only thing that changed was one line — you added an index . Four seconds to four milliseconds is a thousand times faster, from one line of SQL. But here's the part nobody tells you: half the indexes people add do nothing. The query stays slow, the writes get slower, and they can't figure out why. By the end of this you'll know what an index actually is — and the one rule that decides whether yours even gets used. Prefer to watch? Full walkthrough with the B-tree lookup animation: With no index: a full table scan You ask the database for one user by email. With no index, what does it do? It reads the first row. Not a match. The second row. Not a match. It keeps going — every single row — until it finds yours or runs out. A million rows, a million checks. SELECT * FROM users WHERE email = 'vlad@stack.dev' ; With no index, that WHERE line has only one way to run: look at all of them. The work grows with the table — ten times the rows, ten times the wait. That's a full table scan , and that's your four seconds. What an index actually is Most people picture an index as a copy of the table, or some kind of cache. It's neither. An index is a sorted map — just the column you search on, kept in order, with a pointer back to the full row. And the shape it's sorted into has a name: a B-tree (the default index in both Postgres and MySQL — technically a B+ tree). At the top, one node — the root . It splits into a few branches . Each branch splits again, down to the leaves , where the pointers to the rows actually live. Every node is sorted. The root doesn't hold your data — it holds signposts . Emails before "M"? Go left. "N" and after? Go right. Each step throws away half the tree, or more. You're never reading rows. You're following signs. The walk: three hops, not a million rows Watch what the lookup actually does: The root — one hop. A branc